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1 <html>
2 <head>
3 <title>pcrepattern specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepattern man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18 <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19 <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20 <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21 <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
22 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
23 <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24 <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25 <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26 <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27 <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28 <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29 <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30 <li><a name="TOC15" href="#SEC15">REPETITION</a>
31 <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
32 <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33 <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34 <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35 <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36 <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37 <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38 <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39 <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40 <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41 <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42 <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43 <li><a name="TOC28" href="#SEC28">REVISION</a>
44 </ul>
45 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46 <P>
47 The syntax and semantics of the regular expressions that are supported by PCRE
48 are described in detail below. There is a quick-reference syntax summary in the
49 <a href="pcresyntax.html"><b>pcresyntax</b></a>
50 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51 also supports some alternative regular expression syntax (which does not
52 conflict with the Perl syntax) in order to provide some compatibility with
53 regular expressions in Python, .NET, and Oniguruma.
54 </P>
55 <P>
56 Perl's regular expressions are described in its own documentation, and
57 regular expressions in general are covered in a number of books, some of which
58 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59 published by O'Reilly, covers regular expressions in great detail. This
60 description of PCRE's regular expressions is intended as reference material.
61 </P>
62 <P>
63 The original operation of PCRE was on strings of one-byte characters. However,
64 there is now also support for UTF-8 character strings. To use this,
65 PCRE must be built to include UTF-8 support, and you must call
66 <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There
67 is also a special sequence that can be given at the start of a pattern:
68 <pre>
69 (*UTF8)
70 </pre>
71 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
72 option. This feature is not Perl-compatible. How setting UTF-8 mode affects
73 pattern matching is mentioned in several places below. There is also a summary
74 of UTF-8 features in the
75 <a href="pcre.html#utf8support">section on UTF-8 support</a>
76 in the main
77 <a href="pcre.html"><b>pcre</b></a>
78 page.
79 </P>
80 <P>
81 The remainder of this document discusses the patterns that are supported by
82 PCRE when its main matching function, <b>pcre_exec()</b>, is used.
83 From release 6.0, PCRE offers a second matching function,
84 <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
85 Perl-compatible. Some of the features discussed below are not available when
86 <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
87 alternative function, and how it differs from the normal function, are
88 discussed in the
89 <a href="pcrematching.html"><b>pcrematching</b></a>
90 page.
91 </P>
92 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
93 <P>
94 PCRE supports five different conventions for indicating line breaks in
95 strings: a single CR (carriage return) character, a single LF (linefeed)
96 character, the two-character sequence CRLF, any of the three preceding, or any
97 Unicode newline sequence. The
98 <a href="pcreapi.html"><b>pcreapi</b></a>
99 page has
100 <a href="pcreapi.html#newlines">further discussion</a>
101 about newlines, and shows how to set the newline convention in the
102 <i>options</i> arguments for the compiling and matching functions.
103 </P>
104 <P>
105 It is also possible to specify a newline convention by starting a pattern
106 string with one of the following five sequences:
107 <pre>
108 (*CR) carriage return
109 (*LF) linefeed
110 (*CRLF) carriage return, followed by linefeed
111 (*ANYCRLF) any of the three above
112 (*ANY) all Unicode newline sequences
113 </pre>
114 These override the default and the options given to <b>pcre_compile()</b> or
115 <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default
116 newline sequence, the pattern
117 <pre>
118 (*CR)a.b
119 </pre>
120 changes the convention to CR. That pattern matches "a\nb" because LF is no
121 longer a newline. Note that these special settings, which are not
122 Perl-compatible, are recognized only at the very start of a pattern, and that
123 they must be in upper case. If more than one of them is present, the last one
124 is used.
125 </P>
126 <P>
127 The newline convention does not affect what the \R escape sequence matches. By
128 default, this is any Unicode newline sequence, for Perl compatibility. However,
129 this can be changed; see the description of \R in the section entitled
130 <a href="#newlineseq">"Newline sequences"</a>
131 below. A change of \R setting can be combined with a change of newline
132 convention.
133 </P>
134 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
135 <P>
136 A regular expression is a pattern that is matched against a subject string from
137 left to right. Most characters stand for themselves in a pattern, and match the
138 corresponding characters in the subject. As a trivial example, the pattern
139 <pre>
140 The quick brown fox
141 </pre>
142 matches a portion of a subject string that is identical to itself. When
143 caseless matching is specified (the PCRE_CASELESS option), letters are matched
144 independently of case. In UTF-8 mode, PCRE always understands the concept of
145 case for characters whose values are less than 128, so caseless matching is
146 always possible. For characters with higher values, the concept of case is
147 supported if PCRE is compiled with Unicode property support, but not otherwise.
148 If you want to use caseless matching for characters 128 and above, you must
149 ensure that PCRE is compiled with Unicode property support as well as with
150 UTF-8 support.
151 </P>
152 <P>
153 The power of regular expressions comes from the ability to include alternatives
154 and repetitions in the pattern. These are encoded in the pattern by the use of
155 <i>metacharacters</i>, which do not stand for themselves but instead are
156 interpreted in some special way.
157 </P>
158 <P>
159 There are two different sets of metacharacters: those that are recognized
160 anywhere in the pattern except within square brackets, and those that are
161 recognized within square brackets. Outside square brackets, the metacharacters
162 are as follows:
163 <pre>
164 \ general escape character with several uses
165 ^ assert start of string (or line, in multiline mode)
166 $ assert end of string (or line, in multiline mode)
167 . match any character except newline (by default)
168 [ start character class definition
169 | start of alternative branch
170 ( start subpattern
171 ) end subpattern
172 ? extends the meaning of (
173 also 0 or 1 quantifier
174 also quantifier minimizer
175 * 0 or more quantifier
176 + 1 or more quantifier
177 also "possessive quantifier"
178 { start min/max quantifier
179 </pre>
180 Part of a pattern that is in square brackets is called a "character class". In
181 a character class the only metacharacters are:
182 <pre>
183 \ general escape character
184 ^ negate the class, but only if the first character
185 - indicates character range
186 [ POSIX character class (only if followed by POSIX syntax)
187 ] terminates the character class
188 </pre>
189 The following sections describe the use of each of the metacharacters.
190 </P>
191 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
192 <P>
193 The backslash character has several uses. Firstly, if it is followed by a
194 non-alphanumeric character, it takes away any special meaning that character
195 may have. This use of backslash as an escape character applies both inside and
196 outside character classes.
197 </P>
198 <P>
199 For example, if you want to match a * character, you write \* in the pattern.
200 This escaping action applies whether or not the following character would
201 otherwise be interpreted as a metacharacter, so it is always safe to precede a
202 non-alphanumeric with backslash to specify that it stands for itself. In
203 particular, if you want to match a backslash, you write \\.
204 </P>
205 <P>
206 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
207 pattern (other than in a character class) and characters between a # outside
208 a character class and the next newline are ignored. An escaping backslash can
209 be used to include a whitespace or # character as part of the pattern.
210 </P>
211 <P>
212 If you want to remove the special meaning from a sequence of characters, you
213 can do so by putting them between \Q and \E. This is different from Perl in
214 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
215 Perl, $ and @ cause variable interpolation. Note the following examples:
216 <pre>
217 Pattern PCRE matches Perl matches
218
219 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
220 \Qabc\$xyz\E abc\$xyz abc\$xyz
221 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
222 </pre>
223 The \Q...\E sequence is recognized both inside and outside character classes.
224 <a name="digitsafterbackslash"></a></P>
225 <br><b>
226 Non-printing characters
227 </b><br>
228 <P>
229 A second use of backslash provides a way of encoding non-printing characters
230 in patterns in a visible manner. There is no restriction on the appearance of
231 non-printing characters, apart from the binary zero that terminates a pattern,
232 but when a pattern is being prepared by text editing, it is often easier to use
233 one of the following escape sequences than the binary character it represents:
234 <pre>
235 \a alarm, that is, the BEL character (hex 07)
236 \cx "control-x", where x is any character
237 \e escape (hex 1B)
238 \f formfeed (hex 0C)
239 \n linefeed (hex 0A)
240 \r carriage return (hex 0D)
241 \t tab (hex 09)
242 \ddd character with octal code ddd, or back reference
243 \xhh character with hex code hh
244 \x{hhh..} character with hex code hhh..
245 </pre>
246 The precise effect of \cx is as follows: if x is a lower case letter, it
247 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
248 Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
249 7B.
250 </P>
251 <P>
252 After \x, from zero to two hexadecimal digits are read (letters can be in
253 upper or lower case). Any number of hexadecimal digits may appear between \x{
254 and }, but the value of the character code must be less than 256 in non-UTF-8
255 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
256 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
257 point, which is 10FFFF.
258 </P>
259 <P>
260 If characters other than hexadecimal digits appear between \x{ and }, or if
261 there is no terminating }, this form of escape is not recognized. Instead, the
262 initial \x will be interpreted as a basic hexadecimal escape, with no
263 following digits, giving a character whose value is zero.
264 </P>
265 <P>
266 Characters whose value is less than 256 can be defined by either of the two
267 syntaxes for \x. There is no difference in the way they are handled. For
268 example, \xdc is exactly the same as \x{dc}.
269 </P>
270 <P>
271 After \0 up to two further octal digits are read. If there are fewer than two
272 digits, just those that are present are used. Thus the sequence \0\x\07
273 specifies two binary zeros followed by a BEL character (code value 7). Make
274 sure you supply two digits after the initial zero if the pattern character that
275 follows is itself an octal digit.
