1 -----------------------------------------------------------------------------
2 This file contains a concatenation of the PCRE man pages, converted to plain
3 text format for ease of searching with a text editor, or for use on systems
4 that do not have a man page processor. The small individual files that give
5 synopses of each function in the library have not been included. There are
6 separate text files for the pcregrep and pcretest commands.
7 -----------------------------------------------------------------------------
14 PCRE - Perl-compatible regular expressions
19 The PCRE library is a set of functions that implement regular expres-
20 sion pattern matching using the same syntax and semantics as Perl, with
21 just a few differences. Certain features that appeared in Python and
22 PCRE before they appeared in Perl are also available using the Python
23 syntax. There is also some support for certain .NET and Oniguruma syn-
24 tax items, and there is an option for requesting some minor changes
25 that give better JavaScript compatibility.
27 The current implementation of PCRE (release 7.x) corresponds approxi-
28 mately with Perl 5.10, including support for UTF-8 encoded strings and
29 Unicode general category properties. However, UTF-8 and Unicode support
30 has to be explicitly enabled; it is not the default. The Unicode tables
31 correspond to Unicode release 5.0.0.
33 In addition to the Perl-compatible matching function, PCRE contains an
34 alternative matching function that matches the same compiled patterns
35 in a different way. In certain circumstances, the alternative function
36 has some advantages. For a discussion of the two matching algorithms,
37 see the pcrematching page.
39 PCRE is written in C and released as a C library. A number of people
40 have written wrappers and interfaces of various kinds. In particular,
41 Google Inc. have provided a comprehensive C++ wrapper. This is now
42 included as part of the PCRE distribution. The pcrecpp page has details
43 of this interface. Other people's contributions can be found in the
44 Contrib directory at the primary FTP site, which is:
46 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
48 Details of exactly which Perl regular expression features are and are
49 not supported by PCRE are given in separate documents. See the pcrepat-
50 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
53 Some features of PCRE can be included, excluded, or changed when the
54 library is built. The pcre_config() function makes it possible for a
55 client to discover which features are available. The features them-
56 selves are described in the pcrebuild page. Documentation about build-
57 ing PCRE for various operating systems can be found in the README file
58 in the source distribution.
60 The library contains a number of undocumented internal functions and
61 data tables that are used by more than one of the exported external
62 functions, but which are not intended for use by external callers.
63 Their names all begin with "_pcre_", which hopefully will not provoke
64 any name clashes. In some environments, it is possible to control which
65 external symbols are exported when a shared library is built, and in
66 these cases the undocumented symbols are not exported.
71 The user documentation for PCRE comprises a number of different sec-
72 tions. In the "man" format, each of these is a separate "man page". In
73 the HTML format, each is a separate page, linked from the index page.
74 In the plain text format, all the sections are concatenated, for ease
75 of searching. The sections are as follows:
78 pcre-config show PCRE installation configuration information
79 pcreapi details of PCRE's native C API
80 pcrebuild options for building PCRE
81 pcrecallout details of the callout feature
82 pcrecompat discussion of Perl compatibility
83 pcrecpp details of the C++ wrapper
84 pcregrep description of the pcregrep command
85 pcrematching discussion of the two matching algorithms
86 pcrepartial details of the partial matching facility
87 pcrepattern syntax and semantics of supported
89 pcresyntax quick syntax reference
90 pcreperform discussion of performance issues
91 pcreposix the POSIX-compatible C API
92 pcreprecompile details of saving and re-using precompiled patterns
93 pcresample discussion of the sample program
94 pcrestack discussion of stack usage
95 pcretest description of the pcretest testing command
97 In addition, in the "man" and HTML formats, there is a short page for
98 each C library function, listing its arguments and results.
103 There are some size limitations in PCRE but it is hoped that they will
104 never in practice be relevant.
106 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
107 is compiled with the default internal linkage size of 2. If you want to
108 process regular expressions that are truly enormous, you can compile
109 PCRE with an internal linkage size of 3 or 4 (see the README file in
110 the source distribution and the pcrebuild documentation for details).
111 In these cases the limit is substantially larger. However, the speed
112 of execution is slower.
114 All values in repeating quantifiers must be less than 65536.
116 There is no limit to the number of parenthesized subpatterns, but there
117 can be no more than 65535 capturing subpatterns.
119 The maximum length of name for a named subpattern is 32 characters, and
120 the maximum number of named subpatterns is 10000.
122 The maximum length of a subject string is the largest positive number
123 that an integer variable can hold. However, when using the traditional
124 matching function, PCRE uses recursion to handle subpatterns and indef-
125 inite repetition. This means that the available stack space may limit
126 the size of a subject string that can be processed by certain patterns.
127 For a discussion of stack issues, see the pcrestack documentation.
130 UTF-8 AND UNICODE PROPERTY SUPPORT
132 From release 3.3, PCRE has had some support for character strings
133 encoded in the UTF-8 format. For release 4.0 this was greatly extended
134 to cover most common requirements, and in release 5.0 additional sup-
135 port for Unicode general category properties was added.
137 In order process UTF-8 strings, you must build PCRE to include UTF-8
138 support in the code, and, in addition, you must call pcre_compile()
139 with the PCRE_UTF8 option flag. When you do this, both the pattern and
140 any subject strings that are matched against it are treated as UTF-8
141 strings instead of just strings of bytes.
143 If you compile PCRE with UTF-8 support, but do not use it at run time,
144 the library will be a bit bigger, but the additional run time overhead
145 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
148 If PCRE is built with Unicode character property support (which implies
149 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
150 ported. The available properties that can be tested are limited to the
151 general category properties such as Lu for an upper case letter or Nd
152 for a decimal number, the Unicode script names such as Arabic or Han,
153 and the derived properties Any and L&. A full list is given in the
154 pcrepattern documentation. Only the short names for properties are sup-
155 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
156 ter}, is not supported. Furthermore, in Perl, many properties may
157 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
158 does not support this.
160 Validity of UTF-8 strings
162 When you set the PCRE_UTF8 flag, the strings passed as patterns and
163 subjects are (by default) checked for validity on entry to the relevant
164 functions. From release 7.3 of PCRE, the check is according the rules
165 of RFC 3629, which are themselves derived from the Unicode specifica-
166 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
167 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
168 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
171 The excluded code points are the "Low Surrogate Area" of Unicode, of
172 which the Unicode Standard says this: "The Low Surrogate Area does not
173 contain any character assignments, consequently no character code
174 charts or namelists are provided for this area. Surrogates are reserved
175 for use with UTF-16 and then must be used in pairs." The code points
176 that are encoded by UTF-16 pairs are available as independent code
177 points in the UTF-8 encoding. (In other words, the whole surrogate
178 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
180 If an invalid UTF-8 string is passed to PCRE, an error return
181 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
182 that your strings are valid, and therefore want to skip these checks in
183 order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
184 compile time or at run time, PCRE assumes that the pattern or subject
185 it is given (respectively) contains only valid UTF-8 codes. In this
186 case, it does not diagnose an invalid UTF-8 string.
188 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
189 what happens depends on why the string is invalid. If the string con-
190 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
191 string of characters in the range 0 to 0x7FFFFFFF. In other words,
192 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
193 strings according to the more liberal rules of RFC 2279. However, if
194 the string does not even conform to RFC 2279, the result is undefined.
195 Your program may crash.
197 If you want to process strings of values in the full range 0 to
198 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
199 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
200 this situation, you will have to apply your own validity check.
202 General comments about UTF-8 mode
204 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
205 two-byte UTF-8 character if the value is greater than 127.
207 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
208 characters for values greater than \177.
210 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
211 vidual bytes, for example: \x{100}{3}.
213 4. The dot metacharacter matches one UTF-8 character instead of a sin-
216 5. The escape sequence \C can be used to match a single byte in UTF-8
217 mode, but its use can lead to some strange effects. This facility is
218 not available in the alternative matching function, pcre_dfa_exec().
220 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
221 test characters of any code value, but the characters that PCRE recog-
222 nizes as digits, spaces, or word characters remain the same set as
223 before, all with values less than 256. This remains true even when PCRE
224 includes Unicode property support, because to do otherwise would slow
225 down PCRE in many common cases. If you really want to test for a wider
226 sense of, say, "digit", you must use Unicode property tests such as
229 7. Similarly, characters that match the POSIX named character classes
230 are all low-valued characters.
232 8. However, the Perl 5.10 horizontal and vertical whitespace matching
233 escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
236 9. Case-insensitive matching applies only to characters whose values
237 are less than 128, unless PCRE is built with Unicode property support.
238 Even when Unicode property support is available, PCRE still uses its
239 own character tables when checking the case of low-valued characters,
240 so as not to degrade performance. The Unicode property information is
241 used only for characters with higher values. Even when Unicode property
242 support is available, PCRE supports case-insensitive matching only when
243 there is a one-to-one mapping between a letter's cases. There are a
244 small number of many-to-one mappings in Unicode; these are not sup-
251 University Computing Service
252 Cambridge CB2 3QH, England.
254 Putting an actual email address here seems to have been a spam magnet,
255 so I've taken it away. If you want to email me, use my two initials,
256 followed by the two digits 10, at the domain cam.ac.uk.
261 Last updated: 12 April 2008
262 Copyright (c) 1997-2008 University of Cambridge.
263 ------------------------------------------------------------------------------
266 PCREBUILD(3) PCREBUILD(3)
270 PCRE - Perl-compatible regular expressions
273 PCRE BUILD-TIME OPTIONS
275 This document describes the optional features of PCRE that can be
276 selected when the library is compiled. It assumes use of the configure
277 script, where the optional features are selected or deselected by pro-
278 viding options to configure before running the make command. However,
279 the same options can be selected in both Unix-like and non-Unix-like
280 environments using the GUI facility of CMakeSetup if you are using
281 CMake instead of configure to build PCRE.
283 The complete list of options for configure (which includes the standard
284 ones such as the selection of the installation directory) can be
289 The following sections include descriptions of options whose names
290 begin with --enable or --disable. These settings specify changes to the
291 defaults for the configure command. Because of the way that configure
292 works, --enable and --disable always come in pairs, so the complemen-
293 tary option always exists as well, but as it specifies the default, it
299 By default, the configure script will search for a C++ compiler and C++
300 header files. If it finds them, it automatically builds the C++ wrapper
301 library for PCRE. You can disable this by adding
305 to the configure command.
310 To build PCRE with support for UTF-8 character strings, add
314 to the configure command. Of itself, this does not make PCRE treat
315 strings as UTF-8. As well as compiling PCRE with this option, you also
316 have have to set the PCRE_UTF8 option when you call the pcre_compile()
320 UNICODE CHARACTER PROPERTY SUPPORT
322 UTF-8 support allows PCRE to process character values greater than 255
323 in the strings that it handles. On its own, however, it does not pro-
324 vide any facilities for accessing the properties of such characters. If
325 you want to be able to use the pattern escapes \P, \p, and \X, which
326 refer to Unicode character properties, you must add
328 --enable-unicode-properties
330 to the configure command. This implies UTF-8 support, even if you have
331 not explicitly requested it.
333 Including Unicode property support adds around 30K of tables to the
334 PCRE library. Only the general category properties such as Lu and Nd
335 are supported. Details are given in the pcrepattern documentation.
338 CODE VALUE OF NEWLINE
340 By default, PCRE interprets character 10 (linefeed, LF) as indicating
341 the end of a line. This is the normal newline character on Unix-like
342 systems. You can compile PCRE to use character 13 (carriage return, CR)
345 --enable-newline-is-cr
347 to the configure command. There is also a --enable-newline-is-lf
348 option, which explicitly specifies linefeed as the newline character.
350 Alternatively, you can specify that line endings are to be indicated by
351 the two character sequence CRLF. If you want this, add
353 --enable-newline-is-crlf
355 to the configure command. There is a fourth option, specified by
357 --enable-newline-is-anycrlf
359 which causes PCRE to recognize any of the three sequences CR, LF, or
360 CRLF as indicating a line ending. Finally, a fifth option, specified by
362 --enable-newline-is-any
364 causes PCRE to recognize any Unicode newline sequence.
366 Whatever line ending convention is selected when PCRE is built can be
367 overridden when the library functions are called. At build time it is
368 conventional to use the standard for your operating system.
373 By default, the sequence \R in a pattern matches any Unicode newline
374 sequence, whatever has been selected as the line ending sequence. If
379 the default is changed so that \R matches only CR, LF, or CRLF. What-
380 ever is selected when PCRE is built can be overridden when the library
381 functions are called.
384 BUILDING SHARED AND STATIC LIBRARIES
386 The PCRE building process uses libtool to build both shared and static
387 Unix libraries by default. You can suppress one of these by adding one
393 to the configure command, as required.
398 When PCRE is called through the POSIX interface (see the pcreposix doc-
399 umentation), additional working storage is required for holding the
400 pointers to capturing substrings, because PCRE requires three integers
401 per substring, whereas the POSIX interface provides only two. If the
402 number of expected substrings is small, the wrapper function uses space
403 on the stack, because this is faster than using malloc() for each call.
404 The default threshold above which the stack is no longer used is 10; it
405 can be changed by adding a setting such as
407 --with-posix-malloc-threshold=20
409 to the configure command.
412 HANDLING VERY LARGE PATTERNS
414 Within a compiled pattern, offset values are used to point from one
415 part to another (for example, from an opening parenthesis to an alter-
416 nation metacharacter). By default, two-byte values are used for these
417 offsets, leading to a maximum size for a compiled pattern of around
418 64K. This is sufficient to handle all but the most gigantic patterns.
419 Nevertheless, some people do want to process enormous patterns, so it
420 is possible to compile PCRE to use three-byte or four-byte offsets by
421 adding a setting such as
425 to the configure command. The value given must be 2, 3, or 4. Using
426 longer offsets slows down the operation of PCRE because it has to load
427 additional bytes when handling them.
430 AVOIDING EXCESSIVE STACK USAGE
432 When matching with the pcre_exec() function, PCRE implements backtrack-
433 ing by making recursive calls to an internal function called match().
434 In environments where the size of the stack is limited, this can se-
435 verely limit PCRE's operation. (The Unix environment does not usually
436 suffer from this problem, but it may sometimes be necessary to increase
437 the maximum stack size. There is a discussion in the pcrestack docu-
438 mentation.) An alternative approach to recursion that uses memory from
439 the heap to remember data, instead of using recursive function calls,
440 has been implemented to work round the problem of limited stack size.
441 If you want to build a version of PCRE that works this way, add
443 --disable-stack-for-recursion
445 to the configure command. With this configuration, PCRE will use the
446 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
447 ment functions. By default these point to malloc() and free(), but you
448 can replace the pointers so that your own functions are used.
450 Separate functions are provided rather than using pcre_malloc and
451 pcre_free because the usage is very predictable: the block sizes
452 requested are always the same, and the blocks are always freed in
453 reverse order. A calling program might be able to implement optimized
454 functions that perform better than malloc() and free(). PCRE runs
455 noticeably more slowly when built in this way. This option affects only
456 the pcre_exec() function; it is not relevant for the the
457 pcre_dfa_exec() function.
460 LIMITING PCRE RESOURCE USAGE
462 Internally, PCRE has a function called match(), which it calls repeat-
463 edly (sometimes recursively) when matching a pattern with the
464 pcre_exec() function. By controlling the maximum number of times this
465 function may be called during a single matching operation, a limit can
466 be placed on the resources used by a single call to pcre_exec(). The
467 limit can be changed at run time, as described in the pcreapi documen-
468 tation. The default is 10 million, but this can be changed by adding a
471 --with-match-limit=500000
473 to the configure command. This setting has no effect on the
474 pcre_dfa_exec() matching function.
476 In some environments it is desirable to limit the depth of recursive
477 calls of match() more strictly than the total number of calls, in order
478 to restrict the maximum amount of stack (or heap, if --disable-stack-
479 for-recursion is specified) that is used. A second limit controls this;
480 it defaults to the value that is set for --with-match-limit, which
481 imposes no additional constraints. However, you can set a lower limit
482 by adding, for example,
484 --with-match-limit-recursion=10000
486 to the configure command. This value can also be overridden at run
490 CREATING CHARACTER TABLES AT BUILD TIME
492 PCRE uses fixed tables for processing characters whose code values are
493 less than 256. By default, PCRE is built with a set of tables that are
494 distributed in the file pcre_chartables.c.dist. These tables are for
495 ASCII codes only. If you add
497 --enable-rebuild-chartables
499 to the configure command, the distributed tables are no longer used.
500 Instead, a program called dftables is compiled and run. This outputs
501 the source for new set of tables, created in the default locale of your
502 C runtime system. (This method of replacing the tables does not work if
503 you are cross compiling, because dftables is run on the local host. If
504 you need to create alternative tables when cross compiling, you will
505 have to do so "by hand".)
510 PCRE assumes by default that it will run in an environment where the
511 character code is ASCII (or Unicode, which is a superset of ASCII).
512 This is the case for most computer operating systems. PCRE can, how-
513 ever, be compiled to run in an EBCDIC environment by adding
517 to the configure command. This setting implies --enable-rebuild-charta-
518 bles. You should only use it if you know that you are in an EBCDIC
519 environment (for example, an IBM mainframe operating system).
522 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
524 By default, pcregrep reads all files as plain text. You can build it so
525 that it recognizes files whose names end in .gz or .bz2, and reads them
526 with libz or libbz2, respectively, by adding one or both of
528 --enable-pcregrep-libz
529 --enable-pcregrep-libbz2
531 to the configure command. These options naturally require that the rel-
532 evant libraries are installed on your system. Configuration will fail
536 PCRETEST OPTION FOR LIBREADLINE SUPPORT
540 --enable-pcretest-libreadline
542 to the configure command, pcretest is linked with the libreadline
543 library, and when its input is from a terminal, it reads it using the
544 readline() function. This provides line-editing and history facilities.
545 Note that libreadline is GPL-licenced, so if you distribute a binary of
546 pcretest linked in this way, there may be licensing issues.
548 Setting this option causes the -lreadline option to be added to the
549 pcretest build. In many operating environments with a sytem-installed
550 libreadline this is sufficient. However, in some environments (e.g. if
551 an unmodified distribution version of readline is in use), some extra
552 configuration may be necessary. The INSTALL file for libreadline says
555 "Readline uses the termcap functions, but does not link with the
556 termcap or curses library itself, allowing applications which link
557 with readline the to choose an appropriate library."
559 If your environment has not been set up so that an appropriate library
560 is automatically included, you may need to add something like
564 immediately before the configure command.
569 pcreapi(3), pcre_config(3).
575 University Computing Service
576 Cambridge CB2 3QH, England.
581 Last updated: 13 April 2008
582 Copyright (c) 1997-2008 University of Cambridge.
583 ------------------------------------------------------------------------------
586 PCREMATCHING(3) PCREMATCHING(3)
590 PCRE - Perl-compatible regular expressions
593 PCRE MATCHING ALGORITHMS
595 This document describes the two different algorithms that are available
596 in PCRE for matching a compiled regular expression against a given sub-
597 ject string. The "standard" algorithm is the one provided by the
598 pcre_exec() function. This works in the same was as Perl's matching
599 function, and provides a Perl-compatible matching operation.
601 An alternative algorithm is provided by the pcre_dfa_exec() function;
602 this operates in a different way, and is not Perl-compatible. It has
603 advantages and disadvantages compared with the standard algorithm, and
604 these are described below.
606 When there is only one possible way in which a given subject string can
607 match a pattern, the two algorithms give the same answer. A difference
608 arises, however, when there are multiple possibilities. For example, if
613 is matched against the string
615 <something> <something else> <something further>
617 there are three possible answers. The standard algorithm finds only one
618 of them, whereas the alternative algorithm finds all three.
621 REGULAR EXPRESSIONS AS TREES
623 The set of strings that are matched by a regular expression can be rep-
624 resented as a tree structure. An unlimited repetition in the pattern
625 makes the tree of infinite size, but it is still a tree. Matching the
626 pattern to a given subject string (from a given starting point) can be
627 thought of as a search of the tree. There are two ways to search a
628 tree: depth-first and breadth-first, and these correspond to the two
629 matching algorithms provided by PCRE.
632 THE STANDARD MATCHING ALGORITHM
634 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
635 sions", the standard algorithm is an "NFA algorithm". It conducts a
636 depth-first search of the pattern tree. That is, it proceeds along a
637 single path through the tree, checking that the subject matches what is
638 required. When there is a mismatch, the algorithm tries any alterna-
639 tives at the current point, and if they all fail, it backs up to the
640 previous branch point in the tree, and tries the next alternative
641 branch at that level. This often involves backing up (moving to the
642 left) in the subject string as well. The order in which repetition
643 branches are tried is controlled by the greedy or ungreedy nature of
646 If a leaf node is reached, a matching string has been found, and at
647 that point the algorithm stops. Thus, if there is more than one possi-
648 ble match, this algorithm returns the first one that it finds. Whether
649 this is the shortest, the longest, or some intermediate length depends
650 on the way the greedy and ungreedy repetition quantifiers are specified
653 Because it ends up with a single path through the tree, it is rela-
654 tively straightforward for this algorithm to keep track of the sub-
655 strings that are matched by portions of the pattern in parentheses.
656 This provides support for capturing parentheses and back references.
659 THE ALTERNATIVE MATCHING ALGORITHM
661 This algorithm conducts a breadth-first search of the tree. Starting
662 from the first matching point in the subject, it scans the subject
663 string from left to right, once, character by character, and as it does
664 this, it remembers all the paths through the tree that represent valid
665 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
666 though it is not implemented as a traditional finite state machine (it
667 keeps multiple states active simultaneously).
669 The scan continues until either the end of the subject is reached, or
670 there are no more unterminated paths. At this point, terminated paths
671 represent the different matching possibilities (if there are none, the
672 match has failed). Thus, if there is more than one possible match,
673 this algorithm finds all of them, and in particular, it finds the long-
674 est. In PCRE, there is an option to stop the algorithm after the first
675 match (which is necessarily the shortest) has been found.
677 Note that all the matches that are found start at the same point in the
678 subject. If the pattern
682 is matched against the string "the caterpillar catchment", the result
683 will be the three strings "cat", "cater", and "caterpillar" that start
684 at the fourth character of the subject. The algorithm does not automat-
685 ically move on to find matches that start at later positions.
687 There are a number of features of PCRE regular expressions that are not
688 supported by the alternative matching algorithm. They are as follows:
690 1. Because the algorithm finds all possible matches, the greedy or
691 ungreedy nature of repetition quantifiers is not relevant. Greedy and
692 ungreedy quantifiers are treated in exactly the same way. However, pos-
693 sessive quantifiers can make a difference when what follows could also
694 match what is quantified, for example in a pattern like this:
698 This pattern matches "aaab!" but not "aaa!", which would be matched by
699 a non-possessive quantifier. Similarly, if an atomic group is present,
700 it is matched as if it were a standalone pattern at the current point,
701 and the longest match is then "locked in" for the rest of the overall
704 2. When dealing with multiple paths through the tree simultaneously, it
705 is not straightforward to keep track of captured substrings for the
706 different matching possibilities, and PCRE's implementation of this
707 algorithm does not attempt to do this. This means that no captured sub-
708 strings are available.
710 3. Because no substrings are captured, back references within the pat-
711 tern are not supported, and cause errors if encountered.
713 4. For the same reason, conditional expressions that use a backrefer-
714 ence as the condition or test for a specific group recursion are not
717 5. Because many paths through the tree may be active, the \K escape
718 sequence, which resets the start of the match when encountered (but may
719 be on some paths and not on others), is not supported. It causes an
720 error if encountered.
722 6. Callouts are supported, but the value of the capture_top field is
723 always 1, and the value of the capture_last field is always -1.
725 7. The \C escape sequence, which (in the standard algorithm) matches a
726 single byte, even in UTF-8 mode, is not supported because the alterna-
727 tive algorithm moves through the subject string one character at a
728 time, for all active paths through the tree.
730 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
731 are not supported. (*FAIL) is supported, and behaves like a failing
735 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
737 Using the alternative matching algorithm provides the following advan-
740 1. All possible matches (at a single point in the subject) are automat-
741 ically found, and in particular, the longest match is found. To find
742 more than one match using the standard algorithm, you have to do kludgy
743 things with callouts.
745 2. There is much better support for partial matching. The restrictions
746 on the content of the pattern that apply when using the standard algo-
747 rithm for partial matching do not apply to the alternative algorithm.
748 For non-anchored patterns, the starting position of a partial match is
751 3. Because the alternative algorithm scans the subject string just
752 once, and never needs to backtrack, it is possible to pass very long
753 subject strings to the matching function in several pieces, checking
754 for partial matching each time.
757 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
759 The alternative algorithm suffers from a number of disadvantages:
761 1. It is substantially slower than the standard algorithm. This is
762 partly because it has to search for all possible matches, but is also
763 because it is less susceptible to optimization.
765 2. Capturing parentheses and back references are not supported.
767 3. Although atomic groups are supported, their use does not provide the
768 performance advantage that it does for the standard algorithm.
774 University Computing Service
775 Cambridge CB2 3QH, England.
780 Last updated: 19 April 2008
781 Copyright (c) 1997-2008 University of Cambridge.
782 ------------------------------------------------------------------------------
785 PCREAPI(3) PCREAPI(3)
789 PCRE - Perl-compatible regular expressions
796 pcre *pcre_compile(const char *pattern, int options,
797 const char **errptr, int *erroffset,
798 const unsigned char *tableptr);
800 pcre *pcre_compile2(const char *pattern, int options,
802 const char **errptr, int *erroffset,
803 const unsigned char *tableptr);
805 pcre_extra *pcre_study(const pcre *code, int options,
806 const char **errptr);
808 int pcre_exec(const pcre *code, const pcre_extra *extra,
809 const char *subject, int length, int startoffset,
810 int options, int *ovector, int ovecsize);
812 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
813 const char *subject, int length, int startoffset,
814 int options, int *ovector, int ovecsize,
815 int *workspace, int wscount);
817 int pcre_copy_named_substring(const pcre *code,
818 const char *subject, int *ovector,
819 int stringcount, const char *stringname,
820 char *buffer, int buffersize);
822 int pcre_copy_substring(const char *subject, int *ovector,
823 int stringcount, int stringnumber, char *buffer,
826 int pcre_get_named_substring(const pcre *code,
827 const char *subject, int *ovector,
828 int stringcount, const char *stringname,
829 const char **stringptr);
831 int pcre_get_stringnumber(const pcre *code,
834 int pcre_get_stringtable_entries(const pcre *code,
835 const char *name, char **first, char **last);
837 int pcre_get_substring(const char *subject, int *ovector,
838 int stringcount, int stringnumber,
839 const char **stringptr);
841 int pcre_get_substring_list(const char *subject,
842 int *ovector, int stringcount, const char ***listptr);
844 void pcre_free_substring(const char *stringptr);
846 void pcre_free_substring_list(const char **stringptr);
848 const unsigned char *pcre_maketables(void);
850 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
851 int what, void *where);
853 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
855 int pcre_refcount(pcre *code, int adjust);
857 int pcre_config(int what, void *where);
859 char *pcre_version(void);
861 void *(*pcre_malloc)(size_t);
863 void (*pcre_free)(void *);
865 void *(*pcre_stack_malloc)(size_t);
867 void (*pcre_stack_free)(void *);
869 int (*pcre_callout)(pcre_callout_block *);
874 PCRE has its own native API, which is described in this document. There
875 are also some wrapper functions that correspond to the POSIX regular
876 expression API. These are described in the pcreposix documentation.
