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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_regex.syntax.basic_syntax"></a><a class="link" href="basic_syntax.html" title="POSIX Basic Regular Expression Syntax">POSIX Basic Regular
  28       Expression Syntax</a>
  29 </h3></div></div></div>
  30 <h4>
  31 <a name="boost_regex.syntax.basic_syntax.h0"></a>
  32         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.synopsis"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.synopsis">Synopsis</a>
  33       </h4>
  34 <p>
  35         The POSIX-Basic regular expression syntax is used by the Unix utility <code class="computeroutput"><span class="identifier">sed</span></code>, and variations are used by <code class="computeroutput"><span class="identifier">grep</span></code> and <code class="computeroutput"><span class="identifier">emacs</span></code>.
  36         You can construct POSIX basic regular expressions in Boost.Regex by passing
  37         the flag <code class="computeroutput"><span class="identifier">basic</span></code> to the regex
  38         constructor (see <a class="link" href="../ref/syntax_option_type.html" title="syntax_option_type"><code class="computeroutput"><span class="identifier">syntax_option_type</span></code></a>), for example:
  39       </p>
  40 <pre class="programlisting"><span class="comment">// e1 is a case sensitive POSIX-Basic expression:</span>
  41 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e1</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">basic</span><span class="special">);</span>
  42 <span class="comment">// e2 a case insensitive POSIX-Basic expression:</span>
  43 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e2</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">basic</span><span class="special">|</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">icase</span><span class="special">);</span>
  44 </pre>
  45 <a name="boost_regex.posix_basic"></a><h4>
  46 <a name="boost_regex.syntax.basic_syntax.h1"></a>
  47         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.posix_basic_syntax"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.posix_basic_syntax">POSIX
  48         Basic Syntax</a>
  49       </h4>
  50 <p>
  51         In POSIX-Basic regular expressions, all characters are match themselves except
  52         for the following special characters:
  53       </p>
  54 <pre class="programlisting">.[\*^$</pre>
  55 <h5>
  56 <a name="boost_regex.syntax.basic_syntax.h2"></a>
  57         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.wildcard"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.wildcard">Wildcard:</a>
  58       </h5>
  59 <p>
  60         The single character '.' when used outside of a character set will match
  61         any single character except:
  62       </p>
  63 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  64 <li class="listitem">
  65             The NULL character when the flag <code class="computeroutput"><span class="identifier">match_no_dot_null</span></code>
  66             is passed to the matching algorithms.
  67           </li>
  68 <li class="listitem">
  69             The newline character when the flag <code class="computeroutput"><span class="identifier">match_not_dot_newline</span></code>
  70             is passed to the matching algorithms.
  71           </li>
  72 </ul></div>
  73 <h5>
  74 <a name="boost_regex.syntax.basic_syntax.h3"></a>
  75         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.anchors"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.anchors">Anchors:</a>
  76       </h5>
  77 <p>
  78         A '^' character shall match the start of a line when used as the first character
  79         of an expression, or the first character of a sub-expression.
  80       </p>
  81 <p>
  82         A '$' character shall match the end of a line when used as the last character
  83         of an expression, or the last character of a sub-expression.
  84       </p>
  85 <h5>
  86 <a name="boost_regex.syntax.basic_syntax.h4"></a>
  87         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.marked_sub_expressions"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.marked_sub_expressions">Marked sub-expressions:</a>
  88       </h5>
  89 <p>
  90         A section beginning <code class="computeroutput"><span class="special">\(</span></code> and ending
  91         <code class="computeroutput"><span class="special">\)</span></code> acts as a marked sub-expression.
  92         Whatever matched the sub-expression is split out in a separate field by the
  93         matching algorithms. Marked sub-expressions can also repeated, or referred-to
  94         by a back-reference.
  95       </p>
  96 <h5>
  97 <a name="boost_regex.syntax.basic_syntax.h5"></a>
  98         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.repeats"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.repeats">Repeats:</a>
  99       </h5>
 100 <p>
 101         Any atom (a single character, a marked sub-expression, or a character class)
 102         can be repeated with the * operator.