276 </P>
277 <P>
278 The handling of a backslash followed by a digit other than 0 is complicated.
279 Outside a character class, PCRE reads it and any following digits as a decimal
280 number. If the number is less than 10, or if there have been at least that many
281 previous capturing left parentheses in the expression, the entire sequence is
282 taken as a <i>back reference</i>. A description of how this works is given
283 <a href="#backreferences">later,</a>
284 following the discussion of
285 <a href="#subpattern">parenthesized subpatterns.</a>
286 </P>
287 <P>
288 Inside a character class, or if the decimal number is greater than 9 and there
289 have not been that many capturing subpatterns, PCRE re-reads up to three octal
290 digits following the backslash, and uses them to generate a data character. Any
291 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
292 character specified in octal must be less than \400. In UTF-8 mode, values up
293 to \777 are permitted. For example:
294 <pre>
295 \040 is another way of writing a space
296 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
297 \7 is always a back reference
298 \11 might be a back reference, or another way of writing a tab
299 \011 is always a tab
300 \0113 is a tab followed by the character "3"
301 \113 might be a back reference, otherwise the character with octal code 113
302 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
303 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
304 </pre>
305 Note that octal values of 100 or greater must not be introduced by a leading
306 zero, because no more than three octal digits are ever read.
307 </P>
308 <P>
309 All the sequences that define a single character value can be used both inside
310 and outside character classes. In addition, inside a character class, the
311 sequence \b is interpreted as the backspace character (hex 08), and the
312 sequences \R and \X are interpreted as the characters "R" and "X",
313 respectively. Outside a character class, these sequences have different
314 meanings
315 <a href="#uniextseq">(see below).</a>
316 </P>
317 <br><b>
318 Absolute and relative back references
319 </b><br>
320 <P>
321 The sequence \g followed by an unsigned or a negative number, optionally
322 enclosed in braces, is an absolute or relative back reference. A named back
323 reference can be coded as \g{name}. Back references are discussed
324 <a href="#backreferences">later,</a>
325 following the discussion of
326 <a href="#subpattern">parenthesized subpatterns.</a>
327 </P>
328 <br><b>
329 Absolute and relative subroutine calls
330 </b><br>
331 <P>
332 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
333 a number enclosed either in angle brackets or single quotes, is an alternative
334 syntax for referencing a subpattern as a "subroutine". Details are discussed
335 <a href="#onigurumasubroutines">later.</a>
336 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
337 synonymous. The former is a back reference; the latter is a
338 <a href="#subpatternsassubroutines">subroutine</a>
339 call.
340 </P>
341 <br><b>
342 Generic character types
343 </b><br>
344 <P>
345 Another use of backslash is for specifying generic character types. The
346 following are always recognized:
347 <pre>
348 \d any decimal digit
349 \D any character that is not a decimal digit
350 \h any horizontal whitespace character
351 \H any character that is not a horizontal whitespace character
352 \s any whitespace character
353 \S any character that is not a whitespace character
354 \v any vertical whitespace character
355 \V any character that is not a vertical whitespace character
356 \w any "word" character
357 \W any "non-word" character
358 </pre>
359 Each pair of escape sequences partitions the complete set of characters into
360 two disjoint sets. Any given character matches one, and only one, of each pair.
361 </P>
362 <P>
363 These character type sequences can appear both inside and outside character
364 classes. They each match one character of the appropriate type. If the current
365 matching point is at the end of the subject string, all of them fail, since
366 there is no character to match.
367 </P>
368 <P>
369 For compatibility with Perl, \s does not match the VT character (code 11).
370 This makes it different from the the POSIX "space" class. The \s characters
371 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372 included in a Perl script, \s may match the VT character. In PCRE, it never
373 does.
374 </P>
375 <P>
376 In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
377 \w, and always match \D, \S, and \W. This is true even when Unicode
378 character property support is available. These sequences retain their original
379 meanings from before UTF-8 support was available, mainly for efficiency
380 reasons. Note that this also affects \b, because it is defined in terms of \w
381 and \W.
382 </P>
383 <P>
384 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
385 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
386 The horizontal space characters are:
387 <pre>
388 U+0009 Horizontal tab
389 U+0020 Space
390 U+00A0 Non-break space
391 U+1680 Ogham space mark
392 U+180E Mongolian vowel separator
393 U+2000 En quad
394 U+2001 Em quad
395 U+2002 En space
396 U+2003 Em space
397 U+2004 Three-per-em space
398 U+2005 Four-per-em space
399 U+2006 Six-per-em space
400 U+2007 Figure space
401 U+2008 Punctuation space
402 U+2009 Thin space
403 U+200A Hair space
404 U+202F Narrow no-break space
405 U+205F Medium mathematical space
406 U+3000 Ideographic space
407 </pre>
408 The vertical space characters are:
409 <pre>
410 U+000A Linefeed
411 U+000B Vertical tab
412 U+000C Formfeed
413 U+000D Carriage return
414 U+0085 Next line
415 U+2028 Line separator
416 U+2029 Paragraph separator
417 </PRE>
418 </P>
419 <P>
420 A "word" character is an underscore or any character less than 256 that is a
421 letter or digit. The definition of letters and digits is controlled by PCRE's
422 low-valued character tables, and may vary if locale-specific matching is taking
423 place (see
424 <a href="pcreapi.html#localesupport">"Locale support"</a>
425 in the
426 <a href="pcreapi.html"><b>pcreapi</b></a>
427 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
428 or "french" in Windows, some character codes greater than 128 are used for
429 accented letters, and these are matched by \w. The use of locales with Unicode
430 is discouraged.
431 <a name="newlineseq"></a></P>
432 <br><b>
433 Newline sequences
434 </b><br>
435 <P>
436 Outside a character class, by default, the escape sequence \R matches any
437 Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
438 equivalent to the following:
439 <pre>
440 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
441 </pre>
442 This is an example of an "atomic group", details of which are given
443 <a href="#atomicgroup">below.</a>
444 This particular group matches either the two-character sequence CR followed by
445 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
446 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
447 line, U+0085). The two-character sequence is treated as a single unit that
448 cannot be split.
449 </P>
450 <P>
451 In UTF-8 mode, two additional characters whose codepoints are greater than 255
452 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
453 Unicode character property support is not needed for these characters to be
454 recognized.
455 </P>
456 <P>
457 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
458 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
459 either at compile time or when the pattern is matched. (BSR is an abbrevation
460 for "backslash R".) This can be made the default when PCRE is built; if this is
461 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
462 It is also possible to specify these settings by starting a pattern string with
463 one of the following sequences:
464 <pre>
465 (*BSR_ANYCRLF) CR, LF, or CRLF only
466 (*BSR_UNICODE) any Unicode newline sequence
467 </pre>
468 These override the default and the options given to <b>pcre_compile()</b> or
469 <b>pcre_compile2()</b>, but they can be overridden by options given to
470 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,
471 which are not Perl-compatible, are recognized only at the very start of a
472 pattern, and that they must be in upper case. If more than one of them is
473 present, the last one is used. They can be combined with a change of newline
474 convention, for example, a pattern can start with:
475 <pre>
476 (*ANY)(*BSR_ANYCRLF)
477 </pre>
478 Inside a character class, \R matches the letter "R".
479 <a name="uniextseq"></a></P>
480 <br><b>
481 Unicode character properties
482 </b><br>
483 <P>
484 When PCRE is built with Unicode character property support, three additional
485 escape sequences that match characters with specific properties are available.
486 When not in UTF-8 mode, these sequences are of course limited to testing
487 characters whose codepoints are less than 256, but they do work in this mode.
488 The extra escape sequences are:
489 <pre>
490 \p{<i>xx</i>} a character with the <i>xx</i> property
491 \P{<i>xx</i>} a character without the <i>xx</i> property
492 \X an extended Unicode sequence
493 </pre>
494 The property names represented by <i>xx</i> above are limited to the Unicode
495 script names, the general category properties, and "Any", which matches any
496 character (including newline). Other properties such as "InMusicalSymbols" are
497 not currently supported by PCRE. Note that \P{Any} does not match any
498 characters, so always causes a match failure.
499 </P>
500 <P>
501 Sets of Unicode characters are defined as belonging to certain scripts. A
502 character from one of these sets can be matched using a script name. For
503 example:
504 <pre>
505 \p{Greek}
506 \P{Han}
507 </pre>
508 Those that are not part of an identified script are lumped together as
509 "Common". The current list of scripts is:
510 </P>
511 <P>
512 Arabic,
513 Armenian,
514 Avestan,
515 Balinese,
516 Bamum,
517 Bengali,
518 Bopomofo,
519 Braille,
520 Buginese,
521 Buhid,
522 Canadian_Aboriginal,
523 Carian,
524 Cham,
525 Cherokee,
526 Common,
527 Coptic,
528 Cuneiform,
529 Cypriot,
530 Cyrillic,
531 Deseret,
532 Devanagari,
533 Egyptian_Hieroglyphs,
534 Ethiopic,
535 Georgian,
536 Glagolitic,
537 Gothic,
538 Greek,
539 Gujarati,
540 Gurmukhi,
541 Han,
542 Hangul,
543 Hanunoo,
544 Hebrew,
545 Hiragana,
546 Imperial_Aramaic,
547 Inherited,
548 Inscriptional_Pahlavi,
549 Inscriptional_Parthian,
550 Javanese,
551 Kaithi,
552 Kannada,
553 Katakana,
554 Kayah_Li,
555 Kharoshthi,
556 Khmer,
557 Lao,
558 Latin,
559 Lepcha,
560 Limbu,
561 Linear_B,
562 Lisu,
563 Lycian,
564 Lydian,
565 Malayalam,
566 Meetei_Mayek,
567 Mongolian,
568 Myanmar,
569 New_Tai_Lue,
570 Nko,
571 Ogham,
572 Old_Italic,
573 Old_Persian,
574 Old_South_Arabian,
575 Old_Turkic,
576 Ol_Chiki,
577 Oriya,
578 Osmanya,
579 Phags_Pa,
580 Phoenician,
581 Rejang,
582 Runic,
583 Samaritan,
584 Saurashtra,
585 Shavian,
586 Sinhala,
587 Sundanese,
588 Syloti_Nagri,
589 Syriac,
590 Tagalog,
591 Tagbanwa,
592 Tai_Le,
593 Tai_Tham,
594 Tai_Viet,
595 Tamil,
596 Telugu,
597 Thaana,
598 Thai,
599 Tibetan,
600 Tifinagh,
601 Ugaritic,
602 Vai,
603 Yi.
604 </P>
605 <P>
606 Each character has exactly one general category property, specified by a
607 two-letter abbreviation. For compatibility with Perl, negation can be specified
608 by including a circumflex between the opening brace and the property name. For
609 example, \p{^Lu} is the same as \P{Lu}.