877 Both of these APIs define a set of C function calls. A C++ wrapper is
878 distributed with PCRE. It is documented in the pcrecpp page.
880 The native API C function prototypes are defined in the header file
881 pcre.h, and on Unix systems the library itself is called libpcre. It
882 can normally be accessed by adding -lpcre to the command for linking an
883 application that uses PCRE. The header file defines the macros
884 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
885 bers for the library. Applications can use these to include support
886 for different releases of PCRE.
888 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
889 pcre_exec() are used for compiling and matching regular expressions in
890 a Perl-compatible manner. A sample program that demonstrates the sim-
891 plest way of using them is provided in the file called pcredemo.c in
892 the source distribution. The pcresample documentation describes how to
895 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
896 ble, is also provided. This uses a different algorithm for the match-
897 ing. The alternative algorithm finds all possible matches (at a given
898 point in the subject), and scans the subject just once. However, this
899 algorithm does not return captured substrings. A description of the two
900 matching algorithms and their advantages and disadvantages is given in
901 the pcrematching documentation.
903 In addition to the main compiling and matching functions, there are
904 convenience functions for extracting captured substrings from a subject
905 string that is matched by pcre_exec(). They are:
907 pcre_copy_substring()
908 pcre_copy_named_substring()
910 pcre_get_named_substring()
911 pcre_get_substring_list()
912 pcre_get_stringnumber()
913 pcre_get_stringtable_entries()
915 pcre_free_substring() and pcre_free_substring_list() are also provided,
916 to free the memory used for extracted strings.
918 The function pcre_maketables() is used to build a set of character
919 tables in the current locale for passing to pcre_compile(),
920 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
921 provided for specialist use. Most commonly, no special tables are
922 passed, in which case internal tables that are generated when PCRE is
925 The function pcre_fullinfo() is used to find out information about a
926 compiled pattern; pcre_info() is an obsolete version that returns only
927 some of the available information, but is retained for backwards com-
928 patibility. The function pcre_version() returns a pointer to a string
929 containing the version of PCRE and its date of release.
931 The function pcre_refcount() maintains a reference count in a data
932 block containing a compiled pattern. This is provided for the benefit
933 of object-oriented applications.
935 The global variables pcre_malloc and pcre_free initially contain the
936 entry points of the standard malloc() and free() functions, respec-
937 tively. PCRE calls the memory management functions via these variables,
938 so a calling program can replace them if it wishes to intercept the
939 calls. This should be done before calling any PCRE functions.
941 The global variables pcre_stack_malloc and pcre_stack_free are also
942 indirections to memory management functions. These special functions
943 are used only when PCRE is compiled to use the heap for remembering
944 data, instead of recursive function calls, when running the pcre_exec()
945 function. See the pcrebuild documentation for details of how to do
946 this. It is a non-standard way of building PCRE, for use in environ-
947 ments that have limited stacks. Because of the greater use of memory
948 management, it runs more slowly. Separate functions are provided so
949 that special-purpose external code can be used for this case. When
950 used, these functions are always called in a stack-like manner (last
951 obtained, first freed), and always for memory blocks of the same size.
952 There is a discussion about PCRE's stack usage in the pcrestack docu-
955 The global variable pcre_callout initially contains NULL. It can be set
956 by the caller to a "callout" function, which PCRE will then call at
957 specified points during a matching operation. Details are given in the
958 pcrecallout documentation.
963 PCRE supports five different conventions for indicating line breaks in
964 strings: a single CR (carriage return) character, a single LF (line-
965 feed) character, the two-character sequence CRLF, any of the three pre-
966 ceding, or any Unicode newline sequence. The Unicode newline sequences
967 are the three just mentioned, plus the single characters VT (vertical
968 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
969 separator, U+2028), and PS (paragraph separator, U+2029).
971 Each of the first three conventions is used by at least one operating
972 system as its standard newline sequence. When PCRE is built, a default
973 can be specified. The default default is LF, which is the Unix stan-
974 dard. When PCRE is run, the default can be overridden, either when a
975 pattern is compiled, or when it is matched.
977 At compile time, the newline convention can be specified by the options
978 argument of pcre_compile(), or it can be specified by special text at
979 the start of the pattern itself; this overrides any other settings. See
980 the pcrepattern page for details of the special character sequences.
982 In the PCRE documentation the word "newline" is used to mean "the char-
983 acter or pair of characters that indicate a line break". The choice of
984 newline convention affects the handling of the dot, circumflex, and
985 dollar metacharacters, the handling of #-comments in /x mode, and, when
986 CRLF is a recognized line ending sequence, the match position advance-
987 ment for a non-anchored pattern. There is more detail about this in the
988 section on pcre_exec() options below.
990 The choice of newline convention does not affect the interpretation of
991 the \n or \r escape sequences, nor does it affect what \R matches,
992 which is controlled in a similar way, but by separate options.
997 The PCRE functions can be used in multi-threading applications, with
998 the proviso that the memory management functions pointed to by
999 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1000 callout function pointed to by pcre_callout, are shared by all threads.
1002 The compiled form of a regular expression is not altered during match-
1003 ing, so the same compiled pattern can safely be used by several threads
1007 SAVING PRECOMPILED PATTERNS FOR LATER USE
1009 The compiled form of a regular expression can be saved and re-used at a
1010 later time, possibly by a different program, and even on a host other
1011 than the one on which it was compiled. Details are given in the
1012 pcreprecompile documentation. However, compiling a regular expression
1013 with one version of PCRE for use with a different version is not guar-
1014 anteed to work and may cause crashes.
1017 CHECKING BUILD-TIME OPTIONS
1019 int pcre_config(int what, void *where);
1021 The function pcre_config() makes it possible for a PCRE client to dis-
1022 cover which optional features have been compiled into the PCRE library.
1023 The pcrebuild documentation has more details about these optional fea-
1026 The first argument for pcre_config() is an integer, specifying which
1027 information is required; the second argument is a pointer to a variable
1028 into which the information is placed. The following information is
1033 The output is an integer that is set to one if UTF-8 support is avail-
1034 able; otherwise it is set to zero.
1036 PCRE_CONFIG_UNICODE_PROPERTIES
1038 The output is an integer that is set to one if support for Unicode
1039 character properties is available; otherwise it is set to zero.
1043 The output is an integer whose value specifies the default character
1044 sequence that is recognized as meaning "newline". The four values that
1045 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1046 and -1 for ANY. The default should normally be the standard sequence
1047 for your operating system.
1051 The output is an integer whose value indicates what character sequences
1052 the \R escape sequence matches by default. A value of 0 means that \R
1053 matches any Unicode line ending sequence; a value of 1 means that \R
1054 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1055 tern is compiled or matched.
1057 PCRE_CONFIG_LINK_SIZE
1059 The output is an integer that contains the number of bytes used for
1060 internal linkage in compiled regular expressions. The value is 2, 3, or
1061 4. Larger values allow larger regular expressions to be compiled, at
1062 the expense of slower matching. The default value of 2 is sufficient
1063 for all but the most massive patterns, since it allows the compiled
1064 pattern to be up to 64K in size.
1066 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1068 The output is an integer that contains the threshold above which the
1069 POSIX interface uses malloc() for output vectors. Further details are
1070 given in the pcreposix documentation.
1072 PCRE_CONFIG_MATCH_LIMIT
1074 The output is an integer that gives the default limit for the number of
1075 internal matching function calls in a pcre_exec() execution. Further
1076 details are given with pcre_exec() below.
1078 PCRE_CONFIG_MATCH_LIMIT_RECURSION
1080 The output is an integer that gives the default limit for the depth of
1081 recursion when calling the internal matching function in a pcre_exec()
1082 execution. Further details are given with pcre_exec() below.
1084 PCRE_CONFIG_STACKRECURSE
1086 The output is an integer that is set to one if internal recursion when
1087 running pcre_exec() is implemented by recursive function calls that use
1088 the stack to remember their state. This is the usual way that PCRE is
1089 compiled. The output is zero if PCRE was compiled to use blocks of data
1090 on the heap instead of recursive function calls. In this case,
1091 pcre_stack_malloc and pcre_stack_free are called to manage memory
1092 blocks on the heap, thus avoiding the use of the stack.
1097 pcre *pcre_compile(const char *pattern, int options,
1098 const char **errptr, int *erroffset,
1099 const unsigned char *tableptr);
1101 pcre *pcre_compile2(const char *pattern, int options,
1103 const char **errptr, int *erroffset,
1104 const unsigned char *tableptr);
1106 Either of the functions pcre_compile() or pcre_compile2() can be called
1107 to compile a pattern into an internal form. The only difference between
1108 the two interfaces is that pcre_compile2() has an additional argument,
1109 errorcodeptr, via which a numerical error code can be returned.
1111 The pattern is a C string terminated by a binary zero, and is passed in
1112 the pattern argument. A pointer to a single block of memory that is
1113 obtained via pcre_malloc is returned. This contains the compiled code
1114 and related data. The pcre type is defined for the returned block; this
1115 is a typedef for a structure whose contents are not externally defined.
1116 It is up to the caller to free the memory (via pcre_free) when it is no
1119 Although the compiled code of a PCRE regex is relocatable, that is, it
1120 does not depend on memory location, the complete pcre data block is not
1121 fully relocatable, because it may contain a copy of the tableptr argu-
1122 ment, which is an address (see below).
1124 The options argument contains various bit settings that affect the com-
1125 pilation. It should be zero if no options are required. The available
1126 options are described below. Some of them, in particular, those that
1127 are compatible with Perl, can also be set and unset from within the
1128 pattern (see the detailed description in the pcrepattern documenta-
1129 tion). For these options, the contents of the options argument speci-
1130 fies their initial settings at the start of compilation and execution.
1131 The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
1132 of matching as well as at compile time.
1134 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1135 if compilation of a pattern fails, pcre_compile() returns NULL, and
1136 sets the variable pointed to by errptr to point to a textual error mes-
1137 sage. This is a static string that is part of the library. You must not
1138 try to free it. The offset from the start of the pattern to the charac-
1139 ter where the error was discovered is placed in the variable pointed to
1140 by erroffset, which must not be NULL. If it is, an immediate error is
1143 If pcre_compile2() is used instead of pcre_compile(), and the error-
1144 codeptr argument is not NULL, a non-zero error code number is returned
1145 via this argument in the event of an error. This is in addition to the
1146 textual error message. Error codes and messages are listed below.
1148 If the final argument, tableptr, is NULL, PCRE uses a default set of
1149 character tables that are built when PCRE is compiled, using the
1150 default C locale. Otherwise, tableptr must be an address that is the
1151 result of a call to pcre_maketables(). This value is stored with the
1152 compiled pattern, and used again by pcre_exec(), unless another table
1153 pointer is passed to it. For more discussion, see the section on locale
1156 This code fragment shows a typical straightforward call to pcre_com-
1163 "^A.*Z", /* the pattern */
1164 0, /* default options */
1165 &error, /* for error message */
1166 &erroffset, /* for error offset */
1167 NULL); /* use default character tables */
1169 The following names for option bits are defined in the pcre.h header
1174 If this bit is set, the pattern is forced to be "anchored", that is, it
1175 is constrained to match only at the first matching point in the string
1176 that is being searched (the "subject string"). This effect can also be
1177 achieved by appropriate constructs in the pattern itself, which is the
1178 only way to do it in Perl.
1182 If this bit is set, pcre_compile() automatically inserts callout items,
1183 all with number 255, before each pattern item. For discussion of the
1184 callout facility, see the pcrecallout documentation.
1189 These options (which are mutually exclusive) control what the \R escape
1190 sequence matches. The choice is either to match only CR, LF, or CRLF,
1191 or to match any Unicode newline sequence. The default is specified when
1192 PCRE is built. It can be overridden from within the pattern, or by set-
1193 ting an option when a compiled pattern is matched.
1197 If this bit is set, letters in the pattern match both upper and lower
1198 case letters. It is equivalent to Perl's /i option, and it can be
1199 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1200 always understands the concept of case for characters whose values are
1201 less than 128, so caseless matching is always possible. For characters
1202 with higher values, the concept of case is supported if PCRE is com-
1203 piled with Unicode property support, but not otherwise. If you want to
1204 use caseless matching for characters 128 and above, you must ensure
1205 that PCRE is compiled with Unicode property support as well as with
1210 If this bit is set, a dollar metacharacter in the pattern matches only
1211 at the end of the subject string. Without this option, a dollar also
1212 matches immediately before a newline at the end of the string (but not
1213 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1214 if PCRE_MULTILINE is set. There is no equivalent to this option in
1215 Perl, and no way to set it within a pattern.
1219 If this bit is set, a dot metacharater in the pattern matches all char-
1220 acters, including those that indicate newline. Without it, a dot does
1221 not match when the current position is at a newline. This option is
1222 equivalent to Perl's /s option, and it can be changed within a pattern
1223 by a (?s) option setting. A negative class such as [^a] always matches
1224 newline characters, independent of the setting of this option.
1228 If this bit is set, names used to identify capturing subpatterns need
1229 not be unique. This can be helpful for certain types of pattern when it
1230 is known that only one instance of the named subpattern can ever be
1231 matched. There are more details of named subpatterns below; see also
1232 the pcrepattern documentation.
1236 If this bit is set, whitespace data characters in the pattern are
1237 totally ignored except when escaped or inside a character class. White-
1238 space does not include the VT character (code 11). In addition, charac-
1239 ters between an unescaped # outside a character class and the next new-
1240 line, inclusive, are also ignored. This is equivalent to Perl's /x
1241 option, and it can be changed within a pattern by a (?x) option set-
1244 This option makes it possible to include comments inside complicated
1245 patterns. Note, however, that this applies only to data characters.
1246 Whitespace characters may never appear within special character
1247 sequences in a pattern, for example within the sequence (?( which
1248 introduces a conditional subpattern.
1252 This option was invented in order to turn on additional functionality
1253 of PCRE that is incompatible with Perl, but it is currently of very
1254 little use. When set, any backslash in a pattern that is followed by a
1255 letter that has no special meaning causes an error, thus reserving
1256 these combinations for future expansion. By default, as in Perl, a
1257 backslash followed by a letter with no special meaning is treated as a
1258 literal. (Perl can, however, be persuaded to give a warning for this.)
1259 There are at present no other features controlled by this option. It
1260 can also be set by a (?X) option setting within a pattern.
1264 If this option is set, an unanchored pattern is required to match
1265 before or at the first newline in the subject string, though the
1266 matched text may continue over the newline.
1268 PCRE_JAVASCRIPT_COMPAT
1270 If this option is set, PCRE's behaviour is changed in some ways so that
1271 it is compatible with JavaScript rather than Perl. The changes are as
1274 (1) A lone closing square bracket in a pattern causes a compile-time
1275 error, because this is illegal in JavaScript (by default it is treated
1276 as a data character). Thus, the pattern AB]CD becomes illegal when this
1279 (2) At run time, a back reference to an unset subpattern group matches
1280 an empty string (by default this causes the current matching alterna-
1281 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1282 set (assuming it can find an "a" in the subject), whereas it fails by
1283 default, for Perl compatibility.
1287 By default, PCRE treats the subject string as consisting of a single
1288 line of characters (even if it actually contains newlines). The "start
1289 of line" metacharacter (^) matches only at the start of the string,
1290 while the "end of line" metacharacter ($) matches only at the end of
1291 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1292 is set). This is the same as Perl.
1294 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1295 constructs match immediately following or immediately before internal
1296 newlines in the subject string, respectively, as well as at the very
1297 start and end. This is equivalent to Perl's /m option, and it can be
1298 changed within a pattern by a (?m) option setting. If there are no new-
1299 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1300 setting PCRE_MULTILINE has no effect.
1305 PCRE_NEWLINE_ANYCRLF
1308 These options override the default newline definition that was chosen
1309 when PCRE was built. Setting the first or the second specifies that a
1310 newline is indicated by a single character (CR or LF, respectively).
1311 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1312 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1313 that any of the three preceding sequences should be recognized. Setting
1314 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1315 recognized. The Unicode newline sequences are the three just mentioned,
1316 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1317 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1318 (paragraph separator, U+2029). The last two are recognized only in
1321 The newline setting in the options word uses three bits that are
1322 treated as a number, giving eight possibilities. Currently only six are
1323 used (default plus the five values above). This means that if you set
1324 more than one newline option, the combination may or may not be sensi-
1325 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1326 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1329 The only time that a line break is specially recognized when compiling
1330 a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1331 character class is encountered. This indicates a comment that lasts
1332 until after the next line break sequence. In other circumstances, line
1333 break sequences are treated as literal data, except that in
1334 PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1335 and are therefore ignored.
1337 The newline option that is set at compile time becomes the default that
1338 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1340 PCRE_NO_AUTO_CAPTURE
1342 If this option is set, it disables the use of numbered capturing paren-
1343 theses in the pattern. Any opening parenthesis that is not followed by
1344 ? behaves as if it were followed by ?: but named parentheses can still
1345 be used for capturing (and they acquire numbers in the usual way).
1346 There is no equivalent of this option in Perl.
1350 This option inverts the "greediness" of the quantifiers so that they
1351 are not greedy by default, but become greedy if followed by "?". It is
1352 not compatible with Perl. It can also be set by a (?U) option setting
1357 This option causes PCRE to regard both the pattern and the subject as
1358 strings of UTF-8 characters instead of single-byte character strings.
1359 However, it is available only when PCRE is built to include UTF-8 sup-
1360 port. If not, the use of this option provokes an error. Details of how
1361 this option changes the behaviour of PCRE are given in the section on
1362 UTF-8 support in the main pcre page.
1366 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1367 automatically checked. There is a discussion about the validity of
1368 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1369 bytes is found, pcre_compile() returns an error. If you already know
1370 that your pattern is valid, and you want to skip this check for perfor-
1371 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1372 set, the effect of passing an invalid UTF-8 string as a pattern is
1373 undefined. It may cause your program to crash. Note that this option
1374 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1375 UTF-8 validity checking of subject strings.
1378 COMPILATION ERROR CODES
1380 The following table lists the error codes than may be returned by
1381 pcre_compile2(), along with the error messages that may be returned by
1382 both compiling functions. As PCRE has developed, some error codes have
1383 fallen out of use. To avoid confusion, they have not been re-used.
1386 1 \ at end of pattern
1387 2 \c at end of pattern
1388 3 unrecognized character follows \
1389 4 numbers out of order in {} quantifier
1390 5 number too big in {} quantifier
1391 6 missing terminating ] for character class
1392 7 invalid escape sequence in character class
1393 8 range out of order in character class
1395 10 [this code is not in use]
1396 11 internal error: unexpected repeat
1397 12 unrecognized character after (? or (?-
1398 13 POSIX named classes are supported only within a class
1400 15 reference to non-existent subpattern
1401 16 erroffset passed as NULL
1402 17 unknown option bit(s) set
1403 18 missing ) after comment
1404 19 [this code is not in use]
1405 20 regular expression is too large
1406 21 failed to get memory
1407 22 unmatched parentheses
1408 23 internal error: code overflow
1409 24 unrecognized character after (?<
1410 25 lookbehind assertion is not fixed length
1411 26 malformed number or name after (?(
1412 27 conditional group contains more than two branches
1413 28 assertion expected after (?(
1414 29 (?R or (?[+-]digits must be followed by )
1415 30 unknown POSIX class name
1416 31 POSIX collating elements are not supported
1417 32 this version of PCRE is not compiled with PCRE_UTF8 support
1418 33 [this code is not in use]
1419 34 character value in \x{...} sequence is too large
1420 35 invalid condition (?(0)
1421 36 \C not allowed in lookbehind assertion
1422 37 PCRE does not support \L, \l, \N, \U, or \u
1423 38 number after (?C is > 255
1424 39 closing ) for (?C expected
1425 40 recursive call could loop indefinitely
1426 41 unrecognized character after (?P
1427 42 syntax error in subpattern name (missing terminator)
1428 43 two named subpatterns have the same name
1429 44 invalid UTF-8 string
1430 45 support for \P, \p, and \X has not been compiled
1431 46 malformed \P or \p sequence
1432 47 unknown property name after \P or \p
1433 48 subpattern name is too long (maximum 32 characters)
1434 49 too many named subpatterns (maximum 10000)
1435 50 [this code is not in use]
1436 51 octal value is greater than \377 (not in UTF-8 mode)
1437 52 internal error: overran compiling workspace
1438 53 internal error: previously-checked referenced subpattern not
1440 54 DEFINE group contains more than one branch
1441 55 repeating a DEFINE group is not allowed
1442 56 inconsistent NEWLINE options
1443 57 \g is not followed by a braced, angle-bracketed, or quoted
1444 name/number or by a plain number
1445 58 a numbered reference must not be zero
1446 59 (*VERB) with an argument is not supported
1447 60 (*VERB) not recognized
1448 61 number is too big
1449 62 subpattern name expected
1450 63 digit expected after (?+
1451 64 ] is an invalid data character in JavaScript compatibility mode
1453 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1454 values may be used if the limits were changed when PCRE was built.
1459 pcre_extra *pcre_study(const pcre *code, int options
1460 const char **errptr);
1462 If a compiled pattern is going to be used several times, it is worth
1463 spending more time analyzing it in order to speed up the time taken for
1464 matching. The function pcre_study() takes a pointer to a compiled pat-
1465 tern as its first argument. If studying the pattern produces additional
1466 information that will help speed up matching, pcre_study() returns a
1467 pointer to a pcre_extra block, in which the study_data field points to
1468 the results of the study.
1470 The returned value from pcre_study() can be passed directly to
1471 pcre_exec(). However, a pcre_extra block also contains other fields
1472 that can be set by the caller before the block is passed; these are
1473 described below in the section on matching a pattern.
1475 If studying the pattern does not produce any additional information
1476 pcre_study() returns NULL. In that circumstance, if the calling program
1477 wants to pass any of the other fields to pcre_exec(), it must set up
1478 its own pcre_extra block.
1480 The second argument of pcre_study() contains option bits. At present,
1481 no options are defined, and this argument should always be zero.
1483 The third argument for pcre_study() is a pointer for an error message.
1484 If studying succeeds (even if no data is returned), the variable it
1485 points to is set to NULL. Otherwise it is set to point to a textual
1486 error message. This is a static string that is part of the library. You
1487 must not try to free it. You should test the error pointer for NULL
1488 after calling pcre_study(), to be sure that it has run successfully.
1490 This is a typical call to pcre_study():
1494 re, /* result of pcre_compile() */
1495 0, /* no options exist */
1496 &error); /* set to NULL or points to a message */
1498 At present, studying a pattern is useful only for non-anchored patterns
1499 that do not have a single fixed starting character. A bitmap of possi-
1500 ble starting bytes is created.
1505 PCRE handles caseless matching, and determines whether characters are
1506 letters, digits, or whatever, by reference to a set of tables, indexed
1507 by character value. When running in UTF-8 mode, this applies only to
1508 characters with codes less than 128. Higher-valued codes never match
1509 escapes such as \w or \d, but can be tested with \p if PCRE is built
1510 with Unicode character property support. The use of locales with Uni-
1511 code is discouraged. If you are handling characters with codes greater
1512 than 128, you should either use UTF-8 and Unicode, or use locales, but
1513 not try to mix the two.
1515 PCRE contains an internal set of tables that are used when the final
1516 argument of pcre_compile() is NULL. These are sufficient for many
1517 applications. Normally, the internal tables recognize only ASCII char-
1518 acters. However, when PCRE is built, it is possible to cause the inter-
1519 nal tables to be rebuilt in the default "C" locale of the local system,
1520 which may cause them to be different.
1522 The internal tables can always be overridden by tables supplied by the
1523 application that calls PCRE. These may be created in a different locale
1524 from the default. As more and more applications change to using Uni-
1525 code, the need for this locale support is expected to die away.
1527 External tables are built by calling the pcre_maketables() function,
1528 which has no arguments, in the relevant locale. The result can then be
1529 passed to pcre_compile() or pcre_exec() as often as necessary. For
1530 example, to build and use tables that are appropriate for the French
1531 locale (where accented characters with values greater than 128 are
1532 treated as letters), the following code could be used:
1534 setlocale(LC_CTYPE, "fr_FR");
1535 tables = pcre_maketables();
1536 re = pcre_compile(..., tables);
1538 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1539 if you are using Windows, the name for the French locale is "french".
1541 When pcre_maketables() runs, the tables are built in memory that is
1542 obtained via pcre_malloc. It is the caller's responsibility to ensure
1543 that the memory containing the tables remains available for as long as
1546 The pointer that is passed to pcre_compile() is saved with the compiled
1547 pattern, and the same tables are used via this pointer by pcre_study()
1548 and normally also by pcre_exec(). Thus, by default, for any single pat-
1549 tern, compilation, studying and matching all happen in the same locale,
1550 but different patterns can be compiled in different locales.
1552 It is possible to pass a table pointer or NULL (indicating the use of
1553 the internal tables) to pcre_exec(). Although not intended for this
1554 purpose, this facility could be used to match a pattern in a different
1555 locale from the one in which it was compiled. Passing table pointers at
1556 run time is discussed below in the section on matching a pattern.
1559 INFORMATION ABOUT A PATTERN
1561 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1562 int what, void *where);
1564 The pcre_fullinfo() function returns information about a compiled pat-
1565 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1566 less retained for backwards compability (and is documented below).
1568 The first argument for pcre_fullinfo() is a pointer to the compiled
1569 pattern. The second argument is the result of pcre_study(), or NULL if
1570 the pattern was not studied. The third argument specifies which piece
1571 of information is required, and the fourth argument is a pointer to a
1572 variable to receive the data. The yield of the function is zero for
1573 success, or one of the following negative numbers:
1575 PCRE_ERROR_NULL the argument code was NULL
1576 the argument where was NULL
1577 PCRE_ERROR_BADMAGIC the "magic number" was not found
1578 PCRE_ERROR_BADOPTION the value of what was invalid
1580 The "magic number" is placed at the start of each compiled pattern as
1581 an simple check against passing an arbitrary memory pointer. Here is a
1582 typical call of pcre_fullinfo(), to obtain the length of the compiled
1588 re, /* result of pcre_compile() */
1589 pe, /* result of pcre_study(), or NULL */
1590 PCRE_INFO_SIZE, /* what is required */
1591 &length); /* where to put the data */
1593 The possible values for the third argument are defined in pcre.h, and
1596 PCRE_INFO_BACKREFMAX
1598 Return the number of the highest back reference in the pattern. The
1599 fourth argument should point to an int variable. Zero is returned if
1600 there are no back references.
1602 PCRE_INFO_CAPTURECOUNT
1604 Return the number of capturing subpatterns in the pattern. The fourth
1605 argument should point to an int variable.
1607 PCRE_INFO_DEFAULT_TABLES
1609 Return a pointer to the internal default character tables within PCRE.
1610 The fourth argument should point to an unsigned char * variable. This
1611 information call is provided for internal use by the pcre_study() func-
1612 tion. External callers can cause PCRE to use its internal tables by
1613 passing a NULL table pointer.