 103       </p>
 104 <p>
 105         For example <code class="computeroutput"><span class="identifier">a</span><span class="special">*</span></code>
 106         will match any number of letter a's repeated zero or more times (an atom
 107         repeated zero times matches an empty string), so the expression <code class="computeroutput"><span class="identifier">a</span><span class="special">*</span><span class="identifier">b</span></code>
 108         will match any of the following:
 109       </p>
 110 <pre class="programlisting">b
 111 ab
 112 aaaaaaaab
 113 </pre>
 114 <p>
 115         An atom can also be repeated with a bounded repeat:
 116       </p>
 117 <p>
 118         <code class="computeroutput"><span class="identifier">a</span><span class="special">\{</span><span class="identifier">n</span><span class="special">\}</span></code> Matches
 119         'a' repeated exactly n times.
 120       </p>
 121 <p>
 122         <code class="computeroutput"><span class="identifier">a</span><span class="special">\{</span><span class="identifier">n</span><span class="special">,\}</span></code> Matches
 123         'a' repeated n or more times.
 124       </p>
 125 <p>
 126         <code class="computeroutput"><span class="identifier">a</span><span class="special">\{</span><span class="identifier">n</span><span class="special">,</span> <span class="identifier">m</span><span class="special">\}</span></code> Matches 'a' repeated between n and m times
 127         inclusive.
 128       </p>
 129 <p>
 130         For example:
 131       </p>
 132 <pre class="programlisting">^a{2,3}$</pre>
 133 <p>
 134         Will match either of:
 135       </p>
 136 <pre class="programlisting">aa
 137 aaa
 138 </pre>
 139 <p>
 140         But neither of:
 141       </p>
 142 <pre class="programlisting">a
 143 aaaa
 144 </pre>
 145 <p>
 146         It is an error to use a repeat operator, if the preceding construct can not
 147         be repeated, for example:
 148       </p>
 149 <pre class="programlisting">a(*)</pre>
 150 <p>
 151         Will raise an error, as there is nothing for the * operator to be applied
 152         to.
 153       </p>
 154 <h5>
 155 <a name="boost_regex.syntax.basic_syntax.h6"></a>
 156         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.back_references"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.back_references">Back
 157         references:</a>
 158       </h5>
 159 <p>
 160         An escape character followed by a digit <span class="emphasis"><em>n</em></span>, where <span class="emphasis"><em>n</em></span>
 161         is in the range 1-9, matches the same string that was matched by sub-expression
 162         <span class="emphasis"><em>n</em></span>. For example the expression:
 163       </p>
 164 <pre class="programlisting">^\(a*\)[^a]*\1$</pre>
 165 <p>
 166         Will match the string:
 167       </p>
 168 <pre class="programlisting">aaabbaaa</pre>
 169 <p>
 170         But not the string:
 171       </p>
 172 <pre class="programlisting">aaabba</pre>
 173 <h5>
 174 <a name="boost_regex.syntax.basic_syntax.h7"></a>
 175         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.character_sets"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.character_sets">Character
 176         sets:</a>
 177       </h5>
 178 <p>
 179         A character set is a bracket-expression starting with [ and ending with ],
 180         it defines a set of characters, and matches any single character that is
 181         a member of that set.
 182       </p>
 183 <p>
 184         A bracket expression may contain any combination of the following:
 185       </p>
 186 <h6>
 187 <a name="boost_regex.syntax.basic_syntax.h8"></a>
 188         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.single_characters"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.single_characters">Single
 189         characters:</a>
 190       </h6>
 191 <p>
 192         For example <code class="computeroutput"><span class="special">[</span><span class="identifier">abc</span><span class="special">]</span></code>, will match any of the characters 'a', 'b',
 193         or 'c'.
 194       </p>
 195 <h6>
 196 <a name="boost_regex.syntax.basic_syntax.h9"></a>
 197         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.character_ranges"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.character_ranges">Character
 198         ranges:</a>
 199       </h6>
 200 <p>
 201         For example <code class="computeroutput"><span class="special">[</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">]</span></code>
 202         will match any single character in the range 'a' to 'c'. By default, for
 203         POSIX-Basic regular expressions, a character <span class="emphasis"><em>x</em></span> is within
 204         the range <span class="emphasis"><em>y</em></span> to <span class="emphasis"><em>z</em></span>, if it collates
 205         within that range; this results in locale specific behavior. This behavior
 206         can be turned off by unsetting the <code class="computeroutput"><span class="identifier">collate</span></code>
 207         option flag when constructing the regular expression - in which case whether
 208         a character appears within a range is determined by comparing the code points
 209         of the characters only.