610 </P>
611 <P>
612 If only one letter is specified with \p or \P, it includes all the general
613 category properties that start with that letter. In this case, in the absence
614 of negation, the curly brackets in the escape sequence are optional; these two
615 examples have the same effect:
616 <pre>
617 \p{L}
618 \pL
619 </pre>
620 The following general category property codes are supported:
621 <pre>
622 C Other
623 Cc Control
624 Cf Format
625 Cn Unassigned
626 Co Private use
627 Cs Surrogate
628
629 L Letter
630 Ll Lower case letter
631 Lm Modifier letter
632 Lo Other letter
633 Lt Title case letter
634 Lu Upper case letter
635
636 M Mark
637 Mc Spacing mark
638 Me Enclosing mark
639 Mn Non-spacing mark
640
641 N Number
642 Nd Decimal number
643 Nl Letter number
644 No Other number
645
646 P Punctuation
647 Pc Connector punctuation
648 Pd Dash punctuation
649 Pe Close punctuation
650 Pf Final punctuation
651 Pi Initial punctuation
652 Po Other punctuation
653 Ps Open punctuation
654
655 S Symbol
656 Sc Currency symbol
657 Sk Modifier symbol
658 Sm Mathematical symbol
659 So Other symbol
660
661 Z Separator
662 Zl Line separator
663 Zp Paragraph separator
664 Zs Space separator
665 </pre>
666 The special property L& is also supported: it matches a character that has
667 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
668 a modifier or "other".
669 </P>
670 <P>
671 The Cs (Surrogate) property applies only to characters in the range U+D800 to
672 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
673 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
674 (see the discussion of PCRE_NO_UTF8_CHECK in the
675 <a href="pcreapi.html"><b>pcreapi</b></a>
676 page). Perl does not support the Cs property.
677 </P>
678 <P>
679 The long synonyms for property names that Perl supports (such as \p{Letter})
680 are not supported by PCRE, nor is it permitted to prefix any of these
681 properties with "Is".
682 </P>
683 <P>
684 No character that is in the Unicode table has the Cn (unassigned) property.
685 Instead, this property is assumed for any code point that is not in the
686 Unicode table.
687 </P>
688 <P>
689 Specifying caseless matching does not affect these escape sequences. For
690 example, \p{Lu} always matches only upper case letters.
691 </P>
692 <P>
693 The \X escape matches any number of Unicode characters that form an extended
694 Unicode sequence. \X is equivalent to
695 <pre>
696 (?&#62;\PM\pM*)
697 </pre>
698 That is, it matches a character without the "mark" property, followed by zero
699 or more characters with the "mark" property, and treats the sequence as an
700 atomic group
701 <a href="#atomicgroup">(see below).</a>
702 Characters with the "mark" property are typically accents that affect the
703 preceding character. None of them have codepoints less than 256, so in
704 non-UTF-8 mode \X matches any one character.
705 </P>
706 <P>
707 Matching characters by Unicode property is not fast, because PCRE has to search
708 a structure that contains data for over fifteen thousand characters. That is
709 why the traditional escape sequences such as \d and \w do not use Unicode
710 properties in PCRE.
711 <a name="resetmatchstart"></a></P>
712 <br><b>
713 Resetting the match start
714 </b><br>
715 <P>
716 The escape sequence \K, which is a Perl 5.10 feature, causes any previously
717 matched characters not to be included in the final matched sequence. For
718 example, the pattern:
719 <pre>
720 foo\Kbar
721 </pre>
722 matches "foobar", but reports that it has matched "bar". This feature is
723 similar to a lookbehind assertion
724 <a href="#lookbehind">(described below).</a>
725 However, in this case, the part of the subject before the real match does not
726 have to be of fixed length, as lookbehind assertions do. The use of \K does
727 not interfere with the setting of
728 <a href="#subpattern">captured substrings.</a>
729 For example, when the pattern
730 <pre>
731 (foo)\Kbar
732 </pre>
733 matches "foobar", the first substring is still set to "foo".
734 </P>
735 <P>
736 Perl documents that the use of \K within assertions is "not well defined". In
737 PCRE, \K is acted upon when it occurs inside positive assertions, but is
738 ignored in negative assertions.
739 <a name="smallassertions"></a></P>
740 <br><b>
741 Simple assertions
742 </b><br>
743 <P>
744 The final use of backslash is for certain simple assertions. An assertion
745 specifies a condition that has to be met at a particular point in a match,
746 without consuming any characters from the subject string. The use of
747 subpatterns for more complicated assertions is described
748 <a href="#bigassertions">below.</a>
749 The backslashed assertions are:
750 <pre>
751 \b matches at a word boundary
752 \B matches when not at a word boundary
753 \A matches at the start of the subject
754 \Z matches at the end of the subject
755 also matches before a newline at the end of the subject
756 \z matches only at the end of the subject
757 \G matches at the first matching position in the subject
758 </pre>
759 These assertions may not appear in character classes (but note that \b has a
760 different meaning, namely the backspace character, inside a character class).
761 </P>
762 <P>
763 A word boundary is a position in the subject string where the current character
764 and the previous character do not both match \w or \W (i.e. one matches
765 \w and the other matches \W), or the start or end of the string if the
766 first or last character matches \w, respectively. Neither PCRE nor Perl has a
767 separte "start of word" or "end of word" metasequence. However, whatever
768 follows \b normally determines which it is. For example, the fragment
769 \ba matches "a" at the start of a word.
770 </P>
771 <P>
772 The \A, \Z, and \z assertions differ from the traditional circumflex and
773 dollar (described in the next section) in that they only ever match at the very
774 start and end of the subject string, whatever options are set. Thus, they are
775 independent of multiline mode. These three assertions are not affected by the
776 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
777 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
778 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
779 at a point other than the beginning of the subject, \A can never match. The
780 difference between \Z and \z is that \Z matches before a newline at the end
781 of the string as well as at the very end, whereas \z matches only at the end.
782 </P>
783 <P>
784 The \G assertion is true only when the current matching position is at the
785 start point of the match, as specified by the <i>startoffset</i> argument of
786 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
787 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
788 arguments, you can mimic Perl's /g option, and it is in this kind of
789 implementation where \G can be useful.
790 </P>
791 <P>
792 Note, however, that PCRE's interpretation of \G, as the start of the current
793 match, is subtly different from Perl's, which defines it as the end of the
794 previous match. In Perl, these can be different when the previously matched
795 string was empty. Because PCRE does just one match at a time, it cannot
796 reproduce this behaviour.
797 </P>
798 <P>
799 If all the alternatives of a pattern begin with \G, the expression is anchored
800 to the starting match position, and the "anchored" flag is set in the compiled
801 regular expression.
802 </P>
803 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
804 <P>
805 Outside a character class, in the default matching mode, the circumflex
806 character is an assertion that is true only if the current matching point is
807 at the start of the subject string. If the <i>startoffset</i> argument of
808 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
809 option is unset. Inside a character class, circumflex has an entirely different
810 meaning
811 <a href="#characterclass">(see below).</a>
812 </P>
813 <P>
814 Circumflex need not be the first character of the pattern if a number of
815 alternatives are involved, but it should be the first thing in each alternative
816 in which it appears if the pattern is ever to match that branch. If all
817 possible alternatives start with a circumflex, that is, if the pattern is
818 constrained to match only at the start of the subject, it is said to be an
819 "anchored" pattern. (There are also other constructs that can cause a pattern
820 to be anchored.)
821 </P>
822 <P>
823 A dollar character is an assertion that is true only if the current matching
824 point is at the end of the subject string, or immediately before a newline
825 at the end of the string (by default). Dollar need not be the last character of
826 the pattern if a number of alternatives are involved, but it should be the last
827 item in any branch in which it appears. Dollar has no special meaning in a
828 character class.
829 </P>
830 <P>
831 The meaning of dollar can be changed so that it matches only at the very end of
832 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
833 does not affect the \Z assertion.
834 </P>
835 <P>
836 The meanings of the circumflex and dollar characters are changed if the
837 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
838 immediately after internal newlines as well as at the start of the subject
839 string. It does not match after a newline that ends the string. A dollar
840 matches before any newlines in the string, as well as at the very end, when
841 PCRE_MULTILINE is set. When newline is specified as the two-character
842 sequence CRLF, isolated CR and LF characters do not indicate newlines.
843 </P>
844 <P>
845 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
846 \n represents a newline) in multiline mode, but not otherwise. Consequently,
847 patterns that are anchored in single line mode because all branches start with
848 ^ are not anchored in multiline mode, and a match for circumflex is possible
849 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
850 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
851 </P>
852 <P>
853 Note that the sequences \A, \Z, and \z can be used to match the start and
854 end of the subject in both modes, and if all branches of a pattern start with
855 \A it is always anchored, whether or not PCRE_MULTILINE is set.