1617 Return information about the first byte of any matched string, for a
1618 non-anchored pattern. The fourth argument should point to an int vari-
1619 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1620 is still recognized for backwards compatibility.)
1622 If there is a fixed first byte, for example, from a pattern such as
1623 (cat|cow|coyote), its value is returned. Otherwise, if either
1625 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1626 branch starts with "^", or
1628 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1629 set (if it were set, the pattern would be anchored),
1631 -1 is returned, indicating that the pattern matches only at the start
1632 of a subject string or after any newline within the string. Otherwise
1633 -2 is returned. For anchored patterns, -2 is returned.
1635 PCRE_INFO_FIRSTTABLE
1637 If the pattern was studied, and this resulted in the construction of a
1638 256-bit table indicating a fixed set of bytes for the first byte in any
1639 matching string, a pointer to the table is returned. Otherwise NULL is
1640 returned. The fourth argument should point to an unsigned char * vari-
1645 Return 1 if the pattern contains any explicit matches for CR or LF
1646 characters, otherwise 0. The fourth argument should point to an int
1647 variable. An explicit match is either a literal CR or LF character, or
1652 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
1653 otherwise 0. The fourth argument should point to an int variable. (?J)
1654 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1656 PCRE_INFO_LASTLITERAL
1658 Return the value of the rightmost literal byte that must exist in any
1659 matched string, other than at its start, if such a byte has been
1660 recorded. The fourth argument should point to an int variable. If there
1661 is no such byte, -1 is returned. For anchored patterns, a last literal
1662 byte is recorded only if it follows something of variable length. For
1663 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1664 /^a\dz\d/ the returned value is -1.
1667 PCRE_INFO_NAMEENTRYSIZE
1670 PCRE supports the use of named as well as numbered capturing parenthe-
1671 ses. The names are just an additional way of identifying the parenthe-
1672 ses, which still acquire numbers. Several convenience functions such as
1673 pcre_get_named_substring() are provided for extracting captured sub-
1674 strings by name. It is also possible to extract the data directly, by
1675 first converting the name to a number in order to access the correct
1676 pointers in the output vector (described with pcre_exec() below). To do
1677 the conversion, you need to use the name-to-number map, which is
1678 described by these three values.
1680 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1681 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1682 of each entry; both of these return an int value. The entry size
1683 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1684 a pointer to the first entry of the table (a pointer to char). The
1685 first two bytes of each entry are the number of the capturing parenthe-
1686 sis, most significant byte first. The rest of the entry is the corre-
1687 sponding name, zero terminated. The names are in alphabetical order.
1688 When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1689 theses numbers. For example, consider the following pattern (assume
1690 PCRE_EXTENDED is set, so white space - including newlines - is
1693 (?<date> (?<year>(\d\d)?\d\d) -
1694 (?<month>\d\d) - (?<day>\d\d) )
1696 There are four named subpatterns, so the table has four entries, and
1697 each entry in the table is eight bytes long. The table is as follows,
1698 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1702 00 05 d a y 00 ?? ??
1706 When writing code to extract data from named subpatterns using the
1707 name-to-number map, remember that the length of the entries is likely
1708 to be different for each compiled pattern.
1712 Return 1 if the pattern can be used for partial matching, otherwise 0.
1713 The fourth argument should point to an int variable. The pcrepartial
1714 documentation lists the restrictions that apply to patterns when par-
1715 tial matching is used.
1719 Return a copy of the options with which the pattern was compiled. The
1720 fourth argument should point to an unsigned long int variable. These
1721 option bits are those specified in the call to pcre_compile(), modified
1722 by any top-level option settings at the start of the pattern itself. In
1723 other words, they are the options that will be in force when matching
1724 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1725 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1728 A pattern is automatically anchored by PCRE if all of its top-level
1729 alternatives begin with one of the following:
1731 ^ unless PCRE_MULTILINE is set
1734 .* if PCRE_DOTALL is set and there are no back
1735 references to the subpattern in which .* appears
1737 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1742 Return the size of the compiled pattern, that is, the value that was
1743 passed as the argument to pcre_malloc() when PCRE was getting memory in
1744 which to place the compiled data. The fourth argument should point to a
1749 Return the size of the data block pointed to by the study_data field in
1750 a pcre_extra block. That is, it is the value that was passed to
1751 pcre_malloc() when PCRE was getting memory into which to place the data
1752 created by pcre_study(). The fourth argument should point to a size_t
1756 OBSOLETE INFO FUNCTION
1758 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1760 The pcre_info() function is now obsolete because its interface is too
1761 restrictive to return all the available data about a compiled pattern.
1762 New programs should use pcre_fullinfo() instead. The yield of
1763 pcre_info() is the number of capturing subpatterns, or one of the fol-
1764 lowing negative numbers:
1766 PCRE_ERROR_NULL the argument code was NULL
1767 PCRE_ERROR_BADMAGIC the "magic number" was not found
1769 If the optptr argument is not NULL, a copy of the options with which
1770 the pattern was compiled is placed in the integer it points to (see
1771 PCRE_INFO_OPTIONS above).
1773 If the pattern is not anchored and the firstcharptr argument is not
1774 NULL, it is used to pass back information about the first character of
1775 any matched string (see PCRE_INFO_FIRSTBYTE above).
1780 int pcre_refcount(pcre *code, int adjust);
1782 The pcre_refcount() function is used to maintain a reference count in
1783 the data block that contains a compiled pattern. It is provided for the
1784 benefit of applications that operate in an object-oriented manner,
1785 where different parts of the application may be using the same compiled
1786 pattern, but you want to free the block when they are all done.
1788 When a pattern is compiled, the reference count field is initialized to
1789 zero. It is changed only by calling this function, whose action is to
1790 add the adjust value (which may be positive or negative) to it. The
1791 yield of the function is the new value. However, the value of the count
1792 is constrained to lie between 0 and 65535, inclusive. If the new value
1793 is outside these limits, it is forced to the appropriate limit value.
1795 Except when it is zero, the reference count is not correctly preserved
1796 if a pattern is compiled on one host and then transferred to a host
1797 whose byte-order is different. (This seems a highly unlikely scenario.)
1800 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1802 int pcre_exec(const pcre *code, const pcre_extra *extra,
1803 const char *subject, int length, int startoffset,
1804 int options, int *ovector, int ovecsize);
1806 The function pcre_exec() is called to match a subject string against a
1807 compiled pattern, which is passed in the code argument. If the pattern
1808 has been studied, the result of the study should be passed in the extra
1809 argument. This function is the main matching facility of the library,
1810 and it operates in a Perl-like manner. For specialist use there is also
1811 an alternative matching function, which is described below in the sec-
1812 tion about the pcre_dfa_exec() function.
1814 In most applications, the pattern will have been compiled (and option-
1815 ally studied) in the same process that calls pcre_exec(). However, it
1816 is possible to save compiled patterns and study data, and then use them
1817 later in different processes, possibly even on different hosts. For a
1818 discussion about this, see the pcreprecompile documentation.
1820 Here is an example of a simple call to pcre_exec():
1825 re, /* result of pcre_compile() */
1826 NULL, /* we didn't study the pattern */
1827 "some string", /* the subject string */
1828 11, /* the length of the subject string */
1829 0, /* start at offset 0 in the subject */
1830 0, /* default options */
1831 ovector, /* vector of integers for substring information */
1832 30); /* number of elements (NOT size in bytes) */
1834 Extra data for pcre_exec()
1836 If the extra argument is not NULL, it must point to a pcre_extra data
1837 block. The pcre_study() function returns such a block (when it doesn't
1838 return NULL), but you can also create one for yourself, and pass addi-
1839 tional information in it. The pcre_extra block contains the following
1840 fields (not necessarily in this order):
1842 unsigned long int flags;
1844 unsigned long int match_limit;
1845 unsigned long int match_limit_recursion;
1847 const unsigned char *tables;
1849 The flags field is a bitmap that specifies which of the other fields
1850 are set. The flag bits are:
1852 PCRE_EXTRA_STUDY_DATA
1853 PCRE_EXTRA_MATCH_LIMIT
1854 PCRE_EXTRA_MATCH_LIMIT_RECURSION
1855 PCRE_EXTRA_CALLOUT_DATA
1858 Other flag bits should be set to zero. The study_data field is set in
1859 the pcre_extra block that is returned by pcre_study(), together with
1860 the appropriate flag bit. You should not set this yourself, but you may
1861 add to the block by setting the other fields and their corresponding
1864 The match_limit field provides a means of preventing PCRE from using up
1865 a vast amount of resources when running patterns that are not going to
1866 match, but which have a very large number of possibilities in their
1867 search trees. The classic example is the use of nested unlimited
1870 Internally, PCRE uses a function called match() which it calls repeat-
1871 edly (sometimes recursively). The limit set by match_limit is imposed
1872 on the number of times this function is called during a match, which
1873 has the effect of limiting the amount of backtracking that can take
1874 place. For patterns that are not anchored, the count restarts from zero
1875 for each position in the subject string.
1877 The default value for the limit can be set when PCRE is built; the
1878 default default is 10 million, which handles all but the most extreme
1879 cases. You can override the default by suppling pcre_exec() with a
1880 pcre_extra block in which match_limit is set, and
1881 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1882 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1884 The match_limit_recursion field is similar to match_limit, but instead
1885 of limiting the total number of times that match() is called, it limits
1886 the depth of recursion. The recursion depth is a smaller number than
1887 the total number of calls, because not all calls to match() are recur-
1888 sive. This limit is of use only if it is set smaller than match_limit.
1890 Limiting the recursion depth limits the amount of stack that can be
1891 used, or, when PCRE has been compiled to use memory on the heap instead
1892 of the stack, the amount of heap memory that can be used.
1894 The default value for match_limit_recursion can be set when PCRE is
1895 built; the default default is the same value as the default for
1896 match_limit. You can override the default by suppling pcre_exec() with
1897 a pcre_extra block in which match_limit_recursion is set, and
1898 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1899 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1901 The pcre_callout field is used in conjunction with the "callout" fea-
1902 ture, which is described in the pcrecallout documentation.
1904 The tables field is used to pass a character tables pointer to
1905 pcre_exec(); this overrides the value that is stored with the compiled
1906 pattern. A non-NULL value is stored with the compiled pattern only if
1907 custom tables were supplied to pcre_compile() via its tableptr argu-
1908 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1909 PCRE's internal tables to be used. This facility is helpful when re-
1910 using patterns that have been saved after compiling with an external
1911 set of tables, because the external tables might be at a different
1912 address when pcre_exec() is called. See the pcreprecompile documenta-
1913 tion for a discussion of saving compiled patterns for later use.
1915 Option bits for pcre_exec()
1917 The unused bits of the options argument for pcre_exec() must be zero.
1918 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1919 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
1924 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
1925 matching position. If a pattern was compiled with PCRE_ANCHORED, or
1926 turned out to be anchored by virtue of its contents, it cannot be made
1927 unachored at matching time.
1932 These options (which are mutually exclusive) control what the \R escape
1933 sequence matches. The choice is either to match only CR, LF, or CRLF,
1934 or to match any Unicode newline sequence. These options override the
1935 choice that was made or defaulted when the pattern was compiled.
1940 PCRE_NEWLINE_ANYCRLF
1943 These options override the newline definition that was chosen or
1944 defaulted when the pattern was compiled. For details, see the descrip-
1945 tion of pcre_compile() above. During matching, the newline choice
1946 affects the behaviour of the dot, circumflex, and dollar metacharac-
1947 ters. It may also alter the way the match position is advanced after a
1948 match failure for an unanchored pattern.
1950 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
1951 set, and a match attempt for an unanchored pattern fails when the cur-
1952 rent position is at a CRLF sequence, and the pattern contains no
1953 explicit matches for CR or LF characters, the match position is
1954 advanced by two characters instead of one, in other words, to after the
1957 The above rule is a compromise that makes the most common cases work as
1958 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
1959 option is not set), it does not match the string "\r\nA" because, after
1960 failing at the start, it skips both the CR and the LF before retrying.
1961 However, the pattern [\r\n]A does match that string, because it con-
1962 tains an explicit CR or LF reference, and so advances only by one char-
1963 acter after the first failure.
1965 An explicit match for CR of LF is either a literal appearance of one of
1966 those characters, or one of the \r or \n escape sequences. Implicit
1967 matches such as [^X] do not count, nor does \s (which includes CR and
1968 LF in the characters that it matches).
1970 Notwithstanding the above, anomalous effects may still occur when CRLF
1971 is a valid newline sequence and explicit \r or \n escapes appear in the
1976 This option specifies that first character of the subject string is not
1977 the beginning of a line, so the circumflex metacharacter should not
1978 match before it. Setting this without PCRE_MULTILINE (at compile time)
1979 causes circumflex never to match. This option affects only the behav-
1980 iour of the circumflex metacharacter. It does not affect \A.
1984 This option specifies that the end of the subject string is not the end
1985 of a line, so the dollar metacharacter should not match it nor (except
1986 in multiline mode) a newline immediately before it. Setting this with-
1987 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1988 option affects only the behaviour of the dollar metacharacter. It does
1989 not affect \Z or \z.
1993 An empty string is not considered to be a valid match if this option is
1994 set. If there are alternatives in the pattern, they are tried. If all
1995 the alternatives match the empty string, the entire match fails. For
1996 example, if the pattern
2000 is applied to a string not beginning with "a" or "b", it matches the
2001 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2002 match is not valid, so PCRE searches further into the string for occur-
2003 rences of "a" or "b".
2005 Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
2006 cial case of a pattern match of the empty string within its split()
2007 function, and when using the /g modifier. It is possible to emulate
2008 Perl's behaviour after matching a null string by first trying the match
2009 again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
2010 if that fails by advancing the starting offset (see below) and trying
2011 an ordinary match again. There is some code that demonstrates how to do
2012 this in the pcredemo.c sample program.
2016 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2017 UTF-8 string is automatically checked when pcre_exec() is subsequently
2018 called. The value of startoffset is also checked to ensure that it
2019 points to the start of a UTF-8 character. There is a discussion about
2020 the validity of UTF-8 strings in the section on UTF-8 support in the
2021 main pcre page. If an invalid UTF-8 sequence of bytes is found,
2022 pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con-
2023 tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
2025 If you already know that your subject is valid, and you want to skip
2026 these checks for performance reasons, you can set the
2027 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2028 do this for the second and subsequent calls to pcre_exec() if you are
2029 making repeated calls to find all the matches in a single subject
2030 string. However, you should be sure that the value of startoffset
2031 points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
2032 set, the effect of passing an invalid UTF-8 string as a subject, or a
2033 value of startoffset that does not point to the start of a UTF-8 char-
2034 acter, is undefined. Your program may crash.
2038 This option turns on the partial matching feature. If the subject
2039 string fails to match the pattern, but at some point during the match-
2040 ing process the end of the subject was reached (that is, the subject
2041 partially matches the pattern and the failure to match occurred only
2042 because there were not enough subject characters), pcre_exec() returns
2043 PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
2044 used, there are restrictions on what may appear in the pattern. These
2045 are discussed in the pcrepartial documentation.
2047 The string to be matched by pcre_exec()
2049 The subject string is passed to pcre_exec() as a pointer in subject, a
2050 length in length, and a starting byte offset in startoffset. In UTF-8
2051 mode, the byte offset must point to the start of a UTF-8 character.
2052 Unlike the pattern string, the subject may contain binary zero bytes.
2053 When the starting offset is zero, the search for a match starts at the
2054 beginning of the subject, and this is by far the most common case.
2056 A non-zero starting offset is useful when searching for another match
2057 in the same subject by calling pcre_exec() again after a previous suc-
2058 cess. Setting startoffset differs from just passing over a shortened
2059 string and setting PCRE_NOTBOL in the case of a pattern that begins
2060 with any kind of lookbehind. For example, consider the pattern
2064 which finds occurrences of "iss" in the middle of words. (\B matches
2065 only if the current position in the subject is not a word boundary.)
2066 When applied to the string "Mississipi" the first call to pcre_exec()
2067 finds the first occurrence. If pcre_exec() is called again with just
2068 the remainder of the subject, namely "issipi", it does not match,
2069 because \B is always false at the start of the subject, which is deemed
2070 to be a word boundary. However, if pcre_exec() is passed the entire
2071 string again, but with startoffset set to 4, it finds the second occur-
2072 rence of "iss" because it is able to look behind the starting point to
2073 discover that it is preceded by a letter.
2075 If a non-zero starting offset is passed when the pattern is anchored,
2076 one attempt to match at the given offset is made. This can only succeed
2077 if the pattern does not require the match to be at the start of the
2080 How pcre_exec() returns captured substrings
2082 In general, a pattern matches a certain portion of the subject, and in
2083 addition, further substrings from the subject may be picked out by
2084 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2085 this is called "capturing" in what follows, and the phrase "capturing
2086 subpattern" is used for a fragment of a pattern that picks out a sub-
2087 string. PCRE supports several other kinds of parenthesized subpattern
2088 that do not cause substrings to be captured.
2090 Captured substrings are returned to the caller via a vector of integer
2091 offsets whose address is passed in ovector. The number of elements in
2092 the vector is passed in ovecsize, which must be a non-negative number.
2093 Note: this argument is NOT the size of ovector in bytes.
2095 The first two-thirds of the vector is used to pass back captured sub-
2096 strings, each substring using a pair of integers. The remaining third
2097 of the vector is used as workspace by pcre_exec() while matching cap-
2098 turing subpatterns, and is not available for passing back information.
2099 The length passed in ovecsize should always be a multiple of three. If
2100 it is not, it is rounded down.
2102 When a match is successful, information about captured substrings is
2103 returned in pairs of integers, starting at the beginning of ovector,
2104 and continuing up to two-thirds of its length at the most. The first
2105 element of a pair is set to the offset of the first character in a sub-
2106 string, and the second is set to the offset of the first character
2107 after the end of a substring. The first pair, ovector[0] and ovec-
2108 tor[1], identify the portion of the subject string matched by the
2109 entire pattern. The next pair is used for the first capturing subpat-
2110 tern, and so on. The value returned by pcre_exec() is one more than the
2111 highest numbered pair that has been set. For example, if two substrings
2112 have been captured, the returned value is 3. If there are no capturing
2113 subpatterns, the return value from a successful match is 1, indicating
2114 that just the first pair of offsets has been set.
2116 If a capturing subpattern is matched repeatedly, it is the last portion
2117 of the string that it matched that is returned.
2119 If the vector is too small to hold all the captured substring offsets,
2120 it is used as far as possible (up to two-thirds of its length), and the
2121 function returns a value of zero. In particular, if the substring off-
2122 sets are not of interest, pcre_exec() may be called with ovector passed
2123 as NULL and ovecsize as zero. However, if the pattern contains back
2124 references and the ovector is not big enough to remember the related
2125 substrings, PCRE has to get additional memory for use during matching.
2126 Thus it is usually advisable to supply an ovector.
2128 The pcre_info() function can be used to find out how many capturing
2129 subpatterns there are in a compiled pattern. The smallest size for
2130 ovector that will allow for n captured substrings, in addition to the
2131 offsets of the substring matched by the whole pattern, is (n+1)*3.
2133 It is possible for capturing subpattern number n+1 to match some part
2134 of the subject when subpattern n has not been used at all. For example,
2135 if the string "abc" is matched against the pattern (a|(z))(bc) the
2136 return from the function is 4, and subpatterns 1 and 3 are matched, but
2137 2 is not. When this happens, both values in the offset pairs corre-
2138 sponding to unused subpatterns are set to -1.
2140 Offset values that correspond to unused subpatterns at the end of the
2141 expression are also set to -1. For example, if the string "abc" is
2142 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2143 matched. The return from the function is 2, because the highest used
2144 capturing subpattern number is 1. However, you can refer to the offsets
2145 for the second and third capturing subpatterns if you wish (assuming
2146 the vector is large enough, of course).
2148 Some convenience functions are provided for extracting the captured
2149 substrings as separate strings. These are described below.
2151 Error return values from pcre_exec()
2153 If pcre_exec() fails, it returns a negative number. The following are
2154 defined in the header file:
2156 PCRE_ERROR_NOMATCH (-1)
2158 The subject string did not match the pattern.
2160 PCRE_ERROR_NULL (-2)
2162 Either code or subject was passed as NULL, or ovector was NULL and
2163 ovecsize was not zero.
2165 PCRE_ERROR_BADOPTION (-3)
2167 An unrecognized bit was set in the options argument.
2169 PCRE_ERROR_BADMAGIC (-4)
2171 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2172 to catch the case when it is passed a junk pointer and to detect when a
2173 pattern that was compiled in an environment of one endianness is run in
2174 an environment with the other endianness. This is the error that PCRE
2175 gives when the magic number is not present.
2177 PCRE_ERROR_UNKNOWN_OPCODE (-5)
2179 While running the pattern match, an unknown item was encountered in the
2180 compiled pattern. This error could be caused by a bug in PCRE or by
2181 overwriting of the compiled pattern.
2183 PCRE_ERROR_NOMEMORY (-6)
2185 If a pattern contains back references, but the ovector that is passed
2186 to pcre_exec() is not big enough to remember the referenced substrings,
2187 PCRE gets a block of memory at the start of matching to use for this
2188 purpose. If the call via pcre_malloc() fails, this error is given. The
2189 memory is automatically freed at the end of matching.
2191 PCRE_ERROR_NOSUBSTRING (-7)
2193 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2194 and pcre_get_substring_list() functions (see below). It is never
2195 returned by pcre_exec().
2197 PCRE_ERROR_MATCHLIMIT (-8)
2199 The backtracking limit, as specified by the match_limit field in a
2200 pcre_extra structure (or defaulted) was reached. See the description
2203 PCRE_ERROR_CALLOUT (-9)
2205 This error is never generated by pcre_exec() itself. It is provided for
2206 use by callout functions that want to yield a distinctive error code.
2207 See the pcrecallout documentation for details.
2209 PCRE_ERROR_BADUTF8 (-10)
2211 A string that contains an invalid UTF-8 byte sequence was passed as a
2214 PCRE_ERROR_BADUTF8_OFFSET (-11)
2216 The UTF-8 byte sequence that was passed as a subject was valid, but the
2217 value of startoffset did not point to the beginning of a UTF-8 charac-
2220 PCRE_ERROR_PARTIAL (-12)
2222 The subject string did not match, but it did match partially. See the
2223 pcrepartial documentation for details of partial matching.
2225 PCRE_ERROR_BADPARTIAL (-13)
2227 The PCRE_PARTIAL option was used with a compiled pattern containing
2228 items that are not supported for partial matching. See the pcrepartial
2229 documentation for details of partial matching.
2231 PCRE_ERROR_INTERNAL (-14)
2233 An unexpected internal error has occurred. This error could be caused
2234 by a bug in PCRE or by overwriting of the compiled pattern.
2236 PCRE_ERROR_BADCOUNT (-15)
2238 This error is given if the value of the ovecsize argument is negative.
2240 PCRE_ERROR_RECURSIONLIMIT (-21)
2242 The internal recursion limit, as specified by the match_limit_recursion
2243 field in a pcre_extra structure (or defaulted) was reached. See the
2246 PCRE_ERROR_BADNEWLINE (-23)
2248 An invalid combination of PCRE_NEWLINE_xxx options was given.
2250 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2253 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2255 int pcre_copy_substring(const char *subject, int *ovector,
2256 int stringcount, int stringnumber, char *buffer,
2259 int pcre_get_substring(const char *subject, int *ovector,
2260 int stringcount, int stringnumber,
2261 const char **stringptr);
2263 int pcre_get_substring_list(const char *subject,
2264 int *ovector, int stringcount, const char ***listptr);
2266 Captured substrings can be accessed directly by using the offsets
2267 returned by pcre_exec() in ovector. For convenience, the functions
2268 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2269 string_list() are provided for extracting captured substrings as new,
2270 separate, zero-terminated strings. These functions identify substrings
2271 by number. The next section describes functions for extracting named
2274 A substring that contains a binary zero is correctly extracted and has
2275 a further zero added on the end, but the result is not, of course, a C
2276 string. However, you can process such a string by referring to the
2277 length that is returned by pcre_copy_substring() and pcre_get_sub-
2278 string(). Unfortunately, the interface to pcre_get_substring_list() is
2279 not adequate for handling strings containing binary zeros, because the
2280 end of the final string is not independently indicated.
2282 The first three arguments are the same for all three of these func-
2283 tions: subject is the subject string that has just been successfully
2284 matched, ovector is a pointer to the vector of integer offsets that was
2285 passed to pcre_exec(), and stringcount is the number of substrings that
2286 were captured by the match, including the substring that matched the
2287 entire regular expression. This is the value returned by pcre_exec() if
2288 it is greater than zero. If pcre_exec() returned zero, indicating that
2289 it ran out of space in ovector, the value passed as stringcount should
2290 be the number of elements in the vector divided by three.
2292 The functions pcre_copy_substring() and pcre_get_substring() extract a
2293 single substring, whose number is given as stringnumber. A value of
2294 zero extracts the substring that matched the entire pattern, whereas
2295 higher values extract the captured substrings. For pcre_copy_sub-
2296 string(), the string is placed in buffer, whose length is given by
2297 buffersize, while for pcre_get_substring() a new block of memory is
2298 obtained via pcre_malloc, and its address is returned via stringptr.
2299 The yield of the function is the length of the string, not including
2300 the terminating zero, or one of these error codes:
2302 PCRE_ERROR_NOMEMORY (-6)
2304 The buffer was too small for pcre_copy_substring(), or the attempt to
2305 get memory failed for pcre_get_substring().
2307 PCRE_ERROR_NOSUBSTRING (-7)
2309 There is no substring whose number is stringnumber.
2311 The pcre_get_substring_list() function extracts all available sub-
2312 strings and builds a list of pointers to them. All this is done in a
2313 single block of memory that is obtained via pcre_malloc. The address of
2314 the memory block is returned via listptr, which is also the start of
2315 the list of string pointers. The end of the list is marked by a NULL
2316 pointer. The yield of the function is zero if all went well, or the
2319 PCRE_ERROR_NOMEMORY (-6)
2321 if the attempt to get the memory block failed.
2323 When any of these functions encounter a substring that is unset, which
2324 can happen when capturing subpattern number n+1 matches some part of
2325 the subject, but subpattern n has not been used at all, they return an
2326 empty string. This can be distinguished from a genuine zero-length sub-
2327 string by inspecting the appropriate offset in ovector, which is nega-
2328 tive for unset substrings.
2330 The two convenience functions pcre_free_substring() and pcre_free_sub-
2331 string_list() can be used to free the memory returned by a previous
2332 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2333 tively. They do nothing more than call the function pointed to by
2334 pcre_free, which of course could be called directly from a C program.