 210       </p>
 211 <h6>
 212 <a name="boost_regex.syntax.basic_syntax.h10"></a>
 213         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.negation"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.negation">Negation:</a>
 214       </h6>
 215 <p>
 216         If the bracket-expression begins with the ^ character, then it matches the
 217         complement of the characters it contains, for example <code class="computeroutput"><span class="special">[^</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">]</span></code> matches any character that is not in the
 218         range a-c.
 219       </p>
 220 <h6>
 221 <a name="boost_regex.syntax.basic_syntax.h11"></a>
 222         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.character_classes"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.character_classes">Character
 223         classes:</a>
 224       </h6>
 225 <p>
 226         An expression of the form <code class="computeroutput"><span class="special">[[:</span><span class="identifier">name</span><span class="special">:]]</span></code>
 227         matches the named character class "name", for example <code class="computeroutput"><span class="special">[[:</span><span class="identifier">lower</span><span class="special">:]]</span></code> matches any lower case character. See
 228         <a class="link" href="character_classes.html" title="Character Class Names">character class names</a>.
 229       </p>
 230 <h6>
 231 <a name="boost_regex.syntax.basic_syntax.h12"></a>
 232         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.collating_elements"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.collating_elements">Collating
 233         Elements:</a>
 234       </h6>
 235 <p>
 236         An expression of the form <code class="computeroutput"><span class="special">[[.</span><span class="identifier">col</span><span class="special">.]</span></code> matches
 237         the collating element <span class="emphasis"><em>col</em></span>. A collating element is any
 238         single character, or any sequence of characters that collates as a single
 239         unit. Collating elements may also be used as the end point of a range, for
 240         example: <code class="computeroutput"><span class="special">[[.</span><span class="identifier">ae</span><span class="special">.]-</span><span class="identifier">c</span><span class="special">]</span></code>
 241         matches the character sequence "ae", plus any single character
 242         in the range "ae"-c, assuming that "ae" is treated as
 243         a single collating element in the current locale.
 244       </p>
 245 <p>
 246         Collating elements may be used in place of escapes (which are not normally
 247         allowed inside character sets), for example <code class="computeroutput"><span class="special">[[.^.]</span><span class="identifier">abc</span><span class="special">]</span></code> would
 248         match either one of the characters 'abc^'.
 249       </p>
 250 <p>
 251         As an extension, a collating element may also be specified via its symbolic
 252         name, for example:
 253       </p>
 254 <pre class="programlisting">[[.NUL.]]</pre>
 255 <p>
 256         matches a 'NUL' character. See <a class="link" href="collating_names.html" title="Collating Names">collating
 257         element names</a>.
 258       </p>
 259 <h6>
 260 <a name="boost_regex.syntax.basic_syntax.h13"></a>
 261         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.equivalence_classes"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.equivalence_classes">Equivalence
 262         classes:</a>
 263       </h6>
 264 <p>
 265         An expression of the form <code class="computeroutput"><span class="special">[[=</span><span class="identifier">col</span><span class="special">=]]</span></code>,
 266         matches any character or collating element whose primary sort key is the
 267         same as that for collating element <span class="emphasis"><em>col</em></span>, as with collating
 268         elements the name <span class="emphasis"><em>col</em></span> may be a <a class="link" href="collating_names.html" title="Collating Names">collating
 269         symbolic name</a>. A primary sort key is one that ignores case, accentation,
 270         or locale-specific tailorings; so for example <code class="computeroutput"><span class="special">[[=</span><span class="identifier">a</span><span class="special">=]]</span></code> matches
 271         any of the characters: a, &#192;, &#193;, &#194;, &#195;, &#196;, &#197;, A, &#224;, &#225;, &#226;, &#227;, &#228; and &#229;. Unfortunately implementation
 272         of this is reliant on the platform's collation and localisation support;
 273         this feature can not be relied upon to work portably across all platforms,
 274         or even all locales on one platform.