856 </P>
857 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
858 <P>
859 Outside a character class, a dot in the pattern matches any one character in
860 the subject string except (by default) a character that signifies the end of a
861 line. In UTF-8 mode, the matched character may be more than one byte long.
862 </P>
863 <P>
864 When a line ending is defined as a single character, dot never matches that
865 character; when the two-character sequence CRLF is used, dot does not match CR
866 if it is immediately followed by LF, but otherwise it matches all characters
867 (including isolated CRs and LFs). When any Unicode line endings are being
868 recognized, dot does not match CR or LF or any of the other line ending
869 characters.
870 </P>
871 <P>
872 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
873 option is set, a dot matches any one character, without exception. If the
874 two-character sequence CRLF is present in the subject string, it takes two dots
875 to match it.
876 </P>
877 <P>
878 The handling of dot is entirely independent of the handling of circumflex and
879 dollar, the only relationship being that they both involve newlines. Dot has no
880 special meaning in a character class.
881 </P>
882 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
883 <P>
884 Outside a character class, the escape sequence \C matches any one byte, both
885 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
886 characters. The feature is provided in Perl in order to match individual bytes
887 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
888 what remains in the string may be a malformed UTF-8 string. For this reason,
889 the \C escape sequence is best avoided.
890 </P>
891 <P>
892 PCRE does not allow \C to appear in lookbehind assertions
893 <a href="#lookbehind">(described below),</a>
894 because in UTF-8 mode this would make it impossible to calculate the length of
895 the lookbehind.
896 <a name="characterclass"></a></P>
897 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
898 <P>
899 An opening square bracket introduces a character class, terminated by a closing
900 square bracket. A closing square bracket on its own is not special by default.
901 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
902 bracket causes a compile-time error. If a closing square bracket is required as
903 a member of the class, it should be the first data character in the class
904 (after an initial circumflex, if present) or escaped with a backslash.
905 </P>
906 <P>
907 A character class matches a single character in the subject. In UTF-8 mode, the
908 character may be more than one byte long. A matched character must be in the
909 set of characters defined by the class, unless the first character in the class
910 definition is a circumflex, in which case the subject character must not be in
911 the set defined by the class. If a circumflex is actually required as a member
912 of the class, ensure it is not the first character, or escape it with a
913 backslash.
914 </P>
915 <P>
916 For example, the character class [aeiou] matches any lower case vowel, while
917 [^aeiou] matches any character that is not a lower case vowel. Note that a
918 circumflex is just a convenient notation for specifying the characters that
919 are in the class by enumerating those that are not. A class that starts with a
920 circumflex is not an assertion; it still consumes a character from the subject
921 string, and therefore it fails if the current pointer is at the end of the
922 string.
923 </P>
924 <P>
925 In UTF-8 mode, characters with values greater than 255 can be included in a
926 class as a literal string of bytes, or by using the \x{ escaping mechanism.
927 </P>
928 <P>
929 When caseless matching is set, any letters in a class represent both their
930 upper case and lower case versions, so for example, a caseless [aeiou] matches
931 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
932 caseful version would. In UTF-8 mode, PCRE always understands the concept of
933 case for characters whose values are less than 128, so caseless matching is
934 always possible. For characters with higher values, the concept of case is
935 supported if PCRE is compiled with Unicode property support, but not otherwise.
936 If you want to use caseless matching in UTF8-mode for characters 128 and above,
937 you must ensure that PCRE is compiled with Unicode property support as well as
938 with UTF-8 support.
939 </P>
940 <P>
941 Characters that might indicate line breaks are never treated in any special way
942 when matching character classes, whatever line-ending sequence is in use, and
943 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
944 such as [^a] always matches one of these characters.
945 </P>
946 <P>
947 The minus (hyphen) character can be used to specify a range of characters in a
948 character class. For example, [d-m] matches any letter between d and m,
949 inclusive. If a minus character is required in a class, it must be escaped with
950 a backslash or appear in a position where it cannot be interpreted as
951 indicating a range, typically as the first or last character in the class.
952 </P>
953 <P>
954 It is not possible to have the literal character "]" as the end character of a
955 range. A pattern such as [W-]46] is interpreted as a class of two characters
956 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
957 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
958 the end of range, so [W-\]46] is interpreted as a class containing a range
959 followed by two other characters. The octal or hexadecimal representation of
960 "]" can also be used to end a range.
961 </P>
962 <P>
963 Ranges operate in the collating sequence of character values. They can also be
964 used for characters specified numerically, for example [\000-\037]. In UTF-8
965 mode, ranges can include characters whose values are greater than 255, for
966 example [\x{100}-\x{2ff}].
967 </P>
968 <P>
969 If a range that includes letters is used when caseless matching is set, it
970 matches the letters in either case. For example, [W-c] is equivalent to
971 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
972 tables for a French locale are in use, [\xc8-\xcb] matches accented E
973 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
974 characters with values greater than 128 only when it is compiled with Unicode
975 property support.
976 </P>
977 <P>
978 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
979 in a character class, and add the characters that they match to the class. For
980 example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
981 conveniently be used with the upper case character types to specify a more
982 restricted set of characters than the matching lower case type. For example,
983 the class [^\W_] matches any letter or digit, but not underscore.
984 </P>
985 <P>
986 The only metacharacters that are recognized in character classes are backslash,
987 hyphen (only where it can be interpreted as specifying a range), circumflex
988 (only at the start), opening square bracket (only when it can be interpreted as
989 introducing a POSIX class name - see the next section), and the terminating
990 closing square bracket. However, escaping other non-alphanumeric characters
991 does no harm.
992 </P>
993 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
994 <P>
995 Perl supports the POSIX notation for character classes. This uses names
996 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
997 this notation. For example,
998 <pre>
999 [01[:alpha:]%]
1000 </pre>
1001 matches "0", "1", any alphabetic character, or "%". The supported class names
1002 are
1003 <pre>
1004 alnum letters and digits
1005 alpha letters
1006 ascii character codes 0 - 127
1007 blank space or tab only
1008 cntrl control characters
1009 digit decimal digits (same as \d)
1010 graph printing characters, excluding space
1011 lower lower case letters
1012 print printing characters, including space
1013 punct printing characters, excluding letters and digits
1014 space white space (not quite the same as \s)
1015 upper upper case letters
1016 word "word" characters (same as \w)
1017 xdigit hexadecimal digits
1018 </pre>
1019 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1020 space (32). Notice that this list includes the VT character (code 11). This
1021 makes "space" different to \s, which does not include VT (for Perl
1022 compatibility).
1023 </P>
1024 <P>
1025 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1026 5.8. Another Perl extension is negation, which is indicated by a ^ character
1027 after the colon. For example,
1028 <pre>
1029 [12[:^digit:]]
1030 </pre>
1031 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1032 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1033 supported, and an error is given if they are encountered.
1034 </P>
1035 <P>
1036 In UTF-8 mode, characters with values greater than 128 do not match any of
1037 the POSIX character classes.
1038 </P>
1039 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1040 <P>
1041 Vertical bar characters are used to separate alternative patterns. For example,
1042 the pattern
1043 <pre>
1044 gilbert|sullivan
1045 </pre>
1046 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1047 and an empty alternative is permitted (matching the empty string). The matching
1048 process tries each alternative in turn, from left to right, and the first one
1049 that succeeds is used. If the alternatives are within a subpattern
1050 <a href="#subpattern">(defined below),</a>
1051 "succeeds" means matching the rest of the main pattern as well as the
1052 alternative in the subpattern.
1053 </P>
1054 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1055 <P>
1056 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1057 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1058 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1059 The option letters are
1060 <pre>
1061 i for PCRE_CASELESS
1062 m for PCRE_MULTILINE
1063 s for PCRE_DOTALL
1064 x for PCRE_EXTENDED
1065 </pre>
1066 For example, (?im) sets caseless, multiline matching. It is also possible to
1067 unset these options by preceding the letter with a hyphen, and a combined
1068 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1069 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1070 permitted. If a letter appears both before and after the hyphen, the option is
1071 unset.
1072 </P>
1073 <P>
1074 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1075 changed in the same way as the Perl-compatible options by using the characters
1076 J, U and X respectively.
1077 </P>
1078 <P>
1079 When one of these option changes occurs at top level (that is, not inside
1080 subpattern parentheses), the change applies to the remainder of the pattern
1081 that follows. If the change is placed right at the start of a pattern, PCRE
1082 extracts it into the global options (and it will therefore show up in data
1083 extracted by the <b>pcre_fullinfo()</b> function).
1084 </P>
1085 <P>
1086 An option change within a subpattern (see below for a description of
1087 subpatterns) affects only that part of the current pattern that follows it, so
1088 <pre>
1089 (a(?i)b)c
1090 </pre>
1091 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1092 By this means, options can be made to have different settings in different
1093 parts of the pattern. Any changes made in one alternative do carry on
1094 into subsequent branches within the same subpattern. For example,
1095 <pre>
1096 (a(?i)b|c)
1097 </pre>
1098 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1099 branch is abandoned before the option setting. This is because the effects of
1100 option settings happen at compile time. There would be some very weird
1101 behaviour otherwise.
1102 </P>
1103 <P>
1104 <b>Note:</b> There are other PCRE-specific options that can be set by the
1105 application when the compile or match functions are called. In some cases the
1106 pattern can contain special leading sequences such as (*CRLF) to override what
1107 the application has set or what has been defaulted. Details are given in the
1108 section entitled
1109 <a href="#newlineseq">"Newline sequences"</a>
1110 above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1111 mode; this is equivalent to setting the PCRE_UTF8 option.