2335 However, PCRE is used in some situations where it is linked via a spe-
2336 cial interface to another programming language that cannot use
2337 pcre_free directly; it is for these cases that the functions are pro-
2341 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2343 int pcre_get_stringnumber(const pcre *code,
2346 int pcre_copy_named_substring(const pcre *code,
2347 const char *subject, int *ovector,
2348 int stringcount, const char *stringname,
2349 char *buffer, int buffersize);
2351 int pcre_get_named_substring(const pcre *code,
2352 const char *subject, int *ovector,
2353 int stringcount, const char *stringname,
2354 const char **stringptr);
2356 To extract a substring by name, you first have to find associated num-
2357 ber. For example, for this pattern
2361 the number of the subpattern called "xxx" is 2. If the name is known to
2362 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2363 name by calling pcre_get_stringnumber(). The first argument is the com-
2364 piled pattern, and the second is the name. The yield of the function is
2365 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2366 subpattern of that name.
2368 Given the number, you can extract the substring directly, or use one of
2369 the functions described in the previous section. For convenience, there
2370 are also two functions that do the whole job.
2372 Most of the arguments of pcre_copy_named_substring() and
2373 pcre_get_named_substring() are the same as those for the similarly
2374 named functions that extract by number. As these are described in the
2375 previous section, they are not re-described here. There are just two
2378 First, instead of a substring number, a substring name is given. Sec-
2379 ond, there is an extra argument, given at the start, which is a pointer
2380 to the compiled pattern. This is needed in order to gain access to the
2381 name-to-number translation table.
2383 These functions call pcre_get_stringnumber(), and if it succeeds, they
2384 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2385 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2386 behaviour may not be what you want (see the next section).
2389 DUPLICATE SUBPATTERN NAMES
2391 int pcre_get_stringtable_entries(const pcre *code,
2392 const char *name, char **first, char **last);
2394 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2395 subpatterns are not required to be unique. Normally, patterns with
2396 duplicate names are such that in any one match, only one of the named
2397 subpatterns participates. An example is shown in the pcrepattern docu-
2400 When duplicates are present, pcre_copy_named_substring() and
2401 pcre_get_named_substring() return the first substring corresponding to
2402 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2403 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2404 function returns one of the numbers that are associated with the name,
2405 but it is not defined which it is.
2407 If you want to get full details of all captured substrings for a given
2408 name, you must use the pcre_get_stringtable_entries() function. The
2409 first argument is the compiled pattern, and the second is the name. The
2410 third and fourth are pointers to variables which are updated by the
2411 function. After it has run, they point to the first and last entries in
2412 the name-to-number table for the given name. The function itself
2413 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2414 there are none. The format of the table is described above in the sec-
2415 tion entitled Information about a pattern. Given all the relevant
2416 entries for the name, you can extract each of their numbers, and hence
2417 the captured data, if any.
2420 FINDING ALL POSSIBLE MATCHES
2422 The traditional matching function uses a similar algorithm to Perl,
2423 which stops when it finds the first match, starting at a given point in
2424 the subject. If you want to find all possible matches, or the longest
2425 possible match, consider using the alternative matching function (see
2426 below) instead. If you cannot use the alternative function, but still
2427 need to find all possible matches, you can kludge it up by making use
2428 of the callout facility, which is described in the pcrecallout documen-
2431 What you have to do is to insert a callout right at the end of the pat-
2432 tern. When your callout function is called, extract and save the cur-
2433 rent matched substring. Then return 1, which forces pcre_exec() to
2434 backtrack and try other alternatives. Ultimately, when it runs out of
2435 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2438 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2440 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2441 const char *subject, int length, int startoffset,
2442 int options, int *ovector, int ovecsize,
2443 int *workspace, int wscount);
2445 The function pcre_dfa_exec() is called to match a subject string
2446 against a compiled pattern, using a matching algorithm that scans the
2447 subject string just once, and does not backtrack. This has different
2448 characteristics to the normal algorithm, and is not compatible with
2449 Perl. Some of the features of PCRE patterns are not supported. Never-
2450 theless, there are times when this kind of matching can be useful. For
2451 a discussion of the two matching algorithms, see the pcrematching docu-
2454 The arguments for the pcre_dfa_exec() function are the same as for
2455 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2456 ent way, and this is described below. The other common arguments are
2457 used in the same way as for pcre_exec(), so their description is not
2460 The two additional arguments provide workspace for the function. The
2461 workspace vector should contain at least 20 elements. It is used for
2462 keeping track of multiple paths through the pattern tree. More
2463 workspace will be needed for patterns and subjects where there are a
2464 lot of potential matches.
2466 Here is an example of a simple call to pcre_dfa_exec():
2472 re, /* result of pcre_compile() */
2473 NULL, /* we didn't study the pattern */
2474 "some string", /* the subject string */
2475 11, /* the length of the subject string */
2476 0, /* start at offset 0 in the subject */
2477 0, /* default options */
2478 ovector, /* vector of integers for substring information */
2479 10, /* number of elements (NOT size in bytes) */
2480 wspace, /* working space vector */
2481 20); /* number of elements (NOT size in bytes) */
2483 Option bits for pcre_dfa_exec()
2485 The unused bits of the options argument for pcre_dfa_exec() must be
2486 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2487 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2488 PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2489 three of these are the same as for pcre_exec(), so their description is
2494 This has the same general effect as it does for pcre_exec(), but the
2495 details are slightly different. When PCRE_PARTIAL is set for
2496 pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
2497 PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
2498 been no complete matches, but there is still at least one matching pos-
2499 sibility. The portion of the string that provided the partial match is
2500 set as the first matching string.
2504 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2505 stop as soon as it has found one match. Because of the way the alterna-
2506 tive algorithm works, this is necessarily the shortest possible match
2507 at the first possible matching point in the subject string.
2511 When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
2512 returns a partial match, it is possible to call it again, with addi-
2513 tional subject characters, and have it continue with the same match.
2514 The PCRE_DFA_RESTART option requests this action; when it is set, the
2515 workspace and wscount options must reference the same vector as before
2516 because data about the match so far is left in them after a partial
2517 match. There is more discussion of this facility in the pcrepartial
2520 Successful returns from pcre_dfa_exec()
2522 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2523 string in the subject. Note, however, that all the matches from one run
2524 of the function start at the same point in the subject. The shorter
2525 matches are all initial substrings of the longer matches. For example,
2530 is matched against the string
2532 This is <something> <something else> <something further> no more
2534 the three matched strings are
2537 <something> <something else>
2538 <something> <something else> <something further>
2540 On success, the yield of the function is a number greater than zero,
2541 which is the number of matched substrings. The substrings themselves
2542 are returned in ovector. Each string uses two elements; the first is
2543 the offset to the start, and the second is the offset to the end. In
2544 fact, all the strings have the same start offset. (Space could have
2545 been saved by giving this only once, but it was decided to retain some
2546 compatibility with the way pcre_exec() returns data, even though the
2547 meaning of the strings is different.)
2549 The strings are returned in reverse order of length; that is, the long-
2550 est matching string is given first. If there were too many matches to
2551 fit into ovector, the yield of the function is zero, and the vector is
2552 filled with the longest matches.
2554 Error returns from pcre_dfa_exec()
2556 The pcre_dfa_exec() function returns a negative number when it fails.
2557 Many of the errors are the same as for pcre_exec(), and these are
2558 described above. There are in addition the following errors that are
2559 specific to pcre_dfa_exec():
2561 PCRE_ERROR_DFA_UITEM (-16)
2563 This return is given if pcre_dfa_exec() encounters an item in the pat-
2564 tern that it does not support, for instance, the use of \C or a back
2567 PCRE_ERROR_DFA_UCOND (-17)
2569 This return is given if pcre_dfa_exec() encounters a condition item
2570 that uses a back reference for the condition, or a test for recursion
2571 in a specific group. These are not supported.
2573 PCRE_ERROR_DFA_UMLIMIT (-18)
2575 This return is given if pcre_dfa_exec() is called with an extra block
2576 that contains a setting of the match_limit field. This is not supported
2577 (it is meaningless).
2579 PCRE_ERROR_DFA_WSSIZE (-19)
2581 This return is given if pcre_dfa_exec() runs out of space in the
2584 PCRE_ERROR_DFA_RECURSE (-20)
2586 When a recursive subpattern is processed, the matching function calls
2587 itself recursively, using private vectors for ovector and workspace.
2588 This error is given if the output vector is not large enough. This
2589 should be extremely rare, as a vector of size 1000 is used.
2594 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2595 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2601 University Computing Service
2602 Cambridge CB2 3QH, England.
2607 Last updated: 12 April 2008
2608 Copyright (c) 1997-2008 University of Cambridge.
2609 ------------------------------------------------------------------------------
2612 PCRECALLOUT(3) PCRECALLOUT(3)
2616 PCRE - Perl-compatible regular expressions
2621 int (*pcre_callout)(pcre_callout_block *);
2623 PCRE provides a feature called "callout", which is a means of temporar-
2624 ily passing control to the caller of PCRE in the middle of pattern
2625 matching. The caller of PCRE provides an external function by putting
2626 its entry point in the global variable pcre_callout. By default, this
2627 variable contains NULL, which disables all calling out.
2629 Within a regular expression, (?C) indicates the points at which the
2630 external function is to be called. Different callout points can be
2631 identified by putting a number less than 256 after the letter C. The
2632 default value is zero. For example, this pattern has two callout
2637 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
2638 called, PCRE automatically inserts callouts, all with number 255,
2639 before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2640 used with the pattern
2644 it is processed as if it were
2646 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2648 Notice that there is a callout before and after each parenthesis and
2649 alternation bar. Automatic callouts can be used for tracking the
2650 progress of pattern matching. The pcretest command has an option that
2651 sets automatic callouts; when it is used, the output indicates how the
2652 pattern is matched. This is useful information when you are trying to
2653 optimize the performance of a particular pattern.
2658 You should be aware that, because of optimizations in the way PCRE
2659 matches patterns, callouts sometimes do not happen. For example, if the
2664 PCRE knows that any matching string must contain the letter "d". If the
2665 subject string is "abyz", the lack of "d" means that matching doesn't
2666 ever start, and the callout is never reached. However, with "abyd",
2667 though the result is still no match, the callout is obeyed.
2670 THE CALLOUT INTERFACE
2672 During matching, when PCRE reaches a callout point, the external func-
2673 tion defined by pcre_callout is called (if it is set). This applies to
2674 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2675 only argument to the callout function is a pointer to a pcre_callout
2676 block. This structure contains the following fields:
2681 const char *subject;
2684 int current_position;
2688 int pattern_position;
2689 int next_item_length;
2691 The version field is an integer containing the version number of the
2692 block format. The initial version was 0; the current version is 1. The
2693 version number will change again in future if additional fields are
2694 added, but the intention is never to remove any of the existing fields.
2696 The callout_number field contains the number of the callout, as com-
2697 piled into the pattern (that is, the number after ?C for manual call-
2698 outs, and 255 for automatically generated callouts).
2700 The offset_vector field is a pointer to the vector of offsets that was
2701 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2702 pcre_exec() is used, the contents can be inspected in order to extract
2703 substrings that have been matched so far, in the same way as for
2704 extracting substrings after a match has completed. For pcre_dfa_exec()
2705 this field is not useful.
2707 The subject and subject_length fields contain copies of the values that
2708 were passed to pcre_exec().
2710 The start_match field normally contains the offset within the subject
2711 at which the current match attempt started. However, if the escape
2712 sequence \K has been encountered, this value is changed to reflect the
2713 modified starting point. If the pattern is not anchored, the callout
2714 function may be called several times from the same point in the pattern
2715 for different starting points in the subject.
2717 The current_position field contains the offset within the subject of
2718 the current match pointer.
2720 When the pcre_exec() function is used, the capture_top field contains
2721 one more than the number of the highest numbered captured substring so
2722 far. If no substrings have been captured, the value of capture_top is
2723 one. This is always the case when pcre_dfa_exec() is used, because it
2724 does not support captured substrings.
2726 The capture_last field contains the number of the most recently cap-
2727 tured substring. If no substrings have been captured, its value is -1.
2728 This is always the case when pcre_dfa_exec() is used.
2730 The callout_data field contains a value that is passed to pcre_exec()
2731 or pcre_dfa_exec() specifically so that it can be passed back in call-
2732 outs. It is passed in the pcre_callout field of the pcre_extra data
2733 structure. If no such data was passed, the value of callout_data in a
2734 pcre_callout block is NULL. There is a description of the pcre_extra
2735 structure in the pcreapi documentation.
2737 The pattern_position field is present from version 1 of the pcre_call-
2738 out structure. It contains the offset to the next item to be matched in
2741 The next_item_length field is present from version 1 of the pcre_call-
2742 out structure. It contains the length of the next item to be matched in
2743 the pattern string. When the callout immediately precedes an alterna-
2744 tion bar, a closing parenthesis, or the end of the pattern, the length
2745 is zero. When the callout precedes an opening parenthesis, the length
2746 is that of the entire subpattern.
2748 The pattern_position and next_item_length fields are intended to help
2749 in distinguishing between different automatic callouts, which all have
2750 the same callout number. However, they are set for all callouts.
2755 The external callout function returns an integer to PCRE. If the value
2756 is zero, matching proceeds as normal. If the value is greater than
2757 zero, matching fails at the current point, but the testing of other
2758 matching possibilities goes ahead, just as if a lookahead assertion had
2759 failed. If the value is less than zero, the match is abandoned, and
2760 pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2762 Negative values should normally be chosen from the set of
2763 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2764 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2765 reserved for use by callout functions; it will never be used by PCRE
2772 University Computing Service
2773 Cambridge CB2 3QH, England.
2778 Last updated: 29 May 2007
2779 Copyright (c) 1997-2007 University of Cambridge.
2780 ------------------------------------------------------------------------------
2783 PCRECOMPAT(3) PCRECOMPAT(3)
2787 PCRE - Perl-compatible regular expressions
2790 DIFFERENCES BETWEEN PCRE AND PERL
2792 This document describes the differences in the ways that PCRE and Perl
2793 handle regular expressions. The differences described here are mainly
2794 with respect to Perl 5.8, though PCRE versions 7.0 and later contain
2795 some features that are expected to be in the forthcoming Perl 5.10.
2797 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2798 of what it does have are given in the section on UTF-8 support in the
2801 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2802 permits them, but they do not mean what you might think. For example,
2803 (?!a){3} does not assert that the next three characters are not "a". It
2804 just asserts that the next character is not "a" three times.
2806 3. Capturing subpatterns that occur inside negative lookahead asser-
2807 tions are counted, but their entries in the offsets vector are never
2808 set. Perl sets its numerical variables from any such patterns that are
2809 matched before the assertion fails to match something (thereby succeed-
2810 ing), but only if the negative lookahead assertion contains just one
2813 4. Though binary zero characters are supported in the subject string,
2814 they are not allowed in a pattern string because it is passed as a nor-
2815 mal C string, terminated by zero. The escape sequence \0 can be used in
2816 the pattern to represent a binary zero.
2818 5. The following Perl escape sequences are not supported: \l, \u, \L,
2819 \U, and \N. In fact these are implemented by Perl's general string-han-
2820 dling and are not part of its pattern matching engine. If any of these
2821 are encountered by PCRE, an error is generated.
2823 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2824 is built with Unicode character property support. The properties that
2825 can be tested with \p and \P are limited to the general category prop-
2826 erties such as Lu and Nd, script names such as Greek or Han, and the
2827 derived properties Any and L&.
2829 7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2830 ters in between are treated as literals. This is slightly different
2831 from Perl in that $ and @ are also handled as literals inside the
2832 quotes. In Perl, they cause variable interpolation (but of course PCRE
2833 does not have variables). Note the following examples:
2835 Pattern PCRE matches Perl matches
2837 \Qabc$xyz\E abc$xyz abc followed by the
2839 \Qabc\$xyz\E abc\$xyz abc\$xyz
2840 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2842 The \Q...\E sequence is recognized both inside and outside character
2845 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2846 constructions. However, there is support for recursive patterns. This
2847 is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2848 "callout" feature allows an external function to be called during pat-
2849 tern matching. See the pcrecallout documentation for details.
2851 9. Subpatterns that are called recursively or as "subroutines" are
2852 always treated as atomic groups in PCRE. This is like Python, but
2855 10. There are some differences that are concerned with the settings of
2856 captured strings when part of a pattern is repeated. For example,
2857 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
2858 unset, but in PCRE it is set to "b".
2860 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT),
2861 (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
2862 the forms without an argument. PCRE does not support (*MARK). If
2863 (*ACCEPT) is within capturing parentheses, PCRE does not set that cap-
2864 ture group; this is different to Perl.
2866 12. PCRE provides some extensions to the Perl regular expression facil-
2867 ities. Perl 5.10 will include new features that are not in earlier
2868 versions, some of which (such as named parentheses) have been in PCRE
2869 for some time. This list is with respect to Perl 5.10:
2871 (a) Although lookbehind assertions must match fixed length strings,
2872 each alternative branch of a lookbehind assertion can match a different
2873 length of string. Perl requires them all to have the same length.
2875 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2876 meta-character matches only at the very end of the string.
2878 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2879 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
2880 ignored. (Perl can be made to issue a warning.)
2882 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
2883 fiers is inverted, that is, by default they are not greedy, but if fol-
2884 lowed by a question mark they are.
2886 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2887 tried only at the first matching position in the subject string.
2889 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2890 TURE options for pcre_exec() have no Perl equivalents.
2892 (g) The \R escape sequence can be restricted to match only CR, LF, or
2893 CRLF by the PCRE_BSR_ANYCRLF option.
2895 (h) The callout facility is PCRE-specific.
2897 (i) The partial matching facility is PCRE-specific.
2899 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
2900 even on different hosts that have the other endianness.
2902 (k) The alternative matching function (pcre_dfa_exec()) matches in a
2903 different way and is not Perl-compatible.
2905 (l) PCRE recognizes some special sequences such as (*CR) at the start
2906 of a pattern that set overall options that cannot be changed within the
2913 University Computing Service
2914 Cambridge CB2 3QH, England.
2919 Last updated: 11 September 2007
2920 Copyright (c) 1997-2007 University of Cambridge.
2921 ------------------------------------------------------------------------------
2924 PCREPATTERN(3) PCREPATTERN(3)
2928 PCRE - Perl-compatible regular expressions
2931 PCRE REGULAR EXPRESSION DETAILS
2933 The syntax and semantics of the regular expressions that are supported
2934 by PCRE are described in detail below. There is a quick-reference syn-
2935 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
2936 semantics as closely as it can. PCRE also supports some alternative
2937 regular expression syntax (which does not conflict with the Perl syn-
2938 tax) in order to provide some compatibility with regular expressions in
2939 Python, .NET, and Oniguruma.
2941 Perl's regular expressions are described in its own documentation, and
2942 regular expressions in general are covered in a number of books, some
2943 of which have copious examples. Jeffrey Friedl's "Mastering Regular
2944 Expressions", published by O'Reilly, covers regular expressions in
2945 great detail. This description of PCRE's regular expressions is
2946 intended as reference material.
2948 The original operation of PCRE was on strings of one-byte characters.
2949 However, there is now also support for UTF-8 character strings. To use
2950 this, you must build PCRE to include UTF-8 support, and then call
2951 pcre_compile() with the PCRE_UTF8 option. How this affects pattern
2952 matching is mentioned in several places below. There is also a summary
2953 of UTF-8 features in the section on UTF-8 support in the main pcre
2956 The remainder of this document discusses the patterns that are sup-
2957 ported by PCRE when its main matching function, pcre_exec(), is used.
2958 From release 6.0, PCRE offers a second matching function,
2959 pcre_dfa_exec(), which matches using a different algorithm that is not
2960 Perl-compatible. Some of the features discussed below are not available
2961 when pcre_dfa_exec() is used. The advantages and disadvantages of the
2962 alternative function, and how it differs from the normal function, are
2963 discussed in the pcrematching page.
2968 PCRE supports five different conventions for indicating line breaks in
2969 strings: a single CR (carriage return) character, a single LF (line-
2970 feed) character, the two-character sequence CRLF, any of the three pre-
2971 ceding, or any Unicode newline sequence. The pcreapi page has further
2972 discussion about newlines, and shows how to set the newline convention
2973 in the options arguments for the compiling and matching functions.
2975 It is also possible to specify a newline convention by starting a pat-
2976 tern string with one of the following five sequences:
2978 (*CR) carriage return
2980 (*CRLF) carriage return, followed by linefeed
2981 (*ANYCRLF) any of the three above
2982 (*ANY) all Unicode newline sequences
2984 These override the default and the options given to pcre_compile(). For
2985 example, on a Unix system where LF is the default newline sequence, the
2990 changes the convention to CR. That pattern matches "a\nb" because LF is
2991 no longer a newline. Note that these special settings, which are not
2992 Perl-compatible, are recognized only at the very start of a pattern,
2993 and that they must be in upper case. If more than one of them is
2994 present, the last one is used.
2996 The newline convention does not affect what the \R escape sequence
2997 matches. By default, this is any Unicode newline sequence, for Perl
2998 compatibility. However, this can be changed; see the description of \R
2999 in the section entitled "Newline sequences" below. A change of \R set-
3000 ting can be combined with a change of newline convention.
3003 CHARACTERS AND METACHARACTERS
3005 A regular expression is a pattern that is matched against a subject
3006 string from left to right. Most characters stand for themselves in a
3007 pattern, and match the corresponding characters in the subject. As a
3008 trivial example, the pattern
3012 matches a portion of a subject string that is identical to itself. When
3013 caseless matching is specified (the PCRE_CASELESS option), letters are
3014 matched independently of case. In UTF-8 mode, PCRE always understands
3015 the concept of case for characters whose values are less than 128, so
3016 caseless matching is always possible. For characters with higher val-
3017 ues, the concept of case is supported if PCRE is compiled with Unicode
3018 property support, but not otherwise. If you want to use caseless
3019 matching for characters 128 and above, you must ensure that PCRE is
3020 compiled with Unicode property support as well as with UTF-8 support.
3022 The power of regular expressions comes from the ability to include
3023 alternatives and repetitions in the pattern. These are encoded in the
3024 pattern by the use of metacharacters, which do not stand for themselves
3025 but instead are interpreted in some special way.
3027 There are two different sets of metacharacters: those that are recog-
3028 nized anywhere in the pattern except within square brackets, and those
3029 that are recognized within square brackets. Outside square brackets,
3030 the metacharacters are as follows:
3032 \ general escape character with several uses
3033 ^ assert start of string (or line, in multiline mode)
3034 $ assert end of string (or line, in multiline mode)
3035 . match any character except newline (by default)
3036 [ start character class definition
3037 | start of alternative branch
3040 ? extends the meaning of (
3041 also 0 or 1 quantifier
3042 also quantifier minimizer
3043 * 0 or more quantifier
3044 + 1 or more quantifier
3045 also "possessive quantifier"
3046 { start min/max quantifier
3048 Part of a pattern that is in square brackets is called a "character
3049 class". In a character class the only metacharacters are:
3051 \ general escape character
3052 ^ negate the class, but only if the first character
3053 - indicates character range
3054 [ POSIX character class (only if followed by POSIX
3056 ] terminates the character class
3058 The following sections describe the use of each of the metacharacters.
3063 The backslash character has several uses. Firstly, if it is followed by
3064 a non-alphanumeric character, it takes away any special meaning that
3065 character may have. This use of backslash as an escape character
3066 applies both inside and outside character classes.
3068 For example, if you want to match a * character, you write \* in the
3069 pattern. This escaping action applies whether or not the following
3070 character would otherwise be interpreted as a metacharacter, so it is
3071 always safe to precede a non-alphanumeric with backslash to specify
3072 that it stands for itself. In particular, if you want to match a back-
3073 slash, you write \\.
3075 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
3076 the pattern (other than in a character class) and characters between a
3077 # outside a character class and the next newline are ignored. An escap-
3078 ing backslash can be used to include a whitespace or # character as
3079 part of the pattern.
3081 If you want to remove the special meaning from a sequence of charac-
3082 ters, you can do so by putting them between \Q and \E. This is differ-
3083 ent from Perl in that $ and @ are handled as literals in \Q...\E
3084 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
3085 tion. Note the following examples:
3087 Pattern PCRE matches Perl matches
3089 \Qabc$xyz\E abc$xyz abc followed by the
3091 \Qabc\$xyz\E abc\$xyz abc\$xyz
3092 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3094 The \Q...\E sequence is recognized both inside and outside character
3097 Non-printing characters
3099 A second use of backslash provides a way of encoding non-printing char-
3100 acters in patterns in a visible manner. There is no restriction on the
3101 appearance of non-printing characters, apart from the binary zero that
3102 terminates a pattern, but when a pattern is being prepared by text
3103 editing, it is usually easier to use one of the following escape
3104 sequences than the binary character it represents:
3106 \a alarm, that is, the BEL character (hex 07)
3107 \cx "control-x", where x is any character
3109 \f formfeed (hex 0C)
3110 \n linefeed (hex 0A)
3111 \r carriage return (hex 0D)
3113 \ddd character with octal code ddd, or backreference
3114 \xhh character with hex code hh
3115 \x{hhh..} character with hex code hhh..
3117 The precise effect of \cx is as follows: if x is a lower case letter,
3118 it is converted to upper case. Then bit 6 of the character (hex 40) is
3119 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
3122 After \x, from zero to two hexadecimal digits are read (letters can be
3123 in upper or lower case). Any number of hexadecimal digits may appear
3124 between \x{ and }, but the value of the character code must be less
3125 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3126 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3127 than the largest Unicode code point, which is 10FFFF.
3129 If characters other than hexadecimal digits appear between \x{ and },
3130 or if there is no terminating }, this form of escape is not recognized.
3131 Instead, the initial \x will be interpreted as a basic hexadecimal
3132 escape, with no following digits, giving a character whose value is
3135 Characters whose value is less than 256 can be defined by either of the
3136 two syntaxes for \x. There is no difference in the way they are han-
3137 dled. For example, \xdc is exactly the same as \x{dc}.
3139 After \0 up to two further octal digits are read. If there are fewer
3140 than two digits, just those that are present are used. Thus the
3141 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3142 (code value 7). Make sure you supply two digits after the initial zero
3143 if the pattern character that follows is itself an octal digit.
3145 The handling of a backslash followed by a digit other than 0 is compli-
3146 cated. Outside a character class, PCRE reads it and any following dig-
3147 its as a decimal number. If the number is less than 10, or if there
3148 have been at least that many previous capturing left parentheses in the
3149 expression, the entire sequence is taken as a back reference. A
3150 description of how this works is given later, following the discussion
3151 of parenthesized subpatterns.
3153 Inside a character class, or if the decimal number is greater than 9
3154 and there have not been that many capturing subpatterns, PCRE re-reads
3155 up to three octal digits following the backslash, and uses them to gen-
3156 erate a data character. Any subsequent digits stand for themselves. In
3157 non-UTF-8 mode, the value of a character specified in octal must be
3158 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3161 \040 is another way of writing a space
3162 \40 is the same, provided there are fewer than 40
3163 previous capturing subpatterns
3164 \7 is always a back reference
3165 \11 might be a back reference, or another way of
3167 \011 is always a tab
3168 \0113 is a tab followed by the character "3"
3169 \113 might be a back reference, otherwise the
3170 character with octal code 113
3171 \377 might be a back reference, otherwise
3172 the byte consisting entirely of 1 bits
3173 \81 is either a back reference, or a binary zero
3174 followed by the two characters "8" and "1"
3176 Note that octal values of 100 or greater must not be introduced by a
3177 leading zero, because no more than three octal digits are ever read.
