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