1112 <a name="subpattern"></a></P>
1113 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1114 <P>
1115 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1116 Turning part of a pattern into a subpattern does two things:
1117 <br>
1118 <br>
1119 1. It localizes a set of alternatives. For example, the pattern
1120 <pre>
1121 cat(aract|erpillar|)
1122 </pre>
1123 matches one of the words "cat", "cataract", or "caterpillar". Without the
1124 parentheses, it would match "cataract", "erpillar" or an empty string.
1125 <br>
1126 <br>
1127 2. It sets up the subpattern as a capturing subpattern. This means that, when
1128 the whole pattern matches, that portion of the subject string that matched the
1129 subpattern is passed back to the caller via the <i>ovector</i> argument of
1130 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1131 from 1) to obtain numbers for the capturing subpatterns.
1132 </P>
1133 <P>
1134 For example, if the string "the red king" is matched against the pattern
1135 <pre>
1136 the ((red|white) (king|queen))
1137 </pre>
1138 the captured substrings are "red king", "red", and "king", and are numbered 1,
1139 2, and 3, respectively.
1140 </P>
1141 <P>
1142 The fact that plain parentheses fulfil two functions is not always helpful.
1143 There are often times when a grouping subpattern is required without a
1144 capturing requirement. If an opening parenthesis is followed by a question mark
1145 and a colon, the subpattern does not do any capturing, and is not counted when
1146 computing the number of any subsequent capturing subpatterns. For example, if
1147 the string "the white queen" is matched against the pattern
1148 <pre>
1149 the ((?:red|white) (king|queen))
1150 </pre>
1151 the captured substrings are "white queen" and "queen", and are numbered 1 and
1152 2. The maximum number of capturing subpatterns is 65535.
1153 </P>
1154 <P>
1155 As a convenient shorthand, if any option settings are required at the start of
1156 a non-capturing subpattern, the option letters may appear between the "?" and
1157 the ":". Thus the two patterns
1158 <pre>
1159 (?i:saturday|sunday)
1160 (?:(?i)saturday|sunday)
1161 </pre>
1162 match exactly the same set of strings. Because alternative branches are tried
1163 from left to right, and options are not reset until the end of the subpattern
1164 is reached, an option setting in one branch does affect subsequent branches, so
1165 the above patterns match "SUNDAY" as well as "Saturday".
1166 <a name="dupsubpatternnumber"></a></P>
1167 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1168 <P>
1169 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1170 the same numbers for its capturing parentheses. Such a subpattern starts with
1171 (?| and is itself a non-capturing subpattern. For example, consider this
1172 pattern:
1173 <pre>
1174 (?|(Sat)ur|(Sun))day
1175 </pre>
1176 Because the two alternatives are inside a (?| group, both sets of capturing
1177 parentheses are numbered one. Thus, when the pattern matches, you can look
1178 at captured substring number one, whichever alternative matched. This construct
1179 is useful when you want to capture part, but not all, of one of a number of
1180 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1181 number is reset at the start of each branch. The numbers of any capturing
1182 buffers that follow the subpattern start after the highest number used in any
1183 branch. The following example is taken from the Perl documentation.
1184 The numbers underneath show in which buffer the captured content will be
1185 stored.
1186 <pre>
1187 # before ---------------branch-reset----------- after
1188 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1189 # 1 2 2 3 2 3 4
1190 </pre>
1191 A back reference to a numbered subpattern uses the most recent value that is
1192 set for that number by any subpattern. The following pattern matches "abcabc"
1193 or "defdef":
1194 <pre>
1195 /(?|(abc)|(def))\1/
1196 </pre>
1197 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1198 refers to the first one in the pattern with the given number. The following
1199 pattern matches "abcabc" or "defabc":
1200 <pre>
1201 /(?|(abc)|(def))(?1)/
1202 </pre>
1203 If a
1204 <a href="#conditions">condition test</a>
1205 for a subpattern's having matched refers to a non-unique number, the test is
1206 true if any of the subpatterns of that number have matched.
1207 </P>
1208 <P>
1209 An alternative approach to using this "branch reset" feature is to use
1210 duplicate named subpatterns, as described in the next section.
1211 </P>
1212 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1213 <P>
1214 Identifying capturing parentheses by number is simple, but it can be very hard
1215 to keep track of the numbers in complicated regular expressions. Furthermore,
1216 if an expression is modified, the numbers may change. To help with this
1217 difficulty, PCRE supports the naming of subpatterns. This feature was not
1218 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1219 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1220 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1221 have different names, but PCRE does not.
1222 </P>
1223 <P>
1224 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1225 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1226 parentheses from other parts of the pattern, such as
1227 <a href="#backreferences">back references,</a>
1228 <a href="#recursion">recursion,</a>
1229 and
1230 <a href="#conditions">conditions,</a>
1231 can be made by name as well as by number.
1232 </P>
1233 <P>
1234 Names consist of up to 32 alphanumeric characters and underscores. Named
1235 capturing parentheses are still allocated numbers as well as names, exactly as
1236 if the names were not present. The PCRE API provides function calls for
1237 extracting the name-to-number translation table from a compiled pattern. There
1238 is also a convenience function for extracting a captured substring by name.
1239 </P>
1240 <P>
1241 By default, a name must be unique within a pattern, but it is possible to relax
1242 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1243 names are also always permitted for subpatterns with the same number, set up as
1244 described in the previous section.) Duplicate names can be useful for patterns
1245 where only one instance of the named parentheses can match. Suppose you want to
1246 match the name of a weekday, either as a 3-letter abbreviation or as the full
1247 name, and in both cases you want to extract the abbreviation. This pattern
1248 (ignoring the line breaks) does the job:
1249 <pre>
1250 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1251 (?&#60;DN&#62;Tue)(?:sday)?|
1252 (?&#60;DN&#62;Wed)(?:nesday)?|
1253 (?&#60;DN&#62;Thu)(?:rsday)?|
1254 (?&#60;DN&#62;Sat)(?:urday)?
1255 </pre>
1256 There are five capturing substrings, but only one is ever set after a match.
1257 (An alternative way of solving this problem is to use a "branch reset"
1258 subpattern, as described in the previous section.)
1259 </P>
1260 <P>
1261 The convenience function for extracting the data by name returns the substring
1262 for the first (and in this example, the only) subpattern of that name that
1263 matched. This saves searching to find which numbered subpattern it was.
1264 </P>
1265 <P>
1266 If you make a back reference to a non-unique named subpattern from elsewhere in
1267 the pattern, the one that corresponds to the first occurrence of the name is
1268 used. In the absence of duplicate numbers (see the previous section) this is
1269 the one with the lowest number. If you use a named reference in a condition
1270 test (see the
1271 <a href="#conditions">section about conditions</a>
1272 below), either to check whether a subpattern has matched, or to check for
1273 recursion, all subpatterns with the same name are tested. If the condition is
1274 true for any one of them, the overall condition is true. This is the same
1275 behaviour as testing by number. For further details of the interfaces for
1276 handling named subpatterns, see the
1277 <a href="pcreapi.html"><b>pcreapi</b></a>
1278 documentation.
1279 </P>
1280 <P>
1281 <b>Warning:</b> You cannot use different names to distinguish between two
1282 subpatterns with the same number because PCRE uses only the numbers when
1283 matching. For this reason, an error is given at compile time if different names
1284 are given to subpatterns with the same number. However, you can give the same
1285 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1286 </P>
1287 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1288 <P>
1289 Repetition is specified by quantifiers, which can follow any of the following
1290 items:
1291 <pre>
1292 a literal data character
1293 the dot metacharacter
1294 the \C escape sequence
1295 the \X escape sequence (in UTF-8 mode with Unicode properties)
1296 the \R escape sequence
1297 an escape such as \d that matches a single character
1298 a character class
1299 a back reference (see next section)
1300 a parenthesized subpattern (unless it is an assertion)
1301 a recursive or "subroutine" call to a subpattern
1302 </pre>
1303 The general repetition quantifier specifies a minimum and maximum number of
1304 permitted matches, by giving the two numbers in curly brackets (braces),
1305 separated by a comma. The numbers must be less than 65536, and the first must
1306 be less than or equal to the second. For example:
1307 <pre>
1308 z{2,4}
1309 </pre>
1310 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1311 character. If the second number is omitted, but the comma is present, there is
1312 no upper limit; if the second number and the comma are both omitted, the
1313 quantifier specifies an exact number of required matches. Thus
1314 <pre>
1315 [aeiou]{3,}
1316 </pre>
1317 matches at least 3 successive vowels, but may match many more, while
1318 <pre>
1319 \d{8}
1320 </pre>
1321 matches exactly 8 digits. An opening curly bracket that appears in a position
1322 where a quantifier is not allowed, or one that does not match the syntax of a
1323 quantifier, is taken as a literal character. For example, {,6} is not a
1324 quantifier, but a literal string of four characters.
1325 </P>
1326 <P>
1327 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1328 bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1329 which is represented by a two-byte sequence. Similarly, when Unicode property
1330 support is available, \X{3} matches three Unicode extended sequences, each of
1331 which may be several bytes long (and they may be of different lengths).
1332 </P>
1333 <P>
1334 The quantifier {0} is permitted, causing the expression to behave as if the
1335 previous item and the quantifier were not present. This may be useful for
1336 subpatterns that are referenced as
1337 <a href="#subpatternsassubroutines">subroutines</a>
1338 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1339 quantifier are omitted from the compiled pattern.