3179 All the sequences that define a single character value can be used both
3180 inside and outside character classes. In addition, inside a character
3181 class, the sequence \b is interpreted as the backspace character (hex
3182 08), and the sequences \R and \X are interpreted as the characters "R"
3183 and "X", respectively. Outside a character class, these sequences have
3184 different meanings (see below).
3186 Absolute and relative back references
3188 The sequence \g followed by an unsigned or a negative number, option-
3189 ally enclosed in braces, is an absolute or relative back reference. A
3190 named back reference can be coded as \g{name}. Back references are dis-
3191 cussed later, following the discussion of parenthesized subpatterns.
3193 Absolute and relative subroutine calls
3195 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
3196 name or a number enclosed either in angle brackets or single quotes, is
3197 an alternative syntax for referencing a subpattern as a "subroutine".
3198 Details are discussed later. Note that \g{...} (Perl syntax) and
3199 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
3200 reference; the latter is a subroutine call.
3202 Generic character types
3204 Another use of backslash is for specifying generic character types. The
3205 following are always recognized:
3207 \d any decimal digit
3208 \D any character that is not a decimal digit
3209 \h any horizontal whitespace character
3210 \H any character that is not a horizontal whitespace character
3211 \s any whitespace character
3212 \S any character that is not a whitespace character
3213 \v any vertical whitespace character
3214 \V any character that is not a vertical whitespace character
3215 \w any "word" character
3216 \W any "non-word" character
3218 Each pair of escape sequences partitions the complete set of characters
3219 into two disjoint sets. Any given character matches one, and only one,
3222 These character type sequences can appear both inside and outside char-
3223 acter classes. They each match one character of the appropriate type.
3224 If the current matching point is at the end of the subject string, all
3225 of them fail, since there is no character to match.
3227 For compatibility with Perl, \s does not match the VT character (code
3228 11). This makes it different from the the POSIX "space" class. The \s
3229 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3230 "use locale;" is included in a Perl script, \s may match the VT charac-
3231 ter. In PCRE, it never does.
3233 In UTF-8 mode, characters with values greater than 128 never match \d,
3234 \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3235 code character property support is available. These sequences retain
3236 their original meanings from before UTF-8 support was available, mainly
3237 for efficiency reasons.
3239 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3240 the other sequences, these do match certain high-valued codepoints in
3241 UTF-8 mode. The horizontal space characters are:
3243 U+0009 Horizontal tab
3245 U+00A0 Non-break space
3246 U+1680 Ogham space mark
3247 U+180E Mongolian vowel separator
3252 U+2004 Three-per-em space
3253 U+2005 Four-per-em space
3254 U+2006 Six-per-em space
3256 U+2008 Punctuation space
3259 U+202F Narrow no-break space
3260 U+205F Medium mathematical space
3261 U+3000 Ideographic space
3263 The vertical space characters are:
3268 U+000D Carriage return
3270 U+2028 Line separator
3271 U+2029 Paragraph separator
3273 A "word" character is an underscore or any character less than 256 that
3274 is a letter or digit. The definition of letters and digits is con-
3275 trolled by PCRE's low-valued character tables, and may vary if locale-
3276 specific matching is taking place (see "Locale support" in the pcreapi
3277 page). For example, in a French locale such as "fr_FR" in Unix-like
3278 systems, or "french" in Windows, some character codes greater than 128
3279 are used for accented letters, and these are matched by \w. The use of
3280 locales with Unicode is discouraged.
3284 Outside a character class, by default, the escape sequence \R matches
3285 any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3286 mode \R is equivalent to the following:
3288 (?>\r\n|\n|\x0b|\f|\r|\x85)
3290 This is an example of an "atomic group", details of which are given
3291 below. This particular group matches either the two-character sequence
3292 CR followed by LF, or one of the single characters LF (linefeed,
3293 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3294 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3295 is treated as a single unit that cannot be split.
3297 In UTF-8 mode, two additional characters whose codepoints are greater
3298 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3299 rator, U+2029). Unicode character property support is not needed for
3300 these characters to be recognized.
3302 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3303 the complete set of Unicode line endings) by setting the option
3304 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3305 (BSR is an abbrevation for "backslash R".) This can be made the default
3306 when PCRE is built; if this is the case, the other behaviour can be
3307 requested via the PCRE_BSR_UNICODE option. It is also possible to
3308 specify these settings by starting a pattern string with one of the
3309 following sequences:
3311 (*BSR_ANYCRLF) CR, LF, or CRLF only
3312 (*BSR_UNICODE) any Unicode newline sequence
3314 These override the default and the options given to pcre_compile(), but
3315 they can be overridden by options given to pcre_exec(). Note that these
3316 special settings, which are not Perl-compatible, are recognized only at
3317 the very start of a pattern, and that they must be in upper case. If
3318 more than one of them is present, the last one is used. They can be
3319 combined with a change of newline convention, for example, a pattern
3322 (*ANY)(*BSR_ANYCRLF)
3324 Inside a character class, \R matches the letter "R".
3326 Unicode character properties
3328 When PCRE is built with Unicode character property support, three addi-
3329 tional escape sequences that match characters with specific properties
3330 are available. When not in UTF-8 mode, these sequences are of course
3331 limited to testing characters whose codepoints are less than 256, but
3332 they do work in this mode. The extra escape sequences are:
3334 \p{xx} a character with the xx property
3335 \P{xx} a character without the xx property
3336 \X an extended Unicode sequence
3338 The property names represented by xx above are limited to the Unicode
3339 script names, the general category properties, and "Any", which matches
3340 any character (including newline). Other properties such as "InMusical-
3341 Symbols" are not currently supported by PCRE. Note that \P{Any} does
3342 not match any characters, so always causes a match failure.
3344 Sets of Unicode characters are defined as belonging to certain scripts.
3345 A character from one of these sets can be matched using a script name.
3351 Those that are not part of an identified script are lumped together as
3352 "Common". The current list of scripts is:
3354 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
3355 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
3356 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3357 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
3358 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
3359 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
3360 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
3361 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
3362 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3364 Each character has exactly one general category property, specified by
3365 a two-letter abbreviation. For compatibility with Perl, negation can be
3366 specified by including a circumflex between the opening brace and the
3367 property name. For example, \p{^Lu} is the same as \P{Lu}.
3369 If only one letter is specified with \p or \P, it includes all the gen-
3370 eral category properties that start with that letter. In this case, in
3371 the absence of negation, the curly brackets in the escape sequence are
3372 optional; these two examples have the same effect:
3377 The following general category property codes are supported:
3387 Ll Lower case letter
3390 Lt Title case letter
3391 Lu Upper case letter
3404 Pc Connector punctuation
3406 Pe Close punctuation
3407 Pf Final punctuation
3408 Pi Initial punctuation
3409 Po Other punctuation
3415 Sm Mathematical symbol
3420 Zp Paragraph separator
3423 The special property L& is also supported: it matches a character that
3424 has the Lu, Ll, or Lt property, in other words, a letter that is not
3425 classified as a modifier or "other".
3427 The Cs (Surrogate) property applies only to characters in the range
3428 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3429 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3430 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3433 The long synonyms for these properties that Perl supports (such as
3434 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3435 any of these properties with "Is".
3437 No character that is in the Unicode table has the Cn (unassigned) prop-
3438 erty. Instead, this property is assumed for any code point that is not
3439 in the Unicode table.
3441 Specifying caseless matching does not affect these escape sequences.
3442 For example, \p{Lu} always matches only upper case letters.
3444 The \X escape matches any number of Unicode characters that form an
3445 extended Unicode sequence. \X is equivalent to
3449 That is, it matches a character without the "mark" property, followed
3450 by zero or more characters with the "mark" property, and treats the
3451 sequence as an atomic group (see below). Characters with the "mark"
3452 property are typically accents that affect the preceding character.
3453 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3454 matches any one character.
3456 Matching characters by Unicode property is not fast, because PCRE has
3457 to search a structure that contains data for over fifteen thousand
3458 characters. That is why the traditional escape sequences such as \d and
3459 \w do not use Unicode properties in PCRE.
3461 Resetting the match start
3463 The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3464 ously matched characters not to be included in the final matched
3465 sequence. For example, the pattern:
3469 matches "foobar", but reports that it has matched "bar". This feature
3470 is similar to a lookbehind assertion (described below). However, in
3471 this case, the part of the subject before the real match does not have
3472 to be of fixed length, as lookbehind assertions do. The use of \K does
3473 not interfere with the setting of captured substrings. For example,
3478 matches "foobar", the first substring is still set to "foo".
3482 The final use of backslash is for certain simple assertions. An asser-
3483 tion specifies a condition that has to be met at a particular point in
3484 a match, without consuming any characters from the subject string. The
3485 use of subpatterns for more complicated assertions is described below.
3486 The backslashed assertions are:
3488 \b matches at a word boundary
3489 \B matches when not at a word boundary
3490 \A matches at the start of the subject
3491 \Z matches at the end of the subject
3492 also matches before a newline at the end of the subject
3493 \z matches only at the end of the subject
3494 \G matches at the first matching position in the subject
3496 These assertions may not appear in character classes (but note that \b
3497 has a different meaning, namely the backspace character, inside a char-
3500 A word boundary is a position in the subject string where the current
3501 character and the previous character do not both match \w or \W (i.e.
3502 one matches \w and the other matches \W), or the start or end of the
3503 string if the first or last character matches \w, respectively.
3505 The \A, \Z, and \z assertions differ from the traditional circumflex
3506 and dollar (described in the next section) in that they only ever match
3507 at the very start and end of the subject string, whatever options are
3508 set. Thus, they are independent of multiline mode. These three asser-
3509 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3510 affect only the behaviour of the circumflex and dollar metacharacters.
3511 However, if the startoffset argument of pcre_exec() is non-zero, indi-
3512 cating that matching is to start at a point other than the beginning of
3513 the subject, \A can never match. The difference between \Z and \z is
3514 that \Z matches before a newline at the end of the string as well as at
3515 the very end, whereas \z matches only at the end.
3517 The \G assertion is true only when the current matching position is at
3518 the start point of the match, as specified by the startoffset argument
3519 of pcre_exec(). It differs from \A when the value of startoffset is
3520 non-zero. By calling pcre_exec() multiple times with appropriate argu-
3521 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3522 mentation where \G can be useful.
3524 Note, however, that PCRE's interpretation of \G, as the start of the
3525 current match, is subtly different from Perl's, which defines it as the
3526 end of the previous match. In Perl, these can be different when the
3527 previously matched string was empty. Because PCRE does just one match
3528 at a time, it cannot reproduce this behaviour.
3530 If all the alternatives of a pattern begin with \G, the expression is
3531 anchored to the starting match position, and the "anchored" flag is set
3532 in the compiled regular expression.
3535 CIRCUMFLEX AND DOLLAR
3537 Outside a character class, in the default matching mode, the circumflex
3538 character is an assertion that is true only if the current matching
3539 point is at the start of the subject string. If the startoffset argu-
3540 ment of pcre_exec() is non-zero, circumflex can never match if the
3541 PCRE_MULTILINE option is unset. Inside a character class, circumflex
3542 has an entirely different meaning (see below).
3544 Circumflex need not be the first character of the pattern if a number
3545 of alternatives are involved, but it should be the first thing in each
3546 alternative in which it appears if the pattern is ever to match that
3547 branch. If all possible alternatives start with a circumflex, that is,
3548 if the pattern is constrained to match only at the start of the sub-
3549 ject, it is said to be an "anchored" pattern. (There are also other
3550 constructs that can cause a pattern to be anchored.)
3552 A dollar character is an assertion that is true only if the current
3553 matching point is at the end of the subject string, or immediately
3554 before a newline at the end of the string (by default). Dollar need not
3555 be the last character of the pattern if a number of alternatives are
3556 involved, but it should be the last item in any branch in which it
3557 appears. Dollar has no special meaning in a character class.
3559 The meaning of dollar can be changed so that it matches only at the
3560 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3561 compile time. This does not affect the \Z assertion.
3563 The meanings of the circumflex and dollar characters are changed if the
3564 PCRE_MULTILINE option is set. When this is the case, a circumflex
3565 matches immediately after internal newlines as well as at the start of
3566 the subject string. It does not match after a newline that ends the
3567 string. A dollar matches before any newlines in the string, as well as
3568 at the very end, when PCRE_MULTILINE is set. When newline is specified
3569 as the two-character sequence CRLF, isolated CR and LF characters do
3570 not indicate newlines.
3572 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3573 (where \n represents a newline) in multiline mode, but not otherwise.
3574 Consequently, patterns that are anchored in single line mode because
3575 all branches start with ^ are not anchored in multiline mode, and a
3576 match for circumflex is possible when the startoffset argument of
3577 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3578 PCRE_MULTILINE is set.
3580 Note that the sequences \A, \Z, and \z can be used to match the start
3581 and end of the subject in both modes, and if all branches of a pattern
3582 start with \A it is always anchored, whether or not PCRE_MULTILINE is
3586 FULL STOP (PERIOD, DOT)
3588 Outside a character class, a dot in the pattern matches any one charac-
3589 ter in the subject string except (by default) a character that signi-
3590 fies the end of a line. In UTF-8 mode, the matched character may be
3591 more than one byte long.
3593 When a line ending is defined as a single character, dot never matches
3594 that character; when the two-character sequence CRLF is used, dot does
3595 not match CR if it is immediately followed by LF, but otherwise it
3596 matches all characters (including isolated CRs and LFs). When any Uni-
3597 code line endings are being recognized, dot does not match CR or LF or
3598 any of the other line ending characters.
3600 The behaviour of dot with regard to newlines can be changed. If the
3601 PCRE_DOTALL option is set, a dot matches any one character, without
3602 exception. If the two-character sequence CRLF is present in the subject
3603 string, it takes two dots to match it.
3605 The handling of dot is entirely independent of the handling of circum-
3606 flex and dollar, the only relationship being that they both involve
3607 newlines. Dot has no special meaning in a character class.
3610 MATCHING A SINGLE BYTE
3612 Outside a character class, the escape sequence \C matches any one byte,
3613 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3614 line-ending characters. The feature is provided in Perl in order to
3615 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3616 acters into individual bytes, what remains in the string may be a mal-
3617 formed UTF-8 string. For this reason, the \C escape sequence is best
3620 PCRE does not allow \C to appear in lookbehind assertions (described
3621 below), because in UTF-8 mode this would make it impossible to calcu-
3622 late the length of the lookbehind.
3625 SQUARE BRACKETS AND CHARACTER CLASSES
3627 An opening square bracket introduces a character class, terminated by a
3628 closing square bracket. A closing square bracket on its own is not spe-
3629 cial. If a closing square bracket is required as a member of the class,
3630 it should be the first data character in the class (after an initial
3631 circumflex, if present) or escaped with a backslash.
3633 A character class matches a single character in the subject. In UTF-8
3634 mode, the character may occupy more than one byte. A matched character
3635 must be in the set of characters defined by the class, unless the first
3636 character in the class definition is a circumflex, in which case the
3637 subject character must not be in the set defined by the class. If a
3638 circumflex is actually required as a member of the class, ensure it is
3639 not the first character, or escape it with a backslash.
3641 For example, the character class [aeiou] matches any lower case vowel,
3642 while [^aeiou] matches any character that is not a lower case vowel.
3643 Note that a circumflex is just a convenient notation for specifying the
3644 characters that are in the class by enumerating those that are not. A
3645 class that starts with a circumflex is not an assertion: it still con-
3646 sumes a character from the subject string, and therefore it fails if
3647 the current pointer is at the end of the string.
3649 In UTF-8 mode, characters with values greater than 255 can be included
3650 in a class as a literal string of bytes, or by using the \x{ escaping
3653 When caseless matching is set, any letters in a class represent both
3654 their upper case and lower case versions, so for example, a caseless
3655 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3656 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3657 understands the concept of case for characters whose values are less
3658 than 128, so caseless matching is always possible. For characters with
3659 higher values, the concept of case is supported if PCRE is compiled
3660 with Unicode property support, but not otherwise. If you want to use
3661 caseless matching for characters 128 and above, you must ensure that
3662 PCRE is compiled with Unicode property support as well as with UTF-8
3665 Characters that might indicate line breaks are never treated in any
3666 special way when matching character classes, whatever line-ending
3667 sequence is in use, and whatever setting of the PCRE_DOTALL and
3668 PCRE_MULTILINE options is used. A class such as [^a] always matches one
3669 of these characters.
3671 The minus (hyphen) character can be used to specify a range of charac-
3672 ters in a character class. For example, [d-m] matches any letter
3673 between d and m, inclusive. If a minus character is required in a
3674 class, it must be escaped with a backslash or appear in a position
3675 where it cannot be interpreted as indicating a range, typically as the
3676 first or last character in the class.
3678 It is not possible to have the literal character "]" as the end charac-
3679 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3680 two characters ("W" and "-") followed by a literal string "46]", so it
3681 would match "W46]" or "-46]". However, if the "]" is escaped with a
3682 backslash it is interpreted as the end of range, so [W-\]46] is inter-
3683 preted as a class containing a range followed by two other characters.
3684 The octal or hexadecimal representation of "]" can also be used to end
3687 Ranges operate in the collating sequence of character values. They can
3688 also be used for characters specified numerically, for example
3689 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3690 are greater than 255, for example [\x{100}-\x{2ff}].
3692 If a range that includes letters is used when caseless matching is set,
3693 it matches the letters in either case. For example, [W-c] is equivalent
3694 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3695 character tables for a French locale are in use, [\xc8-\xcb] matches
3696 accented E characters in both cases. In UTF-8 mode, PCRE supports the
3697 concept of case for characters with values greater than 128 only when
3698 it is compiled with Unicode property support.
3700 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3701 in a character class, and add the characters that they match to the
3702 class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3703 flex can conveniently be used with the upper case character types to
3704 specify a more restricted set of characters than the matching lower
3705 case type. For example, the class [^\W_] matches any letter or digit,
3708 The only metacharacters that are recognized in character classes are
3709 backslash, hyphen (only where it can be interpreted as specifying a
3710 range), circumflex (only at the start), opening square bracket (only
3711 when it can be interpreted as introducing a POSIX class name - see the
3712 next section), and the terminating closing square bracket. However,
3713 escaping other non-alphanumeric characters does no harm.
3716 POSIX CHARACTER CLASSES
3718 Perl supports the POSIX notation for character classes. This uses names
3719 enclosed by [: and :] within the enclosing square brackets. PCRE also
3720 supports this notation. For example,
3724 matches "0", "1", any alphabetic character, or "%". The supported class
3727 alnum letters and digits
3729 ascii character codes 0 - 127
3730 blank space or tab only
3731 cntrl control characters
3732 digit decimal digits (same as \d)
3733 graph printing characters, excluding space
3734 lower lower case letters
3735 print printing characters, including space
3736 punct printing characters, excluding letters and digits
3737 space white space (not quite the same as \s)
3738 upper upper case letters
3739 word "word" characters (same as \w)
3740 xdigit hexadecimal digits
3742 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3743 and space (32). Notice that this list includes the VT character (code
3744 11). This makes "space" different to \s, which does not include VT (for
3745 Perl compatibility).
3747 The name "word" is a Perl extension, and "blank" is a GNU extension
3748 from Perl 5.8. Another Perl extension is negation, which is indicated
3749 by a ^ character after the colon. For example,
3753 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3754 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3755 these are not supported, and an error is given if they are encountered.
3757 In UTF-8 mode, characters with values greater than 128 do not match any
3758 of the POSIX character classes.
3763 Vertical bar characters are used to separate alternative patterns. For
3764 example, the pattern
3768 matches either "gilbert" or "sullivan". Any number of alternatives may
3769 appear, and an empty alternative is permitted (matching the empty
3770 string). The matching process tries each alternative in turn, from left
3771 to right, and the first one that succeeds is used. If the alternatives
3772 are within a subpattern (defined below), "succeeds" means matching the
3773 rest of the main pattern as well as the alternative in the subpattern.
3776 INTERNAL OPTION SETTING
3778 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3779 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
3780 within the pattern by a sequence of Perl option letters enclosed
3781 between "(?" and ")". The option letters are
3784 m for PCRE_MULTILINE
3788 For example, (?im) sets caseless, multiline matching. It is also possi-
3789 ble to unset these options by preceding the letter with a hyphen, and a
3790 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3791 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3792 is also permitted. If a letter appears both before and after the
3793 hyphen, the option is unset.
3795 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3796 can be changed in the same way as the Perl-compatible options by using
3797 the characters J, U and X respectively.
3799 When an option change occurs at top level (that is, not inside subpat-
3800 tern parentheses), the change applies to the remainder of the pattern
3801 that follows. If the change is placed right at the start of a pattern,
3802 PCRE extracts it into the global options (and it will therefore show up
3803 in data extracted by the pcre_fullinfo() function).
3805 An option change within a subpattern (see below for a description of
3806 subpatterns) affects only that part of the current pattern that follows
3811 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3812 used). By this means, options can be made to have different settings
3813 in different parts of the pattern. Any changes made in one alternative
3814 do carry on into subsequent branches within the same subpattern. For
3819 matches "ab", "aB", "c", and "C", even though when matching "C" the
3820 first branch is abandoned before the option setting. This is because
3821 the effects of option settings happen at compile time. There would be
3822 some very weird behaviour otherwise.
3824 Note: There are other PCRE-specific options that can be set by the
3825 application when the compile or match functions are called. In some
3826 cases the pattern can contain special leading sequences to override
3827 what the application has set or what has been defaulted. Details are
3828 given in the section entitled "Newline sequences" above.
3833 Subpatterns are delimited by parentheses (round brackets), which can be
3834 nested. Turning part of a pattern into a subpattern does two things:
3836 1. It localizes a set of alternatives. For example, the pattern
3838 cat(aract|erpillar|)
3840 matches one of the words "cat", "cataract", or "caterpillar". Without
3841 the parentheses, it would match "cataract", "erpillar" or an empty
3844 2. It sets up the subpattern as a capturing subpattern. This means
3845 that, when the whole pattern matches, that portion of the subject
3846 string that matched the subpattern is passed back to the caller via the
3847 ovector argument of pcre_exec(). Opening parentheses are counted from
3848 left to right (starting from 1) to obtain numbers for the capturing
3851 For example, if the string "the red king" is matched against the pat-
3854 the ((red|white) (king|queen))
3856 the captured substrings are "red king", "red", and "king", and are num-
3857 bered 1, 2, and 3, respectively.
3859 The fact that plain parentheses fulfil two functions is not always
3860 helpful. There are often times when a grouping subpattern is required
3861 without a capturing requirement. If an opening parenthesis is followed
3862 by a question mark and a colon, the subpattern does not do any captur-
3863 ing, and is not counted when computing the number of any subsequent
3864 capturing subpatterns. For example, if the string "the white queen" is
3865 matched against the pattern
3867 the ((?:red|white) (king|queen))
3869 the captured substrings are "white queen" and "queen", and are numbered
3870 1 and 2. The maximum number of capturing subpatterns is 65535.
3872 As a convenient shorthand, if any option settings are required at the
3873 start of a non-capturing subpattern, the option letters may appear
3874 between the "?" and the ":". Thus the two patterns
3876 (?i:saturday|sunday)
3877 (?:(?i)saturday|sunday)
3879 match exactly the same set of strings. Because alternative branches are
3880 tried from left to right, and options are not reset until the end of
3881 the subpattern is reached, an option setting in one branch does affect
3882 subsequent branches, so the above patterns match "SUNDAY" as well as
3886 DUPLICATE SUBPATTERN NUMBERS
3888 Perl 5.10 introduced a feature whereby each alternative in a subpattern
3889 uses the same numbers for its capturing parentheses. Such a subpattern
3890 starts with (?| and is itself a non-capturing subpattern. For example,
3891 consider this pattern:
3893 (?|(Sat)ur|(Sun))day
3895 Because the two alternatives are inside a (?| group, both sets of cap-
3896 turing parentheses are numbered one. Thus, when the pattern matches,
3897 you can look at captured substring number one, whichever alternative
3898 matched. This construct is useful when you want to capture part, but
3899 not all, of one of a number of alternatives. Inside a (?| group, paren-
3900 theses are numbered as usual, but the number is reset at the start of
3901 each branch. The numbers of any capturing buffers that follow the sub-
3902 pattern start after the highest number used in any branch. The follow-
3903 ing example is taken from the Perl documentation. The numbers under-
3904 neath show in which buffer the captured content will be stored.
3906 # before ---------------branch-reset----------- after
3907 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3910 A backreference or a recursive call to a numbered subpattern always
3911 refers to the first one in the pattern with the given number.
3913 An alternative approach to using this "branch reset" feature is to use
3914 duplicate named subpatterns, as described in the next section.
3919 Identifying capturing parentheses by number is simple, but it can be
3920 very hard to keep track of the numbers in complicated regular expres-
3921 sions. Furthermore, if an expression is modified, the numbers may
3922 change. To help with this difficulty, PCRE supports the naming of sub-
3923 patterns. This feature was not added to Perl until release 5.10. Python
3924 had the feature earlier, and PCRE introduced it at release 4.0, using
3925 the Python syntax. PCRE now supports both the Perl and the Python syn-
3928 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
3929 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
3930 to capturing parentheses from other parts of the pattern, such as back-
3931 references, recursion, and conditions, can be made by name as well as
3934 Names consist of up to 32 alphanumeric characters and underscores.
3935 Named capturing parentheses are still allocated numbers as well as
3936 names, exactly as if the names were not present. The PCRE API provides
3937 function calls for extracting the name-to-number translation table from
3938 a compiled pattern. There is also a convenience function for extracting
3939 a captured substring by name.
3941 By default, a name must be unique within a pattern, but it is possible
3942 to relax this constraint by setting the PCRE_DUPNAMES option at compile
3943 time. This can be useful for patterns where only one instance of the
3944 named parentheses can match. Suppose you want to match the name of a
3945 weekday, either as a 3-letter abbreviation or as the full name, and in
3946 both cases you want to extract the abbreviation. This pattern (ignoring
3947 the line breaks) does the job:
3949 (?<DN>Mon|Fri|Sun)(?:day)?|
3950 (?<DN>Tue)(?:sday)?|
3951 (?<DN>Wed)(?:nesday)?|
3952 (?<DN>Thu)(?:rsday)?|
3953 (?<DN>Sat)(?:urday)?
3955 There are five capturing substrings, but only one is ever set after a
3956 match. (An alternative way of solving this problem is to use a "branch
3957 reset" subpattern, as described in the previous section.)
3959 The convenience function for extracting the data by name returns the
3960 substring for the first (and in this example, the only) subpattern of
3961 that name that matched. This saves searching to find which numbered
3962 subpattern it was. If you make a reference to a non-unique named sub-
3963 pattern from elsewhere in the pattern, the one that corresponds to the
3964 lowest number is used. For further details of the interfaces for han-
3965 dling named subpatterns, see the pcreapi documentation.