 275       </p>
 276 <h6>
 277 <a name="boost_regex.syntax.basic_syntax.h14"></a>
 278         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.combinations"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.combinations">Combinations:</a>
 279       </h6>
 280 <p>
 281         All of the above can be combined in one character set declaration, for example:
 282         <code class="computeroutput"><span class="special">[[:</span><span class="identifier">digit</span><span class="special">:]</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">[.</span><span class="identifier">NUL</span><span class="special">.]].</span></code>
 283       </p>
 284 <h5>
 285 <a name="boost_regex.syntax.basic_syntax.h15"></a>
 286         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.escapes"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.escapes">Escapes</a>
 287       </h5>
 288 <p>
 289         With the exception of the escape sequences \{, \}, \(, and \), which are
 290         documented above, an escape followed by any character matches that character.
 291         This can be used to make the special characters
 292       </p>
 293 <pre class="programlisting">.[\*^$</pre>
 294 <p>
 295         "ordinary". Note that the escape character loses its special meaning
 296         inside a character set, so <code class="computeroutput"><span class="special">[\^]</span></code>
 297         will match either a literal '\' or a '^'.
 298       </p>
 299 <h4>
 300 <a name="boost_regex.syntax.basic_syntax.h16"></a>
 301         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.what_gets_matched"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.what_gets_matched">What
 302         Gets Matched</a>
 303       </h4>
 304 <p>
 305         When there is more that one way to match a regular expression, the "best"
 306         possible match is obtained using the <a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">leftmost-longest
 307         rule</a>.
 308       </p>
 309 <h4>
 310 <a name="boost_regex.syntax.basic_syntax.h17"></a>
 311         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.variations"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.variations">Variations</a>
 312       </h4>
 313 <a name="boost_regex.grep_syntax"></a><h5>
 314 <a name="boost_regex.syntax.basic_syntax.h18"></a>
 315         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.grep"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.grep">Grep</a>
 316       </h5>
 317 <p>
 318         When an expression is compiled with the flag <code class="computeroutput"><span class="identifier">grep</span></code>
 319         set, then the expression is treated as a newline separated list of <a class="link" href="basic_syntax.html#boost_regex.posix_basic">POSIX-Basic expressions</a>, a match
 320         is found if any of the expressions in the list match, for example:
 321       </p>
 322 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e</span><span class="special">(</span><span class="string">"abc\ndef"</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">grep</span><span class="special">);</span>
 323 </pre>
 324 <p>
 325         will match either of the <a class="link" href="basic_syntax.html#boost_regex.posix_basic">POSIX-Basic
 326         expressions</a> "abc" or "def".
 327       </p>
 328 <p>
 329         As its name suggests, this behavior is consistent with the Unix utility grep.
 330       </p>
 331 <h5>
 332 <a name="boost_regex.syntax.basic_syntax.h19"></a>
 333         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.emacs"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.emacs">emacs</a>
 334       </h5>
 335 <p>
 336         In addition to the <a class="link" href="basic_syntax.html#boost_regex.posix_basic">POSIX-Basic features</a>
 337         the following characters are also special:
 338       </p>
 339 <div class="informaltable"><table class="table">
 340 <colgroup>
 341 <col>
 342 <col>
 343 </colgroup>
 344 <thead><tr>
 345 <th>
 346                 <p>
 347                   Character
 348                 </p>
 349               </th>
 350 <th>
 351                 <p>
 352                   Description
 353                 </p>
 354               </th>
 355 </tr></thead>
 356 <tbody>
 357 <tr>
 358 <td>
 359                 <p>
 360                   +
 361                 </p>
 362               </td>
 363 <td>
 364                 <p>
 365                   repeats the preceding atom one or more times.
 366                 </p>
 367               </td>
 368 </tr>
 369 <tr>
 370 <td>
 371                 <p>
 372                   ?
 373                 </p>
 374               </td>
 375 <td>
 376                 <p>
 377                   repeats the preceding atom zero or one times.