1340 </P>
1341 <P>
1342 For convenience, the three most common quantifiers have single-character
1343 abbreviations:
1344 <pre>
1345 * is equivalent to {0,}
1346 + is equivalent to {1,}
1347 ? is equivalent to {0,1}
1348 </pre>
1349 It is possible to construct infinite loops by following a subpattern that can
1350 match no characters with a quantifier that has no upper limit, for example:
1351 <pre>
1352 (a?)*
1353 </pre>
1354 Earlier versions of Perl and PCRE used to give an error at compile time for
1355 such patterns. However, because there are cases where this can be useful, such
1356 patterns are now accepted, but if any repetition of the subpattern does in fact
1357 match no characters, the loop is forcibly broken.
1358 </P>
1359 <P>
1360 By default, the quantifiers are "greedy", that is, they match as much as
1361 possible (up to the maximum number of permitted times), without causing the
1362 rest of the pattern to fail. The classic example of where this gives problems
1363 is in trying to match comments in C programs. These appear between /* and */
1364 and within the comment, individual * and / characters may appear. An attempt to
1365 match C comments by applying the pattern
1366 <pre>
1367 /\*.*\*/
1368 </pre>
1369 to the string
1370 <pre>
1371 /* first comment */ not comment /* second comment */
1372 </pre>
1373 fails, because it matches the entire string owing to the greediness of the .*
1374 item.
1375 </P>
1376 <P>
1377 However, if a quantifier is followed by a question mark, it ceases to be
1378 greedy, and instead matches the minimum number of times possible, so the
1379 pattern
1380 <pre>
1381 /\*.*?\*/
1382 </pre>
1383 does the right thing with the C comments. The meaning of the various
1384 quantifiers is not otherwise changed, just the preferred number of matches.
1385 Do not confuse this use of question mark with its use as a quantifier in its
1386 own right. Because it has two uses, it can sometimes appear doubled, as in
1387 <pre>
1388 \d??\d
1389 </pre>
1390 which matches one digit by preference, but can match two if that is the only
1391 way the rest of the pattern matches.
1392 </P>
1393 <P>
1394 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1395 the quantifiers are not greedy by default, but individual ones can be made
1396 greedy by following them with a question mark. In other words, it inverts the
1397 default behaviour.
1398 </P>
1399 <P>
1400 When a parenthesized subpattern is quantified with a minimum repeat count that
1401 is greater than 1 or with a limited maximum, more memory is required for the
1402 compiled pattern, in proportion to the size of the minimum or maximum.
1403 </P>
1404 <P>
1405 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1406 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1407 implicitly anchored, because whatever follows will be tried against every
1408 character position in the subject string, so there is no point in retrying the
1409 overall match at any position after the first. PCRE normally treats such a
1410 pattern as though it were preceded by \A.
1411 </P>
1412 <P>
1413 In cases where it is known that the subject string contains no newlines, it is
1414 worth setting PCRE_DOTALL in order to obtain this optimization, or
1415 alternatively using ^ to indicate anchoring explicitly.
1416 </P>
1417 <P>
1418 However, there is one situation where the optimization cannot be used. When .*
1419 is inside capturing parentheses that are the subject of a back reference
1420 elsewhere in the pattern, a match at the start may fail where a later one
1421 succeeds. Consider, for example:
1422 <pre>
1423 (.*)abc\1
1424 </pre>
1425 If the subject is "xyz123abc123" the match point is the fourth character. For
1426 this reason, such a pattern is not implicitly anchored.
1427 </P>
1428 <P>
1429 When a capturing subpattern is repeated, the value captured is the substring
1430 that matched the final iteration. For example, after
1431 <pre>
1432 (tweedle[dume]{3}\s*)+
1433 </pre>
1434 has matched "tweedledum tweedledee" the value of the captured substring is
1435 "tweedledee". However, if there are nested capturing subpatterns, the
1436 corresponding captured values may have been set in previous iterations. For
1437 example, after
1438 <pre>
1439 /(a|(b))+/
1440 </pre>
1441 matches "aba" the value of the second captured substring is "b".
1442 <a name="atomicgroup"></a></P>
1443 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1444 <P>
1445 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1446 repetition, failure of what follows normally causes the repeated item to be
1447 re-evaluated to see if a different number of repeats allows the rest of the
1448 pattern to match. Sometimes it is useful to prevent this, either to change the
1449 nature of the match, or to cause it fail earlier than it otherwise might, when
1450 the author of the pattern knows there is no point in carrying on.
1451 </P>
1452 <P>
1453 Consider, for example, the pattern \d+foo when applied to the subject line
1454 <pre>
1455 123456bar
1456 </pre>
1457 After matching all 6 digits and then failing to match "foo", the normal
1458 action of the matcher is to try again with only 5 digits matching the \d+
1459 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1460 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1461 that once a subpattern has matched, it is not to be re-evaluated in this way.
1462 </P>
1463 <P>
1464 If we use atomic grouping for the previous example, the matcher gives up
1465 immediately on failing to match "foo" the first time. The notation is a kind of
1466 special parenthesis, starting with (?&#62; as in this example:
1467 <pre>
1468 (?&#62;\d+)foo
1469 </pre>
1470 This kind of parenthesis "locks up" the part of the pattern it contains once
1471 it has matched, and a failure further into the pattern is prevented from
1472 backtracking into it. Backtracking past it to previous items, however, works as
1473 normal.
1474 </P>
1475 <P>
1476 An alternative description is that a subpattern of this type matches the string
1477 of characters that an identical standalone pattern would match, if anchored at
1478 the current point in the subject string.
1479 </P>
1480 <P>
1481 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1482 the above example can be thought of as a maximizing repeat that must swallow
1483 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1484 number of digits they match in order to make the rest of the pattern match,
1485 (?&#62;\d+) can only match an entire sequence of digits.
1486 </P>
1487 <P>
1488 Atomic groups in general can of course contain arbitrarily complicated
1489 subpatterns, and can be nested. However, when the subpattern for an atomic
1490 group is just a single repeated item, as in the example above, a simpler
1491 notation, called a "possessive quantifier" can be used. This consists of an
1492 additional + character following a quantifier. Using this notation, the
1493 previous example can be rewritten as
1494 <pre>
1495 \d++foo
1496 </pre>
1497 Note that a possessive quantifier can be used with an entire group, for
1498 example:
1499 <pre>
1500 (abc|xyz){2,3}+
1501 </pre>
1502 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1503 option is ignored. They are a convenient notation for the simpler forms of
1504 atomic group. However, there is no difference in the meaning of a possessive
1505 quantifier and the equivalent atomic group, though there may be a performance
1506 difference; possessive quantifiers should be slightly faster.
1507 </P>
1508 <P>
1509 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1510 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1511 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1512 package, and PCRE copied it from there. It ultimately found its way into Perl
1513 at release 5.10.
1514 </P>
1515 <P>
1516 PCRE has an optimization that automatically "possessifies" certain simple
1517 pattern constructs. For example, the sequence A+B is treated as A++B because
1518 there is no point in backtracking into a sequence of A's when B must follow.
1519 </P>
1520 <P>
1521 When a pattern contains an unlimited repeat inside a subpattern that can itself
1522 be repeated an unlimited number of times, the use of an atomic group is the
1523 only way to avoid some failing matches taking a very long time indeed. The
1524 pattern
1525 <pre>
1526 (\D+|&#60;\d+&#62;)*[!?]
1527 </pre>
1528 matches an unlimited number of substrings that either consist of non-digits, or
1529 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1530 quickly. However, if it is applied to
1531 <pre>
1532 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1533 </pre>
1534 it takes a long time before reporting failure. This is because the string can
1535 be divided between the internal \D+ repeat and the external * repeat in a
1536 large number of ways, and all have to be tried. (The example uses [!?] rather
1537 than a single character at the end, because both PCRE and Perl have an
1538 optimization that allows for fast failure when a single character is used. They
1539 remember the last single character that is required for a match, and fail early
1540 if it is not present in the string.) If the pattern is changed so that it uses
1541 an atomic group, like this:
1542 <pre>
1543 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1544 </pre>
1545 sequences of non-digits cannot be broken, and failure happens quickly.
1546 <a name="backreferences"></a></P>
1547 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1548 <P>
1549 Outside a character class, a backslash followed by a digit greater than 0 (and
1550 possibly further digits) is a back reference to a capturing subpattern earlier
1551 (that is, to its left) in the pattern, provided there have been that many
1552 previous capturing left parentheses.
1553 </P>
1554 <P>
1555 However, if the decimal number following the backslash is less than 10, it is
1556 always taken as a back reference, and causes an error only if there are not
1557 that many capturing left parentheses in the entire pattern. In other words, the
1558 parentheses that are referenced need not be to the left of the reference for
1559 numbers less than 10. A "forward back reference" of this type can make sense
1560 when a repetition is involved and the subpattern to the right has participated
1561 in an earlier iteration.
1562 </P>
1563 <P>
1564 It is not possible to have a numerical "forward back reference" to a subpattern
1565 whose number is 10 or more using this syntax because a sequence such as \50 is
1566 interpreted as a character defined in octal. See the subsection entitled
1567 "Non-printing characters"
1568 <a href="#digitsafterbackslash">above</a>
1569 for further details of the handling of digits following a backslash. There is
1570 no such problem when named parentheses are used. A back reference to any
1571 subpattern is possible using named parentheses (see below).