3970 Repetition is specified by quantifiers, which can follow any of the
3973 a literal data character
3974 the dot metacharacter
3975 the \C escape sequence
3976 the \X escape sequence (in UTF-8 mode with Unicode properties)
3977 the \R escape sequence
3978 an escape such as \d that matches a single character
3980 a back reference (see next section)
3981 a parenthesized subpattern (unless it is an assertion)
3983 The general repetition quantifier specifies a minimum and maximum num-
3984 ber of permitted matches, by giving the two numbers in curly brackets
3985 (braces), separated by a comma. The numbers must be less than 65536,
3986 and the first must be less than or equal to the second. For example:
3990 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
3991 special character. If the second number is omitted, but the comma is
3992 present, there is no upper limit; if the second number and the comma
3993 are both omitted, the quantifier specifies an exact number of required
3998 matches at least 3 successive vowels, but may match many more, while
4002 matches exactly 8 digits. An opening curly bracket that appears in a
4003 position where a quantifier is not allowed, or one that does not match
4004 the syntax of a quantifier, is taken as a literal character. For exam-
4005 ple, {,6} is not a quantifier, but a literal string of four characters.
4007 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
4008 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4009 acters, each of which is represented by a two-byte sequence. Similarly,
4010 when Unicode property support is available, \X{3} matches three Unicode
4011 extended sequences, each of which may be several bytes long (and they
4012 may be of different lengths).
4014 The quantifier {0} is permitted, causing the expression to behave as if
4015 the previous item and the quantifier were not present. This may be use-
4016 ful for subpatterns that are referenced as subroutines from elsewhere
4017 in the pattern. Items other than subpatterns that have a {0} quantifier
4018 are omitted from the compiled pattern.
4020 For convenience, the three most common quantifiers have single-charac-
4023 * is equivalent to {0,}
4024 + is equivalent to {1,}
4025 ? is equivalent to {0,1}
4027 It is possible to construct infinite loops by following a subpattern
4028 that can match no characters with a quantifier that has no upper limit,
4033 Earlier versions of Perl and PCRE used to give an error at compile time
4034 for such patterns. However, because there are cases where this can be
4035 useful, such patterns are now accepted, but if any repetition of the
4036 subpattern does in fact match no characters, the loop is forcibly bro-
4039 By default, the quantifiers are "greedy", that is, they match as much
4040 as possible (up to the maximum number of permitted times), without
4041 causing the rest of the pattern to fail. The classic example of where
4042 this gives problems is in trying to match comments in C programs. These
4043 appear between /* and */ and within the comment, individual * and /
4044 characters may appear. An attempt to match C comments by applying the
4051 /* first comment */ not comment /* second comment */
4053 fails, because it matches the entire string owing to the greediness of
4056 However, if a quantifier is followed by a question mark, it ceases to
4057 be greedy, and instead matches the minimum number of times possible, so
4062 does the right thing with the C comments. The meaning of the various
4063 quantifiers is not otherwise changed, just the preferred number of
4064 matches. Do not confuse this use of question mark with its use as a
4065 quantifier in its own right. Because it has two uses, it can sometimes
4066 appear doubled, as in
4070 which matches one digit by preference, but can match two if that is the
4071 only way the rest of the pattern matches.
4073 If the PCRE_UNGREEDY option is set (an option that is not available in
4074 Perl), the quantifiers are not greedy by default, but individual ones
4075 can be made greedy by following them with a question mark. In other
4076 words, it inverts the default behaviour.
4078 When a parenthesized subpattern is quantified with a minimum repeat
4079 count that is greater than 1 or with a limited maximum, more memory is
4080 required for the compiled pattern, in proportion to the size of the
4083 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4084 alent to Perl's /s) is set, thus allowing the dot to match newlines,
4085 the pattern is implicitly anchored, because whatever follows will be
4086 tried against every character position in the subject string, so there
4087 is no point in retrying the overall match at any position after the
4088 first. PCRE normally treats such a pattern as though it were preceded
4091 In cases where it is known that the subject string contains no new-
4092 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
4093 mization, or alternatively using ^ to indicate anchoring explicitly.
4095 However, there is one situation where the optimization cannot be used.
4096 When .* is inside capturing parentheses that are the subject of a
4097 backreference elsewhere in the pattern, a match at the start may fail
4098 where a later one succeeds. Consider, for example:
4102 If the subject is "xyz123abc123" the match point is the fourth charac-
4103 ter. For this reason, such a pattern is not implicitly anchored.
4105 When a capturing subpattern is repeated, the value captured is the sub-
4106 string that matched the final iteration. For example, after
4108 (tweedle[dume]{3}\s*)+
4110 has matched "tweedledum tweedledee" the value of the captured substring
4111 is "tweedledee". However, if there are nested capturing subpatterns,
4112 the corresponding captured values may have been set in previous itera-
4113 tions. For example, after
4117 matches "aba" the value of the second captured substring is "b".
4120 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4122 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4123 repetition, failure of what follows normally causes the repeated item
4124 to be re-evaluated to see if a different number of repeats allows the
4125 rest of the pattern to match. Sometimes it is useful to prevent this,
4126 either to change the nature of the match, or to cause it fail earlier
4127 than it otherwise might, when the author of the pattern knows there is
4128 no point in carrying on.
4130 Consider, for example, the pattern \d+foo when applied to the subject
4135 After matching all 6 digits and then failing to match "foo", the normal
4136 action of the matcher is to try again with only 5 digits matching the
4137 \d+ item, and then with 4, and so on, before ultimately failing.
4138 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4139 the means for specifying that once a subpattern has matched, it is not
4140 to be re-evaluated in this way.
4142 If we use atomic grouping for the previous example, the matcher gives
4143 up immediately on failing to match "foo" the first time. The notation
4144 is a kind of special parenthesis, starting with (?> as in this example:
4148 This kind of parenthesis "locks up" the part of the pattern it con-
4149 tains once it has matched, and a failure further into the pattern is
4150 prevented from backtracking into it. Backtracking past it to previous
4151 items, however, works as normal.
4153 An alternative description is that a subpattern of this type matches
4154 the string of characters that an identical standalone pattern would
4155 match, if anchored at the current point in the subject string.
4157 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4158 such as the above example can be thought of as a maximizing repeat that
4159 must swallow everything it can. So, while both \d+ and \d+? are pre-
4160 pared to adjust the number of digits they match in order to make the
4161 rest of the pattern match, (?>\d+) can only match an entire sequence of
4164 Atomic groups in general can of course contain arbitrarily complicated
4165 subpatterns, and can be nested. However, when the subpattern for an
4166 atomic group is just a single repeated item, as in the example above, a
4167 simpler notation, called a "possessive quantifier" can be used. This
4168 consists of an additional + character following a quantifier. Using
4169 this notation, the previous example can be rewritten as
4173 Note that a possessive quantifier can be used with an entire group, for
4178 Possessive quantifiers are always greedy; the setting of the
4179 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4180 simpler forms of atomic group. However, there is no difference in the
4181 meaning of a possessive quantifier and the equivalent atomic group,
4182 though there may be a performance difference; possessive quantifiers
4183 should be slightly faster.
4185 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4186 tax. Jeffrey Friedl originated the idea (and the name) in the first
4187 edition of his book. Mike McCloskey liked it, so implemented it when he
4188 built Sun's Java package, and PCRE copied it from there. It ultimately
4189 found its way into Perl at release 5.10.
4191 PCRE has an optimization that automatically "possessifies" certain sim-
4192 ple pattern constructs. For example, the sequence A+B is treated as
4193 A++B because there is no point in backtracking into a sequence of A's
4196 When a pattern contains an unlimited repeat inside a subpattern that
4197 can itself be repeated an unlimited number of times, the use of an
4198 atomic group is the only way to avoid some failing matches taking a
4199 very long time indeed. The pattern
4203 matches an unlimited number of substrings that either consist of non-
4204 digits, or digits enclosed in <>, followed by either ! or ?. When it
4205 matches, it runs quickly. However, if it is applied to
4207 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4209 it takes a long time before reporting failure. This is because the
4210 string can be divided between the internal \D+ repeat and the external
4211 * repeat in a large number of ways, and all have to be tried. (The
4212 example uses [!?] rather than a single character at the end, because
4213 both PCRE and Perl have an optimization that allows for fast failure
4214 when a single character is used. They remember the last single charac-
4215 ter that is required for a match, and fail early if it is not present
4216 in the string.) If the pattern is changed so that it uses an atomic
4219 ((?>\D+)|<\d+>)*[!?]
4221 sequences of non-digits cannot be broken, and failure happens quickly.
4226 Outside a character class, a backslash followed by a digit greater than
4227 0 (and possibly further digits) is a back reference to a capturing sub-
4228 pattern earlier (that is, to its left) in the pattern, provided there
4229 have been that many previous capturing left parentheses.
4231 However, if the decimal number following the backslash is less than 10,
4232 it is always taken as a back reference, and causes an error only if
4233 there are not that many capturing left parentheses in the entire pat-
4234 tern. In other words, the parentheses that are referenced need not be
4235 to the left of the reference for numbers less than 10. A "forward back
4236 reference" of this type can make sense when a repetition is involved
4237 and the subpattern to the right has participated in an earlier itera-
4240 It is not possible to have a numerical "forward back reference" to a
4241 subpattern whose number is 10 or more using this syntax because a
4242 sequence such as \50 is interpreted as a character defined in octal.
4243 See the subsection entitled "Non-printing characters" above for further
4244 details of the handling of digits following a backslash. There is no
4245 such problem when named parentheses are used. A back reference to any
4246 subpattern is possible using named parentheses (see below).
4248 Another way of avoiding the ambiguity inherent in the use of digits
4249 following a backslash is to use the \g escape sequence, which is a fea-
4250 ture introduced in Perl 5.10. This escape must be followed by an
4251 unsigned number or a negative number, optionally enclosed in braces.
4252 These examples are all identical:
4258 An unsigned number specifies an absolute reference without the ambigu-
4259 ity that is present in the older syntax. It is also useful when literal
4260 digits follow the reference. A negative number is a relative reference.
4261 Consider this example:
4265 The sequence \g{-1} is a reference to the most recently started captur-
4266 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
4267 \g{-2} would be equivalent to \1. The use of relative references can be
4268 helpful in long patterns, and also in patterns that are created by
4269 joining together fragments that contain references within themselves.
4271 A back reference matches whatever actually matched the capturing sub-
4272 pattern in the current subject string, rather than anything matching
4273 the subpattern itself (see "Subpatterns as subroutines" below for a way
4274 of doing that). So the pattern
4276 (sens|respons)e and \1ibility
4278 matches "sense and sensibility" and "response and responsibility", but
4279 not "sense and responsibility". If caseful matching is in force at the
4280 time of the back reference, the case of letters is relevant. For exam-
4285 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4286 original capturing subpattern is matched caselessly.
4288 There are several different ways of writing back references to named
4289 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4290 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4291 unified back reference syntax, in which \g can be used for both numeric
4292 and named references, is also supported. We could rewrite the above
4293 example in any of the following ways:
4295 (?<p1>(?i)rah)\s+\k<p1>
4296 (?'p1'(?i)rah)\s+\k{p1}
4297 (?P<p1>(?i)rah)\s+(?P=p1)
4298 (?<p1>(?i)rah)\s+\g{p1}
4300 A subpattern that is referenced by name may appear in the pattern
4301 before or after the reference.
4303 There may be more than one back reference to the same subpattern. If a
4304 subpattern has not actually been used in a particular match, any back
4305 references to it always fail. For example, the pattern
4309 always fails if it starts to match "a" rather than "bc". Because there
4310 may be many capturing parentheses in a pattern, all digits following
4311 the backslash are taken as part of a potential back reference number.
4312 If the pattern continues with a digit character, some delimiter must be
4313 used to terminate the back reference. If the PCRE_EXTENDED option is
4314 set, this can be whitespace. Otherwise an empty comment (see "Com-
4315 ments" below) can be used.
4317 A back reference that occurs inside the parentheses to which it refers
4318 fails when the subpattern is first used, so, for example, (a\1) never
4319 matches. However, such references can be useful inside repeated sub-
4320 patterns. For example, the pattern
4324 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4325 ation of the subpattern, the back reference matches the character
4326 string corresponding to the previous iteration. In order for this to
4327 work, the pattern must be such that the first iteration does not need
4328 to match the back reference. This can be done using alternation, as in
4329 the example above, or by a quantifier with a minimum of zero.
4334 An assertion is a test on the characters following or preceding the
4335 current matching point that does not actually consume any characters.
4336 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
4339 More complicated assertions are coded as subpatterns. There are two
4340 kinds: those that look ahead of the current position in the subject
4341 string, and those that look behind it. An assertion subpattern is
4342 matched in the normal way, except that it does not cause the current
4343 matching position to be changed.
4345 Assertion subpatterns are not capturing subpatterns, and may not be
4346 repeated, because it makes no sense to assert the same thing several
4347 times. If any kind of assertion contains capturing subpatterns within
4348 it, these are counted for the purposes of numbering the capturing sub-
4349 patterns in the whole pattern. However, substring capturing is carried
4350 out only for positive assertions, because it does not make sense for
4351 negative assertions.
4353 Lookahead assertions
4355 Lookahead assertions start with (?= for positive assertions and (?! for
4356 negative assertions. For example,
4360 matches a word followed by a semicolon, but does not include the semi-
4361 colon in the match, and
4365 matches any occurrence of "foo" that is not followed by "bar". Note
4366 that the apparently similar pattern
4370 does not find an occurrence of "bar" that is preceded by something
4371 other than "foo"; it finds any occurrence of "bar" whatsoever, because
4372 the assertion (?!foo) is always true when the next three characters are
4373 "bar". A lookbehind assertion is needed to achieve the other effect.
4375 If you want to force a matching failure at some point in a pattern, the
4376 most convenient way to do it is with (?!) because an empty string
4377 always matches, so an assertion that requires there not to be an empty
4378 string must always fail.
4380 Lookbehind assertions
4382 Lookbehind assertions start with (?<= for positive assertions and (?<!
4383 for negative assertions. For example,
4387 does find an occurrence of "bar" that is not preceded by "foo". The
4388 contents of a lookbehind assertion are restricted such that all the
4389 strings it matches must have a fixed length. However, if there are sev-
4390 eral top-level alternatives, they do not all have to have the same
4399 causes an error at compile time. Branches that match different length
4400 strings are permitted only at the top level of a lookbehind assertion.
4401 This is an extension compared with Perl (at least for 5.8), which
4402 requires all branches to match the same length of string. An assertion
4407 is not permitted, because its single top-level branch can match two
4408 different lengths, but it is acceptable if rewritten to use two top-
4413 In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4414 instead of a lookbehind assertion; this is not restricted to a fixed-
4417 The implementation of lookbehind assertions is, for each alternative,
4418 to temporarily move the current position back by the fixed length and
4419 then try to match. If there are insufficient characters before the cur-
4420 rent position, the assertion fails.
4422 PCRE does not allow the \C escape (which matches a single byte in UTF-8
4423 mode) to appear in lookbehind assertions, because it makes it impossi-
4424 ble to calculate the length of the lookbehind. The \X and \R escapes,
4425 which can match different numbers of bytes, are also not permitted.
4427 Possessive quantifiers can be used in conjunction with lookbehind
4428 assertions to specify efficient matching at the end of the subject
4429 string. Consider a simple pattern such as
4433 when applied to a long string that does not match. Because matching
4434 proceeds from left to right, PCRE will look for each "a" in the subject
4435 and then see if what follows matches the rest of the pattern. If the
4436 pattern is specified as
4440 the initial .* matches the entire string at first, but when this fails
4441 (because there is no following "a"), it backtracks to match all but the
4442 last character, then all but the last two characters, and so on. Once
4443 again the search for "a" covers the entire string, from right to left,
4444 so we are no better off. However, if the pattern is written as
4448 there can be no backtracking for the .*+ item; it can match only the
4449 entire string. The subsequent lookbehind assertion does a single test
4450 on the last four characters. If it fails, the match fails immediately.
4451 For long strings, this approach makes a significant difference to the
4454 Using multiple assertions
4456 Several assertions (of any sort) may occur in succession. For example,
4458 (?<=\d{3})(?<!999)foo
4460 matches "foo" preceded by three digits that are not "999". Notice that
4461 each of the assertions is applied independently at the same point in
4462 the subject string. First there is a check that the previous three
4463 characters are all digits, and then there is a check that the same
4464 three characters are not "999". This pattern does not match "foo" pre-
4465 ceded by six characters, the first of which are digits and the last
4466 three of which are not "999". For example, it doesn't match "123abc-
4467 foo". A pattern to do that is
4469 (?<=\d{3}...)(?<!999)foo
4471 This time the first assertion looks at the preceding six characters,
4472 checking that the first three are digits, and then the second assertion
4473 checks that the preceding three characters are not "999".
4475 Assertions can be nested in any combination. For example,
4479 matches an occurrence of "baz" that is preceded by "bar" which in turn
4480 is not preceded by "foo", while
4482 (?<=\d{3}(?!999)...)foo
4484 is another pattern that matches "foo" preceded by three digits and any
4485 three characters that are not "999".
4488 CONDITIONAL SUBPATTERNS
4490 It is possible to cause the matching process to obey a subpattern con-
4491 ditionally or to choose between two alternative subpatterns, depending
4492 on the result of an assertion, or whether a previous capturing subpat-
4493 tern matched or not. The two possible forms of conditional subpattern
4496 (?(condition)yes-pattern)
4497 (?(condition)yes-pattern|no-pattern)
4499 If the condition is satisfied, the yes-pattern is used; otherwise the
4500 no-pattern (if present) is used. If there are more than two alterna-
4501 tives in the subpattern, a compile-time error occurs.
4503 There are four kinds of condition: references to subpatterns, refer-
4504 ences to recursion, a pseudo-condition called DEFINE, and assertions.
4506 Checking for a used subpattern by number
4508 If the text between the parentheses consists of a sequence of digits,
4509 the condition is true if the capturing subpattern of that number has
4510 previously matched. An alternative notation is to precede the digits
4511 with a plus or minus sign. In this case, the subpattern number is rela-
4512 tive rather than absolute. The most recently opened parentheses can be
4513 referenced by (?(-1), the next most recent by (?(-2), and so on. In
4514 looping constructs it can also make sense to refer to subsequent groups
4515 with constructs such as (?(+2).
4517 Consider the following pattern, which contains non-significant white
4518 space to make it more readable (assume the PCRE_EXTENDED option) and to
4519 divide it into three parts for ease of discussion:
4521 ( \( )? [^()]+ (?(1) \) )
4523 The first part matches an optional opening parenthesis, and if that
4524 character is present, sets it as the first captured substring. The sec-
4525 ond part matches one or more characters that are not parentheses. The
4526 third part is a conditional subpattern that tests whether the first set
4527 of parentheses matched or not. If they did, that is, if subject started
4528 with an opening parenthesis, the condition is true, and so the yes-pat-
4529 tern is executed and a closing parenthesis is required. Otherwise,
4530 since no-pattern is not present, the subpattern matches nothing. In
4531 other words, this pattern matches a sequence of non-parentheses,
4532 optionally enclosed in parentheses.
4534 If you were embedding this pattern in a larger one, you could use a
4537 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
4539 This makes the fragment independent of the parentheses in the larger
4542 Checking for a used subpattern by name
4544 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
4545 used subpattern by name. For compatibility with earlier versions of
4546 PCRE, which had this facility before Perl, the syntax (?(name)...) is
4547 also recognized. However, there is a possible ambiguity with this syn-
4548 tax, because subpattern names may consist entirely of digits. PCRE
4549 looks first for a named subpattern; if it cannot find one and the name
4550 consists entirely of digits, PCRE looks for a subpattern of that num-
4551 ber, which must be greater than zero. Using subpattern names that con-
4552 sist entirely of digits is not recommended.
4554 Rewriting the above example to use a named subpattern gives this:
4556 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
4559 Checking for pattern recursion
4561 If the condition is the string (R), and there is no subpattern with the
4562 name R, the condition is true if a recursive call to the whole pattern
4563 or any subpattern has been made. If digits or a name preceded by amper-
4564 sand follow the letter R, for example:
4566 (?(R3)...) or (?(R&name)...)
4568 the condition is true if the most recent recursion is into the subpat-
4569 tern whose number or name is given. This condition does not check the
4570 entire recursion stack.
4572 At "top level", all these recursion test conditions are false. Recur-
4573 sive patterns are described below.
4575 Defining subpatterns for use by reference only
4577 If the condition is the string (DEFINE), and there is no subpattern
4578 with the name DEFINE, the condition is always false. In this case,
4579 there may be only one alternative in the subpattern. It is always
4580 skipped if control reaches this point in the pattern; the idea of
4581 DEFINE is that it can be used to define "subroutines" that can be ref-
4582 erenced from elsewhere. (The use of "subroutines" is described below.)
4583 For example, a pattern to match an IPv4 address could be written like
4584 this (ignore whitespace and line breaks):
4586 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4587 \b (?&byte) (\.(?&byte)){3} \b
4589 The first part of the pattern is a DEFINE group inside which a another
4590 group named "byte" is defined. This matches an individual component of
4591 an IPv4 address (a number less than 256). When matching takes place,
4592 this part of the pattern is skipped because DEFINE acts like a false
4595 The rest of the pattern uses references to the named group to match the
4596 four dot-separated components of an IPv4 address, insisting on a word
4597 boundary at each end.
4599 Assertion conditions
4601 If the condition is not in any of the above formats, it must be an
4602 assertion. This may be a positive or negative lookahead or lookbehind
4603 assertion. Consider this pattern, again containing non-significant
4604 white space, and with the two alternatives on the second line:
4607 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
4609 The condition is a positive lookahead assertion that matches an
4610 optional sequence of non-letters followed by a letter. In other words,
4611 it tests for the presence of at least one letter in the subject. If a
4612 letter is found, the subject is matched against the first alternative;
4613 otherwise it is matched against the second. This pattern matches
4614 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4615 letters and dd are digits.
4620 The sequence (?# marks the start of a comment that continues up to the
4621 next closing parenthesis. Nested parentheses are not permitted. The
4622 characters that make up a comment play no part in the pattern matching
4625 If the PCRE_EXTENDED option is set, an unescaped # character outside a
4626 character class introduces a comment that continues to immediately
4627 after the next newline in the pattern.
4632 Consider the problem of matching a string in parentheses, allowing for
4633 unlimited nested parentheses. Without the use of recursion, the best
4634 that can be done is to use a pattern that matches up to some fixed
4635 depth of nesting. It is not possible to handle an arbitrary nesting
4638 For some time, Perl has provided a facility that allows regular expres-
4639 sions to recurse (amongst other things). It does this by interpolating
4640 Perl code in the expression at run time, and the code can refer to the
4641 expression itself. A Perl pattern using code interpolation to solve the
4642 parentheses problem can be created like this:
4644 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4646 The (?p{...}) item interpolates Perl code at run time, and in this case
4647 refers recursively to the pattern in which it appears.
4649 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4650 it supports special syntax for recursion of the entire pattern, and
4651 also for individual subpattern recursion. After its introduction in
4652 PCRE and Python, this kind of recursion was introduced into Perl at
4655 A special item that consists of (? followed by a number greater than
4656 zero and a closing parenthesis is a recursive call of the subpattern of
4657 the given number, provided that it occurs inside that subpattern. (If
4658 not, it is a "subroutine" call, which is described in the next sec-
4659 tion.) The special item (?R) or (?0) is a recursive call of the entire
4662 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
4663 always treated as an atomic group. That is, once it has matched some of
4664 the subject string, it is never re-entered, even if it contains untried
4665 alternatives and there is a subsequent matching failure.
4667 This PCRE pattern solves the nested parentheses problem (assume the
4668 PCRE_EXTENDED option is set so that white space is ignored):
4670 \( ( (?>[^()]+) | (?R) )* \)
4672 First it matches an opening parenthesis. Then it matches any number of
4673 substrings which can either be a sequence of non-parentheses, or a
4674 recursive match of the pattern itself (that is, a correctly parenthe-
4675 sized substring). Finally there is a closing parenthesis.
4677 If this were part of a larger pattern, you would not want to recurse
4678 the entire pattern, so instead you could use this:
4680 ( \( ( (?>[^()]+) | (?1) )* \) )
4682 We have put the pattern into parentheses, and caused the recursion to
4683 refer to them instead of the whole pattern.
4685 In a larger pattern, keeping track of parenthesis numbers can be
4686 tricky. This is made easier by the use of relative references. (A Perl
4687 5.10 feature.) Instead of (?1) in the pattern above you can write
4688 (?-2) to refer to the second most recently opened parentheses preceding
4689 the recursion. In other words, a negative number counts capturing
4690 parentheses leftwards from the point at which it is encountered.
4692 It is also possible to refer to subsequently opened parentheses, by
4693 writing references such as (?+2). However, these cannot be recursive
4694 because the reference is not inside the parentheses that are refer-
4695 enced. They are always "subroutine" calls, as described in the next
4698 An alternative approach is to use named parentheses instead. The Perl
4699 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
4700 supported. We could rewrite the above example as follows:
4702 (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4704 If there is more than one subpattern with the same name, the earliest
4707 This particular example pattern that we have been looking at contains
4708 nested unlimited repeats, and so the use of atomic grouping for match-
4709 ing strings of non-parentheses is important when applying the pattern
4710 to strings that do not match. For example, when this pattern is applied
4713 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4715 it yields "no match" quickly. However, if atomic grouping is not used,
4716 the match runs for a very long time indeed because there are so many
4717 different ways the + and * repeats can carve up the subject, and all
4718 have to be tested before failure can be reported.
4720 At the end of a match, the values set for any capturing subpatterns are
4721 those from the outermost level of the recursion at which the subpattern
4722 value is set. If you want to obtain intermediate values, a callout
4723 function can be used (see below and the pcrecallout documentation). If
4724 the pattern above is matched against
4728 the value for the capturing parentheses is "ef", which is the last
4729 value taken on at the top level. If additional parentheses are added,
4732 \( ( ( (?>[^()]+) | (?R) )* ) \)
4736 the string they capture is "ab(cd)ef", the contents of the top level
4737 parentheses. If there are more than 15 capturing parentheses in a pat-
4738 tern, PCRE has to obtain extra memory to store data during a recursion,
4739 which it does by using pcre_malloc, freeing it via pcre_free after-
4740 wards. If no memory can be obtained, the match fails with the
4741 PCRE_ERROR_NOMEMORY error.
4743 Do not confuse the (?R) item with the condition (R), which tests for
4744 recursion. Consider this pattern, which matches text in angle brack-
4745 ets, allowing for arbitrary nesting. Only digits are allowed in nested
4746 brackets (that is, when recursing), whereas any characters are permit-
4747 ted at the outer level.
4749 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
4751 In this pattern, (?(R) is the start of a conditional subpattern, with
4752 two different alternatives for the recursive and non-recursive cases.
4753 The (?R) item is the actual recursive call.
4756 SUBPATTERNS AS SUBROUTINES
4758 If the syntax for a recursive subpattern reference (either by number or
4759 by name) is used outside the parentheses to which it refers, it oper-
4760 ates like a subroutine in a programming language. The "called" subpat-
4761 tern may be defined before or after the reference. A numbered reference
4762 can be absolute or relative, as in these examples:
4764 (...(absolute)...)...(?2)...
4765 (...(relative)...)...(?-1)...
4766 (...(?+1)...(relative)...