 378                 </p>
 379               </td>
 380 </tr>
 381 <tr>
 382 <td>
 383                 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 384                       ?
 385                     </li></ul></div>
 386               </td>
 387 <td>
 388                 <p>
 389                   A non-greedy version of *.
 390                 </p>
 391               </td>
 392 </tr>
 393 <tr>
 394 <td>
 395                 <p>
 396                   +?
 397                 </p>
 398               </td>
 399 <td>
 400                 <p>
 401                   A non-greedy version of +.
 402                 </p>
 403               </td>
 404 </tr>
 405 <tr>
 406 <td>
 407                 <p>
 408                   ??
 409                 </p>
 410               </td>
 411 <td>
 412                 <p>
 413                   A non-greedy version of ?.
 414                 </p>
 415               </td>
 416 </tr>
 417 </tbody>
 418 </table></div>
 419 <p>
 420         And the following escape sequences are also recognised:
 421       </p>
 422 <div class="informaltable"><table class="table">
 423 <colgroup>
 424 <col>
 425 <col>
 426 </colgroup>
 427 <thead><tr>
 428 <th>
 429                 <p>
 430                   Escape
 431                 </p>
 432               </th>
 433 <th>
 434                 <p>
 435                   Description
 436                 </p>
 437               </th>
 438 </tr></thead>
 439 <tbody>
 440 <tr>
 441 <td>
 442                 <p>
 443                   \|
 444                 </p>
 445               </td>
 446 <td>
 447                 <p>
 448                   specifies an alternative.
 449                 </p>
 450               </td>
 451 </tr>
 452 <tr>
 453 <td>
 454                 <p>
 455                   \(?: ... )
 456                 </p>
 457               </td>
 458 <td>
 459                 <p>
 460                   is a non-marking grouping construct - allows you to lexically group
 461                   something without spitting out an extra sub-expression.
 462                 </p>
 463               </td>
 464 </tr>
 465 <tr>
 466 <td>
 467                 <p>
 468                   \w
 469                 </p>
 470               </td>
 471 <td>
 472                 <p>
 473                   matches any word character.
 474                 </p>
 475               </td>
 476 </tr>
 477 <tr>
 478 <td>
 479                 <p>
 480                   \W
 481                 </p>
 482               </td>
 483 <td>
 484                 <p>
 485                   matches any non-word character.
 486                 </p>
 487               </td>
 488 </tr>
 489 <tr>
 490 <td>
 491                 <p>
 492                   \sx
 493                 </p>
 494               </td>
 495 <td>
 496                 <p>
 497                   matches any character in the syntax group x, the following emacs
 498                   groupings are supported: 's', ' ', '_', 'w', '.', ')', '(', '"',
 499                   '\'', '&gt;' and '&lt;'. Refer to the emacs docs for details.
 500                 </p>
 501               </td>
 502 </tr>
 503 <tr>
 504 <td>
 505                 <p>
 506                   \Sx
 507                 </p>
 508               </td>
 509 <td>
 510                 <p>
 511                   matches any character not in the syntax grouping x.
 512                 </p>
 513               </td>
 514 </tr>
 515 <tr>
 516 <td>
 517                 <p>
 518                   \c and \C
 519                 </p>
 520               </td>
 521 <td>
 522                 <p>
 523                   These are not supported.
 524                 </p>
 525               </td>
 526 </tr>
 527 <tr>
 528 <td>
 529                 <p>
 530                   \`
 531                 </p>
 532               </td>
 533 <td>
 534                 <p>
 535                   matches zero characters only at the start of a buffer (or string
 536                   being matched).
 537                 </p>
 538               </td>
 539 </tr>
 540 <tr>
 541 <td>
 542                 <p>
 543                   \'
 544                 </p>
 545               </td>
 546 <td>
 547                 <p>
 548                   matches zero characters only at the end of a buffer (or string
 549                   being matched).
 550                 </p>
 551               </td>
 552 </tr>
 553 <tr>
 554 <td>
 555                 <p>
 556                   \b
 557                 </p>
 558               </td>
 559 <td>
 560                 <p>
 561                   matches zero characters at a word boundary.