1572 </P>
1573 <P>
1574 Another way of avoiding the ambiguity inherent in the use of digits following a
1575 backslash is to use the \g escape sequence, which is a feature introduced in
1576 Perl 5.10. This escape must be followed by an unsigned number or a negative
1577 number, optionally enclosed in braces. These examples are all identical:
1578 <pre>
1579 (ring), \1
1580 (ring), \g1
1581 (ring), \g{1}
1582 </pre>
1583 An unsigned number specifies an absolute reference without the ambiguity that
1584 is present in the older syntax. It is also useful when literal digits follow
1585 the reference. A negative number is a relative reference. Consider this
1586 example:
1587 <pre>
1588 (abc(def)ghi)\g{-1}
1589 </pre>
1590 The sequence \g{-1} is a reference to the most recently started capturing
1591 subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1592 would be equivalent to \1. The use of relative references can be helpful in
1593 long patterns, and also in patterns that are created by joining together
1594 fragments that contain references within themselves.
1595 </P>
1596 <P>
1597 A back reference matches whatever actually matched the capturing subpattern in
1598 the current subject string, rather than anything matching the subpattern
1599 itself (see
1600 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1601 below for a way of doing that). So the pattern
1602 <pre>
1603 (sens|respons)e and \1ibility
1604 </pre>
1605 matches "sense and sensibility" and "response and responsibility", but not
1606 "sense and responsibility". If caseful matching is in force at the time of the
1607 back reference, the case of letters is relevant. For example,
1608 <pre>
1609 ((?i)rah)\s+\1
1610 </pre>
1611 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1612 capturing subpattern is matched caselessly.
1613 </P>
1614 <P>
1615 There are several different ways of writing back references to named
1616 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1617 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1618 back reference syntax, in which \g can be used for both numeric and named
1619 references, is also supported. We could rewrite the above example in any of
1620 the following ways:
1621 <pre>
1622 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1623 (?'p1'(?i)rah)\s+\k{p1}
1624 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1625 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1626 </pre>
1627 A subpattern that is referenced by name may appear in the pattern before or
1628 after the reference.
1629 </P>
1630 <P>
1631 There may be more than one back reference to the same subpattern. If a
1632 subpattern has not actually been used in a particular match, any back
1633 references to it always fail by default. For example, the pattern
1634 <pre>
1635 (a|(bc))\2
1636 </pre>
1637 always fails if it starts to match "a" rather than "bc". However, if the
1638 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1639 unset value matches an empty string.
1640 </P>
1641 <P>
1642 Because there may be many capturing parentheses in a pattern, all digits
1643 following a backslash are taken as part of a potential back reference number.
1644 If the pattern continues with a digit character, some delimiter must be used to
1645 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1646 whitespace. Otherwise, the \g{ syntax or an empty comment (see
1647 <a href="#comments">"Comments"</a>
1648 below) can be used.
1649 </P>
1650 <br><b>
1651 Recursive back references
1652 </b><br>
1653 <P>
1654 A back reference that occurs inside the parentheses to which it refers fails
1655 when the subpattern is first used, so, for example, (a\1) never matches.
1656 However, such references can be useful inside repeated subpatterns. For
1657 example, the pattern
1658 <pre>
1659 (a|b\1)+
1660 </pre>
1661 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1662 the subpattern, the back reference matches the character string corresponding
1663 to the previous iteration. In order for this to work, the pattern must be such
1664 that the first iteration does not need to match the back reference. This can be
1665 done using alternation, as in the example above, or by a quantifier with a
1666 minimum of zero.
1667 </P>
1668 <P>
1669 Back references of this type cause the group that they reference to be treated
1670 as an
1671 <a href="#atomicgroup">atomic group.</a>
1672 Once the whole group has been matched, a subsequent matching failure cannot
1673 cause backtracking into the middle of the group.
1674 <a name="bigassertions"></a></P>
1675 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1676 <P>
1677 An assertion is a test on the characters following or preceding the current
1678 matching point that does not actually consume any characters. The simple
1679 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1680 <a href="#smallassertions">above.</a>
1681 </P>
1682 <P>
1683 More complicated assertions are coded as subpatterns. There are two kinds:
1684 those that look ahead of the current position in the subject string, and those
1685 that look behind it. An assertion subpattern is matched in the normal way,
1686 except that it does not cause the current matching position to be changed.
1687 </P>
1688 <P>
1689 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1690 because it makes no sense to assert the same thing several times. If any kind
1691 of assertion contains capturing subpatterns within it, these are counted for
1692 the purposes of numbering the capturing subpatterns in the whole pattern.
1693 However, substring capturing is carried out only for positive assertions,
1694 because it does not make sense for negative assertions.
1695 </P>
1696 <br><b>
1697 Lookahead assertions
1698 </b><br>
1699 <P>
1700 Lookahead assertions start with (?= for positive assertions and (?! for
1701 negative assertions. For example,
1702 <pre>
1703 \w+(?=;)
1704 </pre>
1705 matches a word followed by a semicolon, but does not include the semicolon in
1706 the match, and
1707 <pre>
1708 foo(?!bar)
1709 </pre>
1710 matches any occurrence of "foo" that is not followed by "bar". Note that the
1711 apparently similar pattern
1712 <pre>
1713 (?!foo)bar
1714 </pre>
1715 does not find an occurrence of "bar" that is preceded by something other than
1716 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1717 (?!foo) is always true when the next three characters are "bar". A
1718 lookbehind assertion is needed to achieve the other effect.
1719 </P>
1720 <P>
1721 If you want to force a matching failure at some point in a pattern, the most
1722 convenient way to do it is with (?!) because an empty string always matches, so
1723 an assertion that requires there not to be an empty string must always fail.
1724 The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1725 synonym for (?!).
1726 <a name="lookbehind"></a></P>
1727 <br><b>
1728 Lookbehind assertions
1729 </b><br>
1730 <P>
1731 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1732 negative assertions. For example,
1733 <pre>
1734 (?&#60;!foo)bar
1735 </pre>
1736 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1737 a lookbehind assertion are restricted such that all the strings it matches must
1738 have a fixed length. However, if there are several top-level alternatives, they
1739 do not all have to have the same fixed length. Thus
1740 <pre>
1741 (?&#60;=bullock|donkey)
1742 </pre>
1743 is permitted, but
1744 <pre>
1745 (?&#60;!dogs?|cats?)
1746 </pre>
1747 causes an error at compile time. Branches that match different length strings
1748 are permitted only at the top level of a lookbehind assertion. This is an
1749 extension compared with Perl (5.8 and 5.10), which requires all branches to
1750 match the same length of string. An assertion such as
1751 <pre>
1752 (?&#60;=ab(c|de))
1753 </pre>
1754 is not permitted, because its single top-level branch can match two different
1755 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1756 branches:
1757 <pre>
1758 (?&#60;=abc|abde)
1759 </pre>
1760 In some cases, the Perl 5.10 escape sequence \K
1761 <a href="#resetmatchstart">(see above)</a>
1762 can be used instead of a lookbehind assertion to get round the fixed-length
1763 restriction.
1764 </P>
1765 <P>
1766 The implementation of lookbehind assertions is, for each alternative, to
1767 temporarily move the current position back by the fixed length and then try to
1768 match. If there are insufficient characters before the current position, the
1769 assertion fails.
1770 </P>
1771 <P>
1772 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1773 to appear in lookbehind assertions, because it makes it impossible to calculate
1774 the length of the lookbehind. The \X and \R escapes, which can match
1775 different numbers of bytes, are also not permitted.
1776 </P>
1777 <P>
1778 <a href="#subpatternsassubroutines">"Subroutine"</a>
1779 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1780 as the subpattern matches a fixed-length string.
1781 <a href="#recursion">Recursion,</a>
1782 however, is not supported.
1783 </P>
1784 <P>
1785 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1786 specify efficient matching of fixed-length strings at the end of subject
1787 strings. Consider a simple pattern such as
1788 <pre>
1789 abcd$
1790 </pre>
1791 when applied to a long string that does not match. Because matching proceeds
1792 from left to right, PCRE will look for each "a" in the subject and then see if
1793 what follows matches the rest of the pattern. If the pattern is specified as
1794 <pre>
1795 ^.*abcd$
1796 </pre>
1797 the initial .* matches the entire string at first, but when this fails (because
1798 there is no following "a"), it backtracks to match all but the last character,
1799 then all but the last two characters, and so on. Once again the search for "a"
1800 covers the entire string, from right to left, so we are no better off. However,
1801 if the pattern is written as
1802 <pre>
1803 ^.*+(?&#60;=abcd)
1804 </pre>
1805 there can be no backtracking for the .*+ item; it can match only the entire
1806 string. The subsequent lookbehind assertion does a single test on the last four
1807 characters. If it fails, the match fails immediately. For long strings, this
1808 approach makes a significant difference to the processing time.
1809 </P>
1810 <br><b>
1811 Using multiple assertions
1812 </b><br>
1813 <P>
1814 Several assertions (of any sort) may occur in succession. For example,
1815 <pre>
1816 (?&#60;=\d{3})(?&#60;!999)foo
1817 </pre>
1818 matches "foo" preceded by three digits that are not "999". Notice that each of
1819 the assertions is applied independently at the same point in the subject
1820 string. First there is a check that the previous three characters are all
1821 digits, and then there is a check that the same three characters are not "999".
1822 This pattern does <i>not</i> match "foo" preceded by six characters, the first
1823 of which are digits and the last three of which are not "999". For example, it
1824 doesn't match "123abcfoo". A pattern to do that is
1825 <pre>
1826 (?&#60;=\d{3}...)(?&#60;!999)foo
1827 </pre>
1828 This time the first assertion looks at the preceding six characters, checking
1829 that the first three are digits, and then the second assertion checks that the
1830 preceding three characters are not "999".