4768 An earlier example pointed out that the pattern
4770 (sens|respons)e and \1ibility
4772 matches "sense and sensibility" and "response and responsibility", but
4773 not "sense and responsibility". If instead the pattern
4775 (sens|respons)e and (?1)ibility
4777 is used, it does match "sense and responsibility" as well as the other
4778 two strings. Another example is given in the discussion of DEFINE
4781 Like recursive subpatterns, a "subroutine" call is always treated as an
4782 atomic group. That is, once it has matched some of the subject string,
4783 it is never re-entered, even if it contains untried alternatives and
4784 there is a subsequent matching failure.
4786 When a subpattern is used as a subroutine, processing options such as
4787 case-independence are fixed when the subpattern is defined. They cannot
4788 be changed for different calls. For example, consider this pattern:
4792 It matches "abcabc". It does not match "abcABC" because the change of
4793 processing option does not affect the called subpattern.
4796 ONIGURUMA SUBROUTINE SYNTAX
4798 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
4799 name or a number enclosed either in angle brackets or single quotes, is
4800 an alternative syntax for referencing a subpattern as a subroutine,
4801 possibly recursively. Here are two of the examples used above, rewrit-
4802 ten using this syntax:
4804 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
4805 (sens|respons)e and \g'1'ibility
4807 PCRE supports an extension to Oniguruma: if a number is preceded by a
4808 plus or a minus sign it is taken as a relative reference. For example:
4812 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
4813 synonymous. The former is a back reference; the latter is a subroutine
4819 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4820 Perl code to be obeyed in the middle of matching a regular expression.
4821 This makes it possible, amongst other things, to extract different sub-
4822 strings that match the same pair of parentheses when there is a repeti-
4825 PCRE provides a similar feature, but of course it cannot obey arbitrary
4826 Perl code. The feature is called "callout". The caller of PCRE provides
4827 an external function by putting its entry point in the global variable
4828 pcre_callout. By default, this variable contains NULL, which disables
4831 Within a regular expression, (?C) indicates the points at which the
4832 external function is to be called. If you want to identify different
4833 callout points, you can put a number less than 256 after the letter C.
4834 The default value is zero. For example, this pattern has two callout
4839 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4840 automatically installed before each item in the pattern. They are all
4843 During matching, when PCRE reaches a callout point (and pcre_callout is
4844 set), the external function is called. It is provided with the number
4845 of the callout, the position in the pattern, and, optionally, one item
4846 of data originally supplied by the caller of pcre_exec(). The callout
4847 function may cause matching to proceed, to backtrack, or to fail alto-
4848 gether. A complete description of the interface to the callout function
4849 is given in the pcrecallout documentation.
4852 BACKTRACKING CONTROL
4854 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
4855 which are described in the Perl documentation as "experimental and sub-
4856 ject to change or removal in a future version of Perl". It goes on to
4857 say: "Their usage in production code should be noted to avoid problems
4858 during upgrades." The same remarks apply to the PCRE features described
4861 Since these verbs are specifically related to backtracking, most of
4862 them can be used only when the pattern is to be matched using
4863 pcre_exec(), which uses a backtracking algorithm. With the exception of
4864 (*FAIL), which behaves like a failing negative assertion, they cause an
4865 error if encountered by pcre_dfa_exec().
4867 The new verbs make use of what was previously invalid syntax: an open-
4868 ing parenthesis followed by an asterisk. In Perl, they are generally of
4869 the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4870 its general form is just (*VERB). Any number of these verbs may occur
4871 in a pattern. There are two kinds:
4873 Verbs that act immediately
4875 The following verbs act as soon as they are encountered:
4879 This verb causes the match to end successfully, skipping the remainder
4880 of the pattern. When inside a recursion, only the innermost pattern is
4881 ended immediately. PCRE differs from Perl in what happens if the
4882 (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is
4883 captured: in PCRE no data is captured. For example:
4887 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data
4892 This verb causes the match to fail, forcing backtracking to occur. It
4893 is equivalent to (?!) but easier to read. The Perl documentation notes
4894 that it is probably useful only when combined with (?{}) or (??{}).
4895 Those are, of course, Perl features that are not present in PCRE. The
4896 nearest equivalent is the callout feature, as for example in this pat-
4901 A match with the string "aaaa" always fails, but the callout is taken
4902 before each backtrack happens (in this example, 10 times).
4904 Verbs that act after backtracking
4906 The following verbs do nothing when they are encountered. Matching con-
4907 tinues with what follows, but if there is no subsequent match, a fail-
4908 ure is forced. The verbs differ in exactly what kind of failure
4913 This verb causes the whole match to fail outright if the rest of the
4914 pattern does not match. Even if the pattern is unanchored, no further
4915 attempts to find a match by advancing the start point take place. Once
4916 (*COMMIT) has been passed, pcre_exec() is committed to finding a match
4917 at the current starting point, or not at all. For example:
4921 This matches "xxaab" but not "aacaab". It can be thought of as a kind
4922 of dynamic anchor, or "I've started, so I must finish."
4926 This verb causes the match to fail at the current position if the rest
4927 of the pattern does not match. If the pattern is unanchored, the normal
4928 "bumpalong" advance to the next starting character then happens. Back-
4929 tracking can occur as usual to the left of (*PRUNE), or when matching
4930 to the right of (*PRUNE), but if there is no match to the right, back-
4931 tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
4932 is just an alternative to an atomic group or possessive quantifier, but
4933 there are some uses of (*PRUNE) that cannot be expressed in any other
4938 This verb is like (*PRUNE), except that if the pattern is unanchored,
4939 the "bumpalong" advance is not to the next character, but to the posi-
4940 tion in the subject where (*SKIP) was encountered. (*SKIP) signifies
4941 that whatever text was matched leading up to it cannot be part of a
4942 successful match. Consider:
4946 If the subject is "aaaac...", after the first match attempt fails
4947 (starting at the first character in the string), the starting point
4948 skips on to start the next attempt at "c". Note that a possessive quan-
4949 tifer does not have the same effect in this example; although it would
4950 suppress backtracking during the first match attempt, the second
4951 attempt would start at the second character instead of skipping on to
4956 This verb causes a skip to the next alternation if the rest of the pat-
4957 tern does not match. That is, it cancels pending backtracking, but only
4958 within the current alternation. Its name comes from the observation
4959 that it can be used for a pattern-based if-then-else block:
4961 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
4963 If the COND1 pattern matches, FOO is tried (and possibly further items
4964 after the end of the group if FOO succeeds); on failure the matcher
4965 skips to the second alternative and tries COND2, without backtracking
4966 into COND1. If (*THEN) is used outside of any alternation, it acts
4967 exactly like (*PRUNE).
4972 pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
4978 University Computing Service
4979 Cambridge CB2 3QH, England.
4984 Last updated: 19 April 2008
4985 Copyright (c) 1997-2008 University of Cambridge.
4986 ------------------------------------------------------------------------------
4989 PCRESYNTAX(3) PCRESYNTAX(3)
4993 PCRE - Perl-compatible regular expressions
4996 PCRE REGULAR EXPRESSION SYNTAX SUMMARY
4998 The full syntax and semantics of the regular expressions that are sup-
4999 ported by PCRE are described in the pcrepattern documentation. This
5000 document contains just a quick-reference summary of the syntax.
5005 \x where x is non-alphanumeric is a literal x
5006 \Q...\E treat enclosed characters as literal
5011 \a alarm, that is, the BEL character (hex 07)
5012 \cx "control-x", where x is any character
5014 \f formfeed (hex 0C)
5016 \r carriage return (hex 0D)
5018 \ddd character with octal code ddd, or backreference
5019 \xhh character with hex code hh
5020 \x{hhh..} character with hex code hhh..
5025 . any character except newline;
5026 in dotall mode, any character whatsoever
5027 \C one byte, even in UTF-8 mode (best avoided)
5029 \D a character that is not a decimal digit
5030 \h a horizontal whitespace character
5031 \H a character that is not a horizontal whitespace character
5032 \p{xx} a character with the xx property
5033 \P{xx} a character without the xx property
5034 \R a newline sequence
5035 \s a whitespace character
5036 \S a character that is not a whitespace character
5037 \v a vertical whitespace character
5038 \V a character that is not a vertical whitespace character
5039 \w a "word" character
5040 \W a "non-word" character
5041 \X an extended Unicode sequence
5043 In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.
5046 GENERAL CATEGORY PROPERTY CODES FOR \p and \P
5056 Ll Lower case letter
5059 Lt Title case letter
5060 Lu Upper case letter
5074 Pc Connector punctuation
5076 Pe Close punctuation
5077 Pf Final punctuation
5078 Pi Initial punctuation
5079 Po Other punctuation
5085 Sm Mathematical symbol
5090 Zp Paragraph separator
5094 SCRIPT NAMES FOR \p AND \P
5096 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
5097 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
5098 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
5099 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
5100 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
5101 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
5102 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
5103 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
5104 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
5109 [...] positive character class
5110 [^...] negative character class
5111 [x-y] range (can be used for hex characters)
5112 [[:xxx:]] positive POSIX named set
5113 [[:^xxx:]] negative POSIX named set
5119 cntrl control character
5121 graph printing, excluding space
5122 lower lower case letter
5123 print printing, including space
5124 punct printing, excluding alphanumeric
5126 upper upper case letter
5128 xdigit hexadecimal digit
5130 In PCRE, POSIX character set names recognize only ASCII characters. You
5131 can use \Q...\E inside a character class.
5137 ?+ 0 or 1, possessive
5140 *+ 0 or more, possessive
5143 ++ 1 or more, possessive
5146 {n,m} at least n, no more than m, greedy
5147 {n,m}+ at least n, no more than m, possessive
5148 {n,m}? at least n, no more than m, lazy
5149 {n,} n or more, greedy
5150 {n,}+ n or more, possessive
5151 {n,}? n or more, lazy
5154 ANCHORS AND SIMPLE ASSERTIONS
5157 \B not a word boundary
5159 also after internal newline in multiline mode
5162 also before newline at end of subject
5163 also before internal newline in multiline mode
5165 also before newline at end of subject
5167 \G first matching position in subject
5172 \K reset start of match
5182 (...) capturing group
5183 (?<name>...) named capturing group (Perl)
5184 (?'name'...) named capturing group (Perl)
5185 (?P<name>...) named capturing group (Python)
5186 (?:...) non-capturing group
5187 (?|...) non-capturing group; reset group numbers for
5188 capturing groups in each alternative
5193 (?>...) atomic, non-capturing group
5198 (?#....) comment (not nestable)
5204 (?J) allow duplicate names
5206 (?s) single line (dotall)
5207 (?U) default ungreedy (lazy)
5208 (?x) extended (ignore white space)
5209 (?-...) unset option(s)
5212 LOOKAHEAD AND LOOKBEHIND ASSERTIONS
5214 (?=...) positive look ahead
5215 (?!...) negative look ahead
5216 (?<=...) positive look behind
5217 (?<!...) negative look behind
5219 Each top-level branch of a look behind must be of a fixed length.
5224 \n reference by number (can be ambiguous)
5225 \gn reference by number
5226 \g{n} reference by number
5227 \g{-n} relative reference by number
5228 \k<name> reference by name (Perl)
5229 \k'name' reference by name (Perl)
5230 \g{name} reference by name (Perl)
5231 \k{name} reference by name (.NET)
5232 (?P=name) reference by name (Python)
5235 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
5237 (?R) recurse whole pattern
5238 (?n) call subpattern by absolute number
5239 (?+n) call subpattern by relative number
5240 (?-n) call subpattern by relative number
5241 (?&name) call subpattern by name (Perl)
5242 (?P>name) call subpattern by name (Python)
5243 \g<name> call subpattern by name (Oniguruma)
5244 \g'name' call subpattern by name (Oniguruma)
5245 \g<n> call subpattern by absolute number (Oniguruma)
5246 \g'n' call subpattern by absolute number (Oniguruma)
5247 \g<+n> call subpattern by relative number (PCRE extension)
5248 \g'+n' call subpattern by relative number (PCRE extension)
5249 \g<-n> call subpattern by relative number (PCRE extension)
5250 \g'-n' call subpattern by relative number (PCRE extension)
5253 CONDITIONAL PATTERNS
5255 (?(condition)yes-pattern)
5256 (?(condition)yes-pattern|no-pattern)
5258 (?(n)... absolute reference condition
5259 (?(+n)... relative reference condition
5260 (?(-n)... relative reference condition
5261 (?(<name>)... named reference condition (Perl)
5262 (?('name')... named reference condition (Perl)
5263 (?(name)... named reference condition (PCRE)
5264 (?(R)... overall recursion condition
5265 (?(Rn)... specific group recursion condition
5266 (?(R&name)... specific recursion condition
5267 (?(DEFINE)... define subpattern for reference
5268 (?(assert)... assertion condition
5271 BACKTRACKING CONTROL
5273 The following act immediately they are reached:
5275 (*ACCEPT) force successful match
5276 (*FAIL) force backtrack; synonym (*F)
5278 The following act only when a subsequent match failure causes a back-
5279 track to reach them. They all force a match failure, but they differ in
5280 what happens afterwards. Those that advance the start-of-match point do
5281 so only if the pattern is not anchored.
5283 (*COMMIT) overall failure, no advance of starting point
5284 (*PRUNE) advance to next starting character
5285 (*SKIP) advance start to current matching position
5286 (*THEN) local failure, backtrack to next alternation
5291 These are recognized only at the very start of the pattern or after a
5303 These are recognized only at the very start of the pattern or after a
5304 (*...) option that sets the newline convention.
5313 (?Cn) callout with data n
5318 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
5324 University Computing Service
5325 Cambridge CB2 3QH, England.
5330 Last updated: 09 April 2008
5331 Copyright (c) 1997-2008 University of Cambridge.
5332 ------------------------------------------------------------------------------
5335 PCREPARTIAL(3) PCREPARTIAL(3)
5339 PCRE - Perl-compatible regular expressions
5342 PARTIAL MATCHING IN PCRE
5344 In normal use of PCRE, if the subject string that is passed to
5345 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
5346 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
5347 There are circumstances where it might be helpful to distinguish this
5348 case from other cases in which there is no match.
5350 Consider, for example, an application where a human is required to type
5351 in data for a field with specific formatting requirements. An example
5352 might be a date in the form ddmmmyy, defined by this pattern:
5354 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
5356 If the application sees the user's keystrokes one by one, and can check
5357 that what has been typed so far is potentially valid, it is able to
5358 raise an error as soon as a mistake is made, possibly beeping and not
5359 reflecting the character that has been typed. This immediate feedback
5360 is likely to be a better user interface than a check that is delayed
5361 until the entire string has been entered.
5363 PCRE supports the concept of partial matching by means of the PCRE_PAR-
5364 TIAL option, which can be set when calling pcre_exec() or
5365 pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
5366 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
5367 during the matching process the last part of the subject string matched
5368 part of the pattern. Unfortunately, for non-anchored matching, it is
5369 not possible to obtain the position of the start of the partial match.
5370 No captured data is set when PCRE_ERROR_PARTIAL is returned.
5372 When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code
5373 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of
5374 the subject is reached, there have been no complete matches, but there
5375 is still at least one matching possibility. The portion of the string
5376 that provided the partial match is set as the first matching string.
5378 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
5379 the last literal byte in a pattern, and abandons matching immediately
5380 if such a byte is not present in the subject string. This optimization
5381 cannot be used for a subject string that might match only partially.
5384 RESTRICTED PATTERNS FOR PCRE_PARTIAL
5386 Because of the way certain internal optimizations are implemented in
5387 the pcre_exec() function, the PCRE_PARTIAL option cannot be used with
5388 all patterns. These restrictions do not apply when pcre_dfa_exec() is
5389 used. For pcre_exec(), repeated single characters such as
5393 and repeated single metasequences such as
5397 are not permitted if the maximum number of occurrences is greater than
5398 one. Optional items such as \d? (where the maximum is one) are permit-
5399 ted. Quantifiers with any values are permitted after parentheses, so
5400 the invalid examples above can be coded thus:
5405 These constructions run more slowly, but for the kinds of application
5406 that are envisaged for this facility, this is not felt to be a major
5409 If PCRE_PARTIAL is set for a pattern that does not conform to the
5410 restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
5411 (-13). You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to
5412 find out if a compiled pattern can be used for partial matching.
5415 EXAMPLE OF PARTIAL MATCHING USING PCRETEST
5417 If the escape sequence \P is present in a pcretest data line, the
5418 PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
5419 uses the date example quoted above:
5421 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5434 The first data string is matched completely, so pcretest shows the
5435 matched substrings. The remaining four strings do not match the com-
5436 plete pattern, but the first two are partial matches. The same test,
5437 using pcre_dfa_exec() matching (by means of the \D escape sequence),
5438 produces the following output:
5440 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5444 Partial match: 23dec3
5452 Notice that in this case the portion of the string that was matched is
5456 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
5458 When a partial match has been found using pcre_dfa_exec(), it is possi-
5459 ble to continue the match by providing additional subject data and
5460 calling pcre_dfa_exec() again with the same compiled regular expres-
5461 sion, this time setting the PCRE_DFA_RESTART option. You must also pass
5462 the same working space as before, because this is where details of the
5463 previous partial match are stored. Here is an example using pcretest,
5464 using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
5467 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5473 The first call has "23ja" as the subject, and requests partial match-
5474 ing; the second call has "n05" as the subject for the continued
5475 (restarted) match. Notice that when the match is complete, only the
5476 last part is shown; PCRE does not retain the previously partially-
5477 matched string. It is up to the calling program to do that if it needs
5480 You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial
5481 matching over multiple segments. This facility can be used to pass very
5482 long subject strings to pcre_dfa_exec(). However, some care is needed
5483 for certain types of pattern.
5485 1. If the pattern contains tests for the beginning or end of a line,
5486 you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
5487 ate, when the subject string for any call does not contain the begin-
5488 ning or end of a line.
5490 2. If the pattern contains backward assertions (including \b or \B),
5491 you need to arrange for some overlap in the subject strings to allow
5492 for this. For example, you could pass the subject in chunks that are
5493 500 bytes long, but in a buffer of 700 bytes, with the starting offset
5494 set to 200 and the previous 200 bytes at the start of the buffer.
5496 3. Matching a subject string that is split into multiple segments does
5497 not always produce exactly the same result as matching over one single
5498 long string. The difference arises when there are multiple matching
5499 possibilities, because a partial match result is given only when there
5500 are no completed matches in a call to pcre_dfa_exec(). This means that
5501 as soon as the shortest match has been found, continuation to a new
5502 subject segment is no longer possible. Consider this pcretest example:
5513 The pattern matches the words "dog" or "dogsbody". When the subject is
5514 presented in several parts ("do" and "gsb" being the first two) the
5515 match stops when "dog" has been found, and it is not possible to con-
5516 tinue. On the other hand, if "dogsbody" is presented as a single
5517 string, both matches are found.
5519 Because of this phenomenon, it does not usually make sense to end a
5520 pattern that is going to be matched in this way with a variable repeat.
5522 4. Patterns that contain alternatives at the top level which do not all
5523 start with the same pattern item may not work as expected. For example,
5524 consider this pattern:
5528 If the first part of the subject is "ABC123", a partial match of the
5529 first alternative is found at offset 3. There is no partial match for
5530 the second alternative, because such a match does not start at the same
5531 point in the subject string. Attempting to continue with the string
5532 "789" does not yield a match because only those alternatives that match
5533 at one point in the subject are remembered. The problem arises because
5534 the start of the second alternative matches within the first alterna-
5535 tive. There is no problem with anchored patterns or patterns such as:
5539 where no string can be a partial match for both alternatives.
5545 University Computing Service
5546 Cambridge CB2 3QH, England.
5551 Last updated: 04 June 2007
5552 Copyright (c) 1997-2007 University of Cambridge.
5553 ------------------------------------------------------------------------------
5556 PCREPRECOMPILE(3) PCREPRECOMPILE(3)
5560 PCRE - Perl-compatible regular expressions
5563 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
5565 If you are running an application that uses a large number of regular
5566 expression patterns, it may be useful to store them in a precompiled
5567 form instead of having to compile them every time the application is
5568 run. If you are not using any private character tables (see the
5569 pcre_maketables() documentation), this is relatively straightforward.
5570 If you are using private tables, it is a little bit more complicated.
5572 If you save compiled patterns to a file, you can copy them to a differ-
5573 ent host and run them there. This works even if the new host has the
5574 opposite endianness to the one on which the patterns were compiled.
5575 There may be a small performance penalty, but it should be insignifi-
5576 cant. However, compiling regular expressions with one version of PCRE
5577 for use with a different version is not guaranteed to work and may
5581 SAVING A COMPILED PATTERN
5582 The value returned by pcre_compile() points to a single block of memory
5583 that holds the compiled pattern and associated data. You can find the
5584 length of this block in bytes by calling pcre_fullinfo() with an argu-
5585 ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
5586 manner. Here is sample code that compiles a pattern and writes it to a
5587 file. It assumes that the variable fd refers to a file that is open for
5590 int erroroffset, rc, size;
5594 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
5595 if (re == NULL) { ... handle errors ... }
5596 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
5597 if (rc < 0) { ... handle errors ... }
5598 rc = fwrite(re, 1, size, fd);
5599 if (rc != size) { ... handle errors ... }
5601 In this example, the bytes that comprise the compiled pattern are
5602 copied exactly. Note that this is binary data that may contain any of
5603 the 256 possible byte values. On systems that make a distinction
5604 between binary and non-binary data, be sure that the file is opened for
5607 If you want to write more than one pattern to a file, you will have to
5608 devise a way of separating them. For binary data, preceding each pat-
5609 tern with its length is probably the most straightforward approach.
5610 Another possibility is to write out the data in hexadecimal instead of
5611 binary, one pattern to a line.
5613 Saving compiled patterns in a file is only one possible way of storing
5614 them for later use. They could equally well be saved in a database, or
5615 in the memory of some daemon process that passes them via sockets to
5616 the processes that want them.
5618 If the pattern has been studied, it is also possible to save the study
5619 data in a similar way to the compiled pattern itself. When studying
5620 generates additional information, pcre_study() returns a pointer to a
5621 pcre_extra data block. Its format is defined in the section on matching
5622 a pattern in the pcreapi documentation. The study_data field points to
5623 the binary study data, and this is what you must save (not the
5624 pcre_extra block itself). The length of the study data can be obtained
5625 by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
5626 Remember to check that pcre_study() did return a non-NULL value before
5627 trying to save the study data.
5630 RE-USING A PRECOMPILED PATTERN
5632 Re-using a precompiled pattern is straightforward. Having reloaded it
5633 into main memory, you pass its pointer to pcre_exec() or
5634 pcre_dfa_exec() in the usual way. This should work even on another
5635 host, and even if that host has the opposite endianness to the one
5636 where the pattern was compiled.
5638 However, if you passed a pointer to custom character tables when the
5639 pattern was compiled (the tableptr argument of pcre_compile()), you
5640 must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
5641 because the value saved with the compiled pattern will obviously be
5642 nonsense. A field in a pcre_extra() block is used to pass this data, as
5643 described in the section on matching a pattern in the pcreapi documen-
5646 If you did not provide custom character tables when the pattern was
5647 compiled, the pointer in the compiled pattern is NULL, which causes
5648 pcre_exec() to use PCRE's internal tables. Thus, you do not need to
5649 take any special action at run time in this case.
5651 If you saved study data with the compiled pattern, you need to create
5652 your own pcre_extra data block and set the study_data field to point to
5653 the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
5654 bit in the flags field to indicate that study data is present. Then
5655 pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
5659 COMPATIBILITY WITH DIFFERENT PCRE RELEASES
5661 In general, it is safest to recompile all saved patterns when you
5662 update to a new PCRE release, though not all updates actually require
5663 this. Recompiling is definitely needed for release 7.2.
5669 University Computing Service
5670 Cambridge CB2 3QH, England.
5675 Last updated: 13 June 2007
5676 Copyright (c) 1997-2007 University of Cambridge.
5677 ------------------------------------------------------------------------------
5680 PCREPERFORM(3) PCREPERFORM(3)
5684 PCRE - Perl-compatible regular expressions
5689 Two aspects of performance are discussed below: memory usage and pro-
5690 cessing time. The way you express your pattern as a regular expression
5691 can affect both of them.
5696 Patterns are compiled by PCRE into a reasonably efficient byte code, so
5697 that most simple patterns do not use much memory. However, there is one
5698 case where memory usage can be unexpectedly large. When a parenthesized
5699 subpattern has a quantifier with a minimum greater than 1 and/or a lim-
5700 ited maximum, the whole subpattern is repeated in the compiled code.
5701 For example, the pattern
5705 is compiled as if it were
5707 (abc|def)(abc|def)((abc|def)(abc|def)?)?
5709 (Technical aside: It is done this way so that backtrack points within
5710 each of the repetitions can be independently maintained.)
5712 For regular expressions whose quantifiers use only small numbers, this
5713 is not usually a problem. However, if the numbers are large, and par-
5714 ticularly if such repetitions are nested, the memory usage can become
5715 an embarrassment. For example, the very simple pattern
5717 ((ab){1,1000}c){1,3}
5719 uses 51K bytes when compiled. When PCRE is compiled with its default
5720 internal pointer size of two bytes, the size limit on a compiled pat-
5721 tern is 64K, and this is reached with the above pattern if the outer
5722 repetition is increased from 3 to 4. PCRE can be compiled to use larger
5723 internal pointers and thus handle larger compiled patterns, but it is
5724 better to try to rewrite your pattern to use less memory if you can.
5726 One way of reducing the memory usage for such patterns is to make use
5727 of PCRE's "subroutine" facility. Re-writing the above pattern as
5729 ((ab)(?2){0,999}c)(?1){0,2}
5731 reduces the memory requirements to 18K, and indeed it remains under 20K
5732 even with the outer repetition increased to 100. However, this pattern
5733 is not exactly equivalent, because the "subroutine" calls are treated
5734 as atomic groups into which there can be no backtracking if there is a
5735 subsequent matching failure. Therefore, PCRE cannot do this kind of
5736 rewriting automatically. Furthermore, there is a noticeable loss of
5737 speed when executing the modified pattern. Nevertheless, if the atomic
5738 grouping is not a problem and the loss of speed is acceptable, this
5739 kind of rewriting will allow you to process patterns that PCRE cannot
5745 Certain items in regular expression patterns are processed more effi-
5746 ciently than others. It is more efficient to use a character class like
5747 [aeiou] than a set of single-character alternatives such as
5748 (a|e|i|o|u). In general, the simplest construction that provides the
5749 required behaviour is usually the most efficient. Jeffrey Friedl's book
5750 contains a lot of useful general discussion about optimizing regular
5751 expressions for efficient performance. This document contains a few
5752 observations about PCRE.
5754 Using Unicode character properties (the \p, \P, and \X escapes) is
5755 slow, because PCRE has to scan a structure that contains data for over
5756 fifteen thousand characters whenever it needs a character's property.
5757 If you can find an alternative pattern that does not use character
5758 properties, it will probably be faster.