 562                 </p>
 563               </td>
 564 </tr>
 565 <tr>
 566 <td>
 567                 <p>
 568                   \B
 569                 </p>
 570               </td>
 571 <td>
 572                 <p>
 573                   matches zero characters, not at a word boundary.
 574                 </p>
 575               </td>
 576 </tr>
 577 <tr>
 578 <td>
 579                 <p>
 580                   \&lt;
 581                 </p>
 582               </td>
 583 <td>
 584                 <p>
 585                   matches zero characters only at the start of a word.
 586                 </p>
 587               </td>
 588 </tr>
 589 <tr>
 590 <td>
 591                 <p>
 592                   \&gt;
 593                 </p>
 594               </td>
 595 <td>
 596                 <p>
 597                   matches zero characters only at the end of a word.
 598                 </p>
 599               </td>
 600 </tr>
 601 </tbody>
 602 </table></div>
 603 <p>
 604         Finally, you should note that emacs style regular expressions are matched
 605         according to the <a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.what_gets_matched">Perl
 606         "depth first search" rules</a>. Emacs expressions are matched
 607         this way because they contain Perl-like extensions, that do not interact
 608         well with the <a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">POSIX-style
 609         leftmost-longest rule</a>.
 610       </p>
 611 <h4>
 612 <a name="boost_regex.syntax.basic_syntax.h20"></a>
 613         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.options"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.options">Options</a>
 614       </h4>
 615 <p>
 616         There are a <a class="link" href="../ref/syntax_option_type/syntax_option_type_basic.html" title="Options for POSIX Basic Regular Expressions">variety
 617         of flags</a> that may be combined with the <code class="computeroutput"><span class="identifier">basic</span></code>
 618         and <code class="computeroutput"><span class="identifier">grep</span></code> options when constructing
 619         the regular expression, in particular note that the <a class="link" href="../ref/syntax_option_type/syntax_option_type_basic.html" title="Options for POSIX Basic Regular Expressions"><code class="computeroutput"><span class="identifier">newline_alt</span></code>, <code class="computeroutput"><span class="identifier">no_char_classes</span></code>,
 620         <code class="computeroutput"><span class="identifier">no</span><span class="special">-</span><span class="identifier">intervals</span></code>, <code class="computeroutput"><span class="identifier">bk_plus_qm</span></code>
 621         and <code class="computeroutput"><span class="identifier">bk_plus_vbar</span></code></a> options
 622         all alter the syntax, while the <a class="link" href="../ref/syntax_option_type/syntax_option_type_basic.html" title="Options for POSIX Basic Regular Expressions"><code class="computeroutput"><span class="identifier">collate</span></code> and <code class="computeroutput"><span class="identifier">icase</span></code>
 623         options</a> modify how the case and locale sensitivity are to be applied.
 624       </p>
 625 <h4>
 626 <a name="boost_regex.syntax.basic_syntax.h21"></a>
 627         <span class="phrase"><a name="boost_regex.syntax.basic_syntax.references"></a></span><a class="link" href="basic_syntax.html#boost_regex.syntax.basic_syntax.references">References</a>
 628       </h4>
 629 <p>
 630         <a href="http://www.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap09.html" target="_top">IEEE
 631         Std 1003.1-2001, Portable Operating System Interface (POSIX ), Base Definitions
 632         and Headers, Section 9, Regular Expressions (FWD.1).</a>
 633       </p>
 634 <p>
 635         <a href="http://www.opengroup.org/onlinepubs/000095399/utilities/grep.html" target="_top">IEEE
 636         Std 1003.1-2001, Portable Operating System Interface (POSIX ), Shells and
 637         Utilities, Section 4, Utilities, grep (FWD.1).</a>
 638       </p>
 639 <p>
 640         <a href="http://www.gnu.org/software/emacs/" target="_top">Emacs Version 21.3.</a>
 641       </p>
 642 </div>
 643 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 644 <td align="left"></td>
 645 <td align="right"><div class="copyright-footer">Copyright &#169; 1998-2013 John Maddock<p>
 646         Distributed under the Boost Software License, Version 1.0. (See accompanying
 647         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 648       </p>
 649 </div></td>
 650 </tr></table>
 651 <hr>
 652 <div class="spirit-nav">
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