1831 </P>
1832 <P>
1833 Assertions can be nested in any combination. For example,
1834 <pre>
1835 (?&#60;=(?&#60;!foo)bar)baz
1836 </pre>
1837 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1838 preceded by "foo", while
1839 <pre>
1840 (?&#60;=\d{3}(?!999)...)foo
1841 </pre>
1842 is another pattern that matches "foo" preceded by three digits and any three
1843 characters that are not "999".
1844 <a name="conditions"></a></P>
1845 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1846 <P>
1847 It is possible to cause the matching process to obey a subpattern
1848 conditionally or to choose between two alternative subpatterns, depending on
1849 the result of an assertion, or whether a specific capturing subpattern has
1850 already been matched. The two possible forms of conditional subpattern are:
1851 <pre>
1852 (?(condition)yes-pattern)
1853 (?(condition)yes-pattern|no-pattern)
1854 </pre>
1855 If the condition is satisfied, the yes-pattern is used; otherwise the
1856 no-pattern (if present) is used. If there are more than two alternatives in the
1857 subpattern, a compile-time error occurs.
1858 </P>
1859 <P>
1860 There are four kinds of condition: references to subpatterns, references to
1861 recursion, a pseudo-condition called DEFINE, and assertions.
1862 </P>
1863 <br><b>
1864 Checking for a used subpattern by number
1865 </b><br>
1866 <P>
1867 If the text between the parentheses consists of a sequence of digits, the
1868 condition is true if a capturing subpattern of that number has previously
1869 matched. If there is more than one capturing subpattern with the same number
1870 (see the earlier
1871 <a href="#recursion">section about duplicate subpattern numbers),</a>
1872 the condition is true if any of them have been set. An alternative notation is
1873 to precede the digits with a plus or minus sign. In this case, the subpattern
1874 number is relative rather than absolute. The most recently opened parentheses
1875 can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1876 looping constructs it can also make sense to refer to subsequent groups with
1877 constructs such as (?(+2).
1878 </P>
1879 <P>
1880 Consider the following pattern, which contains non-significant white space to
1881 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1882 three parts for ease of discussion:
1883 <pre>
1884 ( \( )? [^()]+ (?(1) \) )
1885 </pre>
1886 The first part matches an optional opening parenthesis, and if that
1887 character is present, sets it as the first captured substring. The second part
1888 matches one or more characters that are not parentheses. The third part is a
1889 conditional subpattern that tests whether the first set of parentheses matched
1890 or not. If they did, that is, if subject started with an opening parenthesis,
1891 the condition is true, and so the yes-pattern is executed and a closing
1892 parenthesis is required. Otherwise, since no-pattern is not present, the
1893 subpattern matches nothing. In other words, this pattern matches a sequence of
1894 non-parentheses, optionally enclosed in parentheses.
1895 </P>
1896 <P>
1897 If you were embedding this pattern in a larger one, you could use a relative
1898 reference:
1899 <pre>
1900 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
1901 </pre>
1902 This makes the fragment independent of the parentheses in the larger pattern.
1903 </P>
1904 <br><b>
1905 Checking for a used subpattern by name
1906 </b><br>
1907 <P>
1908 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1909 subpattern by name. For compatibility with earlier versions of PCRE, which had
1910 this facility before Perl, the syntax (?(name)...) is also recognized. However,
1911 there is a possible ambiguity with this syntax, because subpattern names may
1912 consist entirely of digits. PCRE looks first for a named subpattern; if it
1913 cannot find one and the name consists entirely of digits, PCRE looks for a
1914 subpattern of that number, which must be greater than zero. Using subpattern
1915 names that consist entirely of digits is not recommended.
1916 </P>
1917 <P>
1918 Rewriting the above example to use a named subpattern gives this:
1919 <pre>
1920 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
1921 </pre>
1922 If the name used in a condition of this kind is a duplicate, the test is
1923 applied to all subpatterns of the same name, and is true if any one of them has
1924 matched.
1925 </P>
1926 <br><b>
1927 Checking for pattern recursion
1928 </b><br>
1929 <P>
1930 If the condition is the string (R), and there is no subpattern with the name R,
1931 the condition is true if a recursive call to the whole pattern or any
1932 subpattern has been made. If digits or a name preceded by ampersand follow the
1933 letter R, for example:
1934 <pre>
1935 (?(R3)...) or (?(R&name)...)
1936 </pre>
1937 the condition is true if the most recent recursion is into a subpattern whose
1938 number or name is given. This condition does not check the entire recursion
1939 stack. If the name used in a condition of this kind is a duplicate, the test is
1940 applied to all subpatterns of the same name, and is true if any one of them is
1941 the most recent recursion.
1942 </P>
1943 <P>
1944 At "top level", all these recursion test conditions are false.
1945 <a href="#recursion">The syntax for recursive patterns</a>
1946 is described below.
1947 </P>
1948 <br><b>
1949 Defining subpatterns for use by reference only
1950 </b><br>
1951 <P>
1952 If the condition is the string (DEFINE), and there is no subpattern with the
1953 name DEFINE, the condition is always false. In this case, there may be only one
1954 alternative in the subpattern. It is always skipped if control reaches this
1955 point in the pattern; the idea of DEFINE is that it can be used to define
1956 "subroutines" that can be referenced from elsewhere. (The use of
1957 <a href="#subpatternsassubroutines">"subroutines"</a>
1958 is described below.) For example, a pattern to match an IPv4 address could be
1959 written like this (ignore whitespace and line breaks):
1960 <pre>
1961 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1962 \b (?&byte) (\.(?&byte)){3} \b
1963 </pre>
1964 The first part of the pattern is a DEFINE group inside which a another group
1965 named "byte" is defined. This matches an individual component of an IPv4
1966 address (a number less than 256). When matching takes place, this part of the
1967 pattern is skipped because DEFINE acts like a false condition. The rest of the
1968 pattern uses references to the named group to match the four dot-separated
1969 components of an IPv4 address, insisting on a word boundary at each end.
1970 </P>
1971 <br><b>
1972 Assertion conditions
1973 </b><br>
1974 <P>
1975 If the condition is not in any of the above formats, it must be an assertion.
1976 This may be a positive or negative lookahead or lookbehind assertion. Consider
1977 this pattern, again containing non-significant white space, and with the two
1978 alternatives on the second line:
1979 <pre>
1980 (?(?=[^a-z]*[a-z])
1981 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1982 </pre>
1983 The condition is a positive lookahead assertion that matches an optional
1984 sequence of non-letters followed by a letter. In other words, it tests for the
1985 presence of at least one letter in the subject. If a letter is found, the
1986 subject is matched against the first alternative; otherwise it is matched
1987 against the second. This pattern matches strings in one of the two forms
1988 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1989 <a name="comments"></a></P>
1990 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
1991 <P>
1992 The sequence (?# marks the start of a comment that continues up to the next
1993 closing parenthesis. Nested parentheses are not permitted. The characters
1994 that make up a comment play no part in the pattern matching at all.
1995 </P>
1996 <P>
1997 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1998 character class introduces a comment that continues to immediately after the
1999 next newline in the pattern.
2000 <a name="recursion"></a></P>
2001 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2002 <P>
2003 Consider the problem of matching a string in parentheses, allowing for
2004 unlimited nested parentheses. Without the use of recursion, the best that can
2005 be done is to use a pattern that matches up to some fixed depth of nesting. It
2006 is not possible to handle an arbitrary nesting depth.
2007 </P>
2008 <P>
2009 For some time, Perl has provided a facility that allows regular expressions to
2010 recurse (amongst other things). It does this by interpolating Perl code in the
2011 expression at run time, and the code can refer to the expression itself. A Perl
2012 pattern using code interpolation to solve the parentheses problem can be
2013 created like this:
2014 <pre>
2015 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2016 </pre>
2017 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2018 recursively to the pattern in which it appears.
2019 </P>
2020 <P>
2021 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2022 supports special syntax for recursion of the entire pattern, and also for
2023 individual subpattern recursion. After its introduction in PCRE and Python,
2024 this kind of recursion was subsequently introduced into Perl at release 5.10.
2025 </P>
2026 <P>
2027 A special item that consists of (? followed by a number greater than zero and a
2028 closing parenthesis is a recursive call of the subpattern of the given number,
2029 provided that it occurs inside that subpattern. (If not, it is a
2030 <a href="#subpatternsassubroutines">"subroutine"</a>
2031 call, which is described in the next section.) The special item (?R) or (?0) is
2032 a recursive call of the entire regular expression.
2033 </P>
2034 <P>
2035 This PCRE pattern solves the nested parentheses problem (assume the
2036 PCRE_EXTENDED option is set so that white space is ignored):
2037 <pre>
2038 \( ( [^()]++ | (?R) )* \)
2039 </pre>
2040 First it matches an opening parenthesis. Then it matches any number of
2041 substrings which can either be a sequence of non-parentheses, or a recursive
2042 match of the pattern itself (that is, a correctly parenthesized substring).
2043 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2044 to avoid backtracking into sequences of non-parentheses.
2045 </P>
2046 <P>
2047 If this were part of a larger pattern, you would not want to recurse the entire
2048 pattern, so instead you could use this:
2049 <pre>
2050 ( \( ( [^()]++ | (?1) )* \) )
2051 </pre>
2052 We have put the pattern into parentheses, and caused the recursion to refer to
2053 them instead of the whole pattern.
2054 </P>
2055 <P>
2056 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2057 is made easier by the use of relative references (a Perl 5.10 feature).
2058 Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2059 most recently opened parentheses preceding the recursion. In other words, a
2060 negative number counts capturing parentheses leftwards from the point at which
2061 it is encountered.
2062 </P>
2063 <P>
2064 It is also possible to refer to subsequently opened parentheses, by writing
2