5760 When a pattern begins with .* not in parentheses, or in parentheses
5761 that are not the subject of a backreference, and the PCRE_DOTALL option
5762 is set, the pattern is implicitly anchored by PCRE, since it can match
5763 only at the start of a subject string. However, if PCRE_DOTALL is not
5764 set, PCRE cannot make this optimization, because the . metacharacter
5765 does not then match a newline, and if the subject string contains new-
5766 lines, the pattern may match from the character immediately following
5767 one of them instead of from the very start. For example, the pattern
5771 matches the subject "first\nand second" (where \n stands for a newline
5772 character), with the match starting at the seventh character. In order
5773 to do this, PCRE has to retry the match starting after every newline in
5776 If you are using such a pattern with subject strings that do not con-
5777 tain newlines, the best performance is obtained by setting PCRE_DOTALL,
5778 or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
5779 ing. That saves PCRE from having to scan along the subject looking for
5780 a newline to restart at.
5782 Beware of patterns that contain nested indefinite repeats. These can
5783 take a long time to run when applied to a string that does not match.
5784 Consider the pattern fragment
5788 This can match "aaaa" in 16 different ways, and this number increases
5789 very rapidly as the string gets longer. (The * repeat can match 0, 1,
5790 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
5791 repeats can match different numbers of times.) When the remainder of
5792 the pattern is such that the entire match is going to fail, PCRE has in
5793 principle to try every possible variation, and this can take an
5794 extremely long time, even for relatively short strings.
5796 An optimization catches some of the more simple cases such as
5800 where a literal character follows. Before embarking on the standard
5801 matching procedure, PCRE checks that there is a "b" later in the sub-
5802 ject string, and if there is not, it fails the match immediately. How-
5803 ever, when there is no following literal this optimization cannot be
5804 used. You can see the difference by comparing the behaviour of
5808 with the pattern above. The former gives a failure almost instantly
5809 when applied to a whole line of "a" characters, whereas the latter
5810 takes an appreciable time with strings longer than about 20 characters.
5812 In many cases, the solution to this kind of performance issue is to use
5813 an atomic group or a possessive quantifier.
5819 University Computing Service
5820 Cambridge CB2 3QH, England.
5825 Last updated: 06 March 2007
5826 Copyright (c) 1997-2007 University of Cambridge.
5827 ------------------------------------------------------------------------------
5830 PCREPOSIX(3) PCREPOSIX(3)
5834 PCRE - Perl-compatible regular expressions.
5837 SYNOPSIS OF POSIX API
5839 #include <pcreposix.h>
5841 int regcomp(regex_t *preg, const char *pattern,
5844 int regexec(regex_t *preg, const char *string,
5845 size_t nmatch, regmatch_t pmatch[], int eflags);
5847 size_t regerror(int errcode, const regex_t *preg,
5848 char *errbuf, size_t errbuf_size);
5850 void regfree(regex_t *preg);
5855 This set of functions provides a POSIX-style API to the PCRE regular
5856 expression package. See the pcreapi documentation for a description of
5857 PCRE's native API, which contains much additional functionality.
5859 The functions described here are just wrapper functions that ultimately
5860 call the PCRE native API. Their prototypes are defined in the
5861 pcreposix.h header file, and on Unix systems the library itself is
5862 called pcreposix.a, so can be accessed by adding -lpcreposix to the
5863 command for linking an application that uses them. Because the POSIX
5864 functions call the native ones, it is also necessary to add -lpcre.
5866 I have implemented only those option bits that can be reasonably mapped
5867 to PCRE native options. In addition, the option REG_EXTENDED is defined
5868 with the value zero. This has no effect, but since programs that are
5869 written to the POSIX interface often use it, this makes it easier to
5870 slot in PCRE as a replacement library. Other POSIX options are not even
5873 When PCRE is called via these functions, it is only the API that is
5874 POSIX-like in style. The syntax and semantics of the regular expres-
5875 sions themselves are still those of Perl, subject to the setting of
5876 various PCRE options, as described below. "POSIX-like in style" means
5877 that the API approximates to the POSIX definition; it is not fully
5878 POSIX-compatible, and in multi-byte encoding domains it is probably
5879 even less compatible.
5881 The header for these functions is supplied as pcreposix.h to avoid any
5882 potential clash with other POSIX libraries. It can, of course, be
5883 renamed or aliased as regex.h, which is the "correct" name. It provides
5884 two structure types, regex_t for compiled internal forms, and reg-
5885 match_t for returning captured substrings. It also defines some con-
5886 stants whose names start with "REG_"; these are used for setting
5887 options and identifying error codes.
5892 The function regcomp() is called to compile a pattern into an internal
5893 form. The pattern is a C string terminated by a binary zero, and is
5894 passed in the argument pattern. The preg argument is a pointer to a
5895 regex_t structure that is used as a base for storing information about
5896 the compiled regular expression.
5898 The argument cflags is either zero, or contains one or more of the bits
5899 defined by the following macros:
5903 The PCRE_DOTALL option is set when the regular expression is passed for
5904 compilation to the native function. Note that REG_DOTALL is not part of
5909 The PCRE_CASELESS option is set when the regular expression is passed
5910 for compilation to the native function.
5914 The PCRE_MULTILINE option is set when the regular expression is passed
5915 for compilation to the native function. Note that this does not mimic
5916 the defined POSIX behaviour for REG_NEWLINE (see the following sec-
5921 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
5922 passed for compilation to the native function. In addition, when a pat-
5923 tern that is compiled with this flag is passed to regexec() for match-
5924 ing, the nmatch and pmatch arguments are ignored, and no captured
5925 strings are returned.
5929 The PCRE_UTF8 option is set when the regular expression is passed for
5930 compilation to the native function. This causes the pattern itself and
5931 all data strings used for matching it to be treated as UTF-8 strings.
5932 Note that REG_UTF8 is not part of the POSIX standard.
5934 In the absence of these flags, no options are passed to the native
5935 function. This means the the regex is compiled with PCRE default
5936 semantics. In particular, the way it handles newline characters in the
5937 subject string is the Perl way, not the POSIX way. Note that setting
5938 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
5939 It does not affect the way newlines are matched by . (they aren't) or
5940 by a negative class such as [^a] (they are).
5942 The yield of regcomp() is zero on success, and non-zero otherwise. The
5943 preg structure is filled in on success, and one member of the structure
5944 is public: re_nsub contains the number of capturing subpatterns in the
5945 regular expression. Various error codes are defined in the header file.
5948 MATCHING NEWLINE CHARACTERS
5950 This area is not simple, because POSIX and Perl take different views of
5951 things. It is not possible to get PCRE to obey POSIX semantics, but
5952 then PCRE was never intended to be a POSIX engine. The following table
5953 lists the different possibilities for matching newline characters in
5958 . matches newline no PCRE_DOTALL
5959 newline matches [^a] yes not changeable
5960 $ matches \n at end yes PCRE_DOLLARENDONLY
5961 $ matches \n in middle no PCRE_MULTILINE
5962 ^ matches \n in middle no PCRE_MULTILINE
5964 This is the equivalent table for POSIX:
5968 . matches newline yes REG_NEWLINE
5969 newline matches [^a] yes REG_NEWLINE
5970 $ matches \n at end no REG_NEWLINE
5971 $ matches \n in middle no REG_NEWLINE
5972 ^ matches \n in middle no REG_NEWLINE
5974 PCRE's behaviour is the same as Perl's, except that there is no equiva-
5975 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
5976 no way to stop newline from matching [^a].
5978 The default POSIX newline handling can be obtained by setting
5979 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
5980 behave exactly as for the REG_NEWLINE action.
5985 The function regexec() is called to match a compiled pattern preg
5986 against a given string, which is by default terminated by a zero byte
5987 (but see REG_STARTEND below), subject to the options in eflags. These
5992 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
5997 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
6002 The string is considered to start at string + pmatch[0].rm_so and to
6003 have a terminating NUL located at string + pmatch[0].rm_eo (there need
6004 not actually be a NUL at that location), regardless of the value of
6005 nmatch. This is a BSD extension, compatible with but not specified by
6006 IEEE Standard 1003.2 (POSIX.2), and should be used with caution in
6007 software intended to be portable to other systems. Note that a non-zero
6008 rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
6009 of the string, not how it is matched.
6011 If the pattern was compiled with the REG_NOSUB flag, no data about any
6012 matched strings is returned. The nmatch and pmatch arguments of
6013 regexec() are ignored.
6015 Otherwise,the portion of the string that was matched, and also any cap-
6016 tured substrings, are returned via the pmatch argument, which points to
6017 an array of nmatch structures of type regmatch_t, containing the mem-
6018 bers rm_so and rm_eo. These contain the offset to the first character
6019 of each substring and the offset to the first character after the end
6020 of each substring, respectively. The 0th element of the vector relates
6021 to the entire portion of string that was matched; subsequent elements
6022 relate to the capturing subpatterns of the regular expression. Unused
6023 entries in the array have both structure members set to -1.
6025 A successful match yields a zero return; various error codes are
6026 defined in the header file, of which REG_NOMATCH is the "expected"
6032 The regerror() function maps a non-zero errorcode from either regcomp()
6033 or regexec() to a printable message. If preg is not NULL, the error
6034 should have arisen from the use of that structure. A message terminated
6035 by a binary zero is placed in errbuf. The length of the message,
6036 including the zero, is limited to errbuf_size. The yield of the func-
6037 tion is the size of buffer needed to hold the whole message.
6042 Compiling a regular expression causes memory to be allocated and asso-
6043 ciated with the preg structure. The function regfree() frees all such
6044 memory, after which preg may no longer be used as a compiled expres-
6051 University Computing Service
6052 Cambridge CB2 3QH, England.
6057 Last updated: 05 April 2008
6058 Copyright (c) 1997-2008 University of Cambridge.
6059 ------------------------------------------------------------------------------
6062 PCRECPP(3) PCRECPP(3)
6066 PCRE - Perl-compatible regular expressions.
6069 SYNOPSIS OF C++ WRAPPER
6071 #include <pcrecpp.h>
6076 The C++ wrapper for PCRE was provided by Google Inc. Some additional
6077 functionality was added by Giuseppe Maxia. This brief man page was con-
6078 structed from the notes in the pcrecpp.h file, which should be con-
6079 sulted for further details.
6084 The "FullMatch" operation checks that supplied text matches a supplied
6085 pattern exactly. If pointer arguments are supplied, it copies matched
6086 sub-strings that match sub-patterns into them.
6088 Example: successful match
6089 pcrecpp::RE re("h.*o");
6090 re.FullMatch("hello");
6092 Example: unsuccessful match (requires full match):
6093 pcrecpp::RE re("e");
6094 !re.FullMatch("hello");
6096 Example: creating a temporary RE object:
6097 pcrecpp::RE("h.*o").FullMatch("hello");
6099 You can pass in a "const char*" or a "string" for "text". The examples
6100 below tend to use a const char*. You can, as in the different examples
6101 above, store the RE object explicitly in a variable or use a temporary
6102 RE object. The examples below use one mode or the other arbitrarily.
6103 Either could correctly be used for any of these examples.
6105 You must supply extra pointer arguments to extract matched subpieces.
6107 Example: extracts "ruby" into "s" and 1234 into "i"
6110 pcrecpp::RE re("(\\w+):(\\d+)");
6111 re.FullMatch("ruby:1234", &s, &i);
6113 Example: does not try to extract any extra sub-patterns
6114 re.FullMatch("ruby:1234", &s);
6116 Example: does not try to extract into NULL
6117 re.FullMatch("ruby:1234", NULL, &i);
6119 Example: integer overflow causes failure
6120 !re.FullMatch("ruby:1234567891234", NULL, &i);
6122 Example: fails because there aren't enough sub-patterns:
6123 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
6125 Example: fails because string cannot be stored in integer
6126 !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
6128 The provided pointer arguments can be pointers to any scalar numeric
6131 string (matched piece is copied to string)
6132 StringPiece (StringPiece is mutated to point to matched piece)
6133 T (where "bool T::ParseFrom(const char*, int)" exists)
6134 NULL (the corresponding matched sub-pattern is not copied)
6136 The function returns true iff all of the following conditions are sat-
6139 a. "text" matches "pattern" exactly;
6141 b. The number of matched sub-patterns is >= number of supplied
6144 c. The "i"th argument has a suitable type for holding the
6145 string captured as the "i"th sub-pattern. If you pass in
6146 void * NULL for the "i"th argument, or a non-void * NULL
6147 of the correct type, or pass fewer arguments than the
6148 number of sub-patterns, "i"th captured sub-pattern is
6151 CAVEAT: An optional sub-pattern that does not exist in the matched
6152 string is assigned the empty string. Therefore, the following will
6153 return false (because the empty string is not a valid number):
6156 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
6158 The matching interface supports at most 16 arguments per call. If you
6159 need more, consider using the more general interface
6160 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
6163 QUOTING METACHARACTERS
6165 You can use the "QuoteMeta" operation to insert backslashes before all
6166 potentially meaningful characters in a string. The returned string,
6167 used as a regular expression, will exactly match the original string.
6170 string quoted = RE::QuoteMeta(unquoted);
6172 Note that it's legal to escape a character even if it has no special
6173 meaning in a regular expression -- so this function does that. (This
6174 also makes it identical to the perl function of the same name; see
6175 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
6181 You can use the "PartialMatch" operation when you want the pattern to
6182 match any substring of the text.
6184 Example: simple search for a string:
6185 pcrecpp::RE("ell").PartialMatch("hello");
6187 Example: find first number in a string:
6189 pcrecpp::RE re("(\\d+)");
6190 re.PartialMatch("x*100 + 20", &number);
6191 assert(number == 100);
6194 UTF-8 AND THE MATCHING INTERFACE
6196 By default, pattern and text are plain text, one byte per character.
6197 The UTF8 flag, passed to the constructor, causes both pattern and
6198 string to be treated as UTF-8 text, still a byte stream but potentially
6199 multiple bytes per character. In practice, the text is likelier to be
6200 UTF-8 than the pattern, but the match returned may depend on the UTF8
6201 flag, so always use it when matching UTF8 text. For example, "." will
6202 match one byte normally but with UTF8 set may match up to three bytes
6203 of a multi-byte character.
6206 pcrecpp::RE_Options options;
6208 pcrecpp::RE re(utf8_pattern, options);
6209 re.FullMatch(utf8_string);
6211 Example: using the convenience function UTF8():
6212 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
6213 re.FullMatch(utf8_string);
6215 NOTE: The UTF8 flag is ignored if pcre was not configured with the
6219 PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
6221 PCRE defines some modifiers to change the behavior of the regular
6222 expression engine. The C++ wrapper defines an auxiliary class,
6223 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
6224 rently, the following modifiers are supported:
6226 modifier description Perl corresponding
6228 PCRE_CASELESS case insensitive match /i
6229 PCRE_MULTILINE multiple lines match /m
6230 PCRE_DOTALL dot matches newlines /s
6231 PCRE_DOLLAR_ENDONLY $ matches only at end N/A
6232 PCRE_EXTRA strict escape parsing N/A
6233 PCRE_EXTENDED ignore whitespaces /x
6234 PCRE_UTF8 handles UTF8 chars built-in
6235 PCRE_UNGREEDY reverses * and *? N/A
6236 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
6238 (*) Both Perl and PCRE allow non capturing parentheses by means of the
6239 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
6240 ture, while (ab|cd) does.
6242 For a full account on how each modifier works, please check the PCRE
6245 For each modifier, there are two member functions whose name is made
6246 out of the modifier in lowercase, without the "PCRE_" prefix. For
6247 instance, PCRE_CASELESS is handled by
6251 which returns true if the modifier is set, and
6253 RE_Options & set_caseless(bool)
6255 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
6256 be accessed through the set_match_limit() and match_limit() member
6257 functions. Setting match_limit to a non-zero value will limit the exe-
6258 cution of pcre to keep it from doing bad things like blowing the stack
6259 or taking an eternity to return a result. A value of 5000 is good
6260 enough to stop stack blowup in a 2MB thread stack. Setting match_limit
6261 to zero disables match limiting. Alternatively, you can call
6262 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
6263 limit how much PCRE recurses. match_limit() limits the number of
6264 matches PCRE does; match_limit_recursion() limits the depth of internal
6265 recursion, and therefore the amount of stack that is used.
6267 Normally, to pass one or more modifiers to a RE class, you declare a
6268 RE_Options object, set the appropriate options, and pass this object to
6269 a RE constructor. Example:
6272 opt.set_caseless(true);
6273 if (RE("HELLO", opt).PartialMatch("hello world")) ...
6275 RE_options has two constructors. The default constructor takes no argu-
6276 ments and creates a set of flags that are off by default. The optional
6277 parameter option_flags is to facilitate transfer of legacy code from C
6278 programs. This lets you do
6281 RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);
6283 However, new code is better off doing
6286 RE_Options().set_caseless(true).set_multiline(true))
6289 If you are going to pass one of the most used modifiers, there are some
6290 convenience functions that return a RE_Options class with the appropri-
6291 ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
6294 If you need to set several options at once, and you don't want to go
6295 through the pains of declaring a RE_Options object and setting several
6296 options, there is a parallel method that give you such ability on the
6297 fly. You can concatenate several set_xxxxx() member functions, since
6298 each of them returns a reference to its class object. For example, to
6299 pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
6300 statement, you may write:
6302 RE(" ^ xyz \\s+ .* blah$",
6306 .set_multiline(true)).PartialMatch(sometext);
6309 SCANNING TEXT INCREMENTALLY
6311 The "Consume" operation may be useful if you want to repeatedly match
6312 regular expressions at the front of a string and skip over them as they
6313 match. This requires use of the "StringPiece" type, which represents a
6314 sub-range of a real string. Like RE, StringPiece is defined in the
6317 Example: read lines of the form "var = value" from a string.
6318 string contents = ...; // Fill string somehow
6319 pcrecpp::StringPiece input(contents); // Wrap in a StringPiece
6323 pcrecpp::RE re("(\\w+) = (\\d+)\n");
6324 while (re.Consume(&input, &var, &value)) {
6328 Each successful call to "Consume" will set "var/value", and also
6329 advance "input" so it points past the matched text.
6331 The "FindAndConsume" operation is similar to "Consume" but does not
6332 anchor your match at the beginning of the string. For example, you
6333 could extract all words from a string by repeatedly calling
6335 pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)
6338 PARSING HEX/OCTAL/C-RADIX NUMBERS
6340 By default, if you pass a pointer to a numeric value, the corresponding
6341 text is interpreted as a base-10 number. You can instead wrap the
6342 pointer with a call to one of the operators Hex(), Octal(), or CRadix()
6343 to interpret the text in another base. The CRadix operator interprets
6344 C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to
6349 pcrecpp::RE re("(.*) (.*) (.*) (.*)");
6350 re.FullMatch("100 40 0100 0x40",
6351 pcrecpp::Octal(&a), pcrecpp::Hex(&b),
6352 pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));
6354 will leave 64 in a, b, c, and d.
6357 REPLACING PARTS OF STRINGS
6359 You can replace the first match of "pattern" in "str" with "rewrite".
6360 Within "rewrite", backslash-escaped digits (\1 to \9) can be used to
6361 insert text matching corresponding parenthesized group from the pat-
6362 tern. \0 in "rewrite" refers to the entire matching text. For example:
6364 string s = "yabba dabba doo";
6365 pcrecpp::RE("b+").Replace("d", &s);
6367 will leave "s" containing "yada dabba doo". The result is true if the
6368 pattern matches and a replacement occurs, false otherwise.
6370 GlobalReplace is like Replace except that it replaces all occurrences
6371 of the pattern in the string with the rewrite. Replacements are not
6372 subject to re-matching. For example:
6374 string s = "yabba dabba doo";
6375 pcrecpp::RE("b+").GlobalReplace("d", &s);
6377 will leave "s" containing "yada dada doo". It returns the number of
6380 Extract is like Replace, except that if the pattern matches, "rewrite"
6381 is copied into "out" (an additional argument) with substitutions. The
6382 non-matching portions of "text" are ignored. Returns true iff a match
6383 occurred and the extraction happened successfully; if no match occurs,
6384 the string is left unaffected.
6389 The C++ wrapper was contributed by Google Inc.
6390 Copyright (c) 2007 Google Inc.
6395 Last updated: 12 November 2007
6396 ------------------------------------------------------------------------------
6399 PCRESAMPLE(3) PCRESAMPLE(3)
6403 PCRE - Perl-compatible regular expressions
6408 A simple, complete demonstration program, to get you started with using
6409 PCRE, is supplied in the file pcredemo.c in the PCRE distribution.
6411 The program compiles the regular expression that is its first argument,
6412 and matches it against the subject string in its second argument. No
6413 PCRE options are set, and default character tables are used. If match-
6414 ing succeeds, the program outputs the portion of the subject that
6415 matched, together with the contents of any captured substrings.
6417 If the -g option is given on the command line, the program then goes on
6418 to check for further matches of the same regular expression in the same
6419 subject string. The logic is a little bit tricky because of the possi-
6420 bility of matching an empty string. Comments in the code explain what
6423 If PCRE is installed in the standard include and library directories
6424 for your system, you should be able to compile the demonstration pro-
6425 gram using this command:
6427 gcc -o pcredemo pcredemo.c -lpcre
6429 If PCRE is installed elsewhere, you may need to add additional options
6430 to the command line. For example, on a Unix-like system that has PCRE
6431 installed in /usr/local, you can compile the demonstration program
6432 using a command like this:
6434 gcc -o pcredemo -I/usr/local/include pcredemo.c \
6435 -L/usr/local/lib -lpcre
6437 Once you have compiled the demonstration program, you can run simple
6440 ./pcredemo 'cat|dog' 'the cat sat on the mat'
6441 ./pcredemo -g 'cat|dog' 'the dog sat on the cat'
6443 Note that there is a much more comprehensive test program, called
6444 pcretest, which supports many more facilities for testing regular
6445 expressions and the PCRE library. The pcredemo program is provided as a
6446 simple coding example.
6448 On some operating systems (e.g. Solaris), when PCRE is not installed in
6449 the standard library directory, you may get an error like this when you
6450 try to run pcredemo:
6452 ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or
6455 This is caused by the way shared library support works on those sys-
6456 tems. You need to add
6460 (for example) to the compile command to get round this problem.
6466 University Computing Service
6467 Cambridge CB2 3QH, England.
6472 Last updated: 23 January 2008
6473 Copyright (c) 1997-2008 University of Cambridge.
6474 ------------------------------------------------------------------------------
6475 PCRESTACK(3) PCRESTACK(3)
6479 PCRE - Perl-compatible regular expressions
6482 PCRE DISCUSSION OF STACK USAGE
6484 When you call pcre_exec(), it makes use of an internal function called
6485 match(). This calls itself recursively at branch points in the pattern,
6486 in order to remember the state of the match so that it can back up and
6487 try a different alternative if the first one fails. As matching pro-
6488 ceeds deeper and deeper into the tree of possibilities, the recursion
6491 Not all calls of match() increase the recursion depth; for an item such
6492 as a* it may be called several times at the same level, after matching
6493 different numbers of a's. Furthermore, in a number of cases where the
6494 result of the recursive call would immediately be passed back as the
6495 result of the current call (a "tail recursion"), the function is just
6498 The pcre_dfa_exec() function operates in an entirely different way, and
6499 hardly uses recursion at all. The limit on its complexity is the amount
6500 of workspace it is given. The comments that follow do NOT apply to
6501 pcre_dfa_exec(); they are relevant only for pcre_exec().
6503 You can set limits on the number of times that match() is called, both
6504 in total and recursively. If the limit is exceeded, an error occurs.
6505 For details, see the section on extra data for pcre_exec() in the
6506 pcreapi documentation.
6508 Each time that match() is actually called recursively, it uses memory
6509 from the process stack. For certain kinds of pattern and data, very
6510 large amounts of stack may be needed, despite the recognition of "tail
6511 recursion". You can often reduce the amount of recursion, and there-
6512 fore the amount of stack used, by modifying the pattern that is being
6513 matched. Consider, for example, this pattern:
6517 It matches from wherever it starts until it encounters "<inet" or the
6518 end of the data, and is the kind of pattern that might be used when
6519 processing an XML file. Each iteration of the outer parentheses matches
6520 either one character that is not "<" or a "<" that is not followed by
6521 "inet". However, each time a parenthesis is processed, a recursion
6522 occurs, so this formulation uses a stack frame for each matched charac-
6523 ter. For a long string, a lot of stack is required. Consider now this
6524 rewritten pattern, which matches exactly the same strings:
6528 This uses very much less stack, because runs of characters that do not
6529 contain "<" are "swallowed" in one item inside the parentheses. Recur-
6530 sion happens only when a "<" character that is not followed by "inet"
6531 is encountered (and we assume this is relatively rare). A possessive
6532 quantifier is used to stop any backtracking into the runs of non-"<"
6533 characters, but that is not related to stack usage.
6535 This example shows that one way of avoiding stack problems when match-
6536 ing long subject strings is to write repeated parenthesized subpatterns
6537 to match more than one character whenever possible.
6539 In environments where stack memory is constrained, you might want to
6540 compile PCRE to use heap memory instead of stack for remembering back-
6541 up points. This makes it run a lot more slowly, however. Details of how
6542 to do this are given in the pcrebuild documentation. When built in this
6543 way, instead of using the stack, PCRE obtains and frees memory by call-
6544 ing the functions that are pointed to by the pcre_stack_malloc and
6545 pcre_stack_free variables. By default, these point to malloc() and
6546 free(), but you can replace the pointers to cause PCRE to use your own
6547 functions. Since the block sizes are always the same, and are always
6548 freed in reverse order, it may be possible to implement customized mem-
6549 ory handlers that are more efficient than the standard functions.
6551 In Unix-like environments, there is not often a problem with the stack
6552 unless very long strings are involved, though the default limit on
6553 stack size varies from system to system. Values from 8Mb to 64Mb are
6554 common. You can find your default limit by running the command:
6558 Unfortunately, the effect of running out of stack is often SIGSEGV,
6559 though sometimes a more explicit error message is given. You can nor-
6560 mally increase the limit on stack size by code such as this:
6563 getrlimit(RLIMIT_STACK, &rlim);
6564 rlim.rlim_cur = 100*1024*1024;
6565 setrlimit(RLIMIT_STACK, &rlim);
6567 This reads the current limits (soft and hard) using getrlimit(), then
6568 attempts to increase the soft limit to 100Mb using setrlimit(). You
6569 must do this before calling pcre_exec().
6571 PCRE has an internal counter that can be used to limit the depth of
6572 recursion, and thus cause pcre_exec() to give an error code before it
6573 runs out of stack. By default, the limit is very large, and unlikely
6574 ever to operate. It can be changed when PCRE is built, and it can also
6575 be set when pcre_exec() is called. For details of these interfaces, see
6576 the pcrebuild and pcreapi documentation.
6578 As a very rough rule of thumb, you should reckon on about 500 bytes per
6579 recursion. Thus, if you want to limit your stack usage to 8Mb, you
6580 should set the limit at 16000 recursions. A 64Mb stack, on the other
6581 hand, can support around 128000 recursions. The pcretest test program
6582 has a command line option (-S) that can be used to increase the size of
6589 University Computing Service
6590 Cambridge CB2 3QH, England.
6595 Last updated: 05 June 2007
6596 Copyright (c) 1997-2007 University of Cambridge.
6597 ------------------------------------------------------------------------------
projects / framework / web / pcre.git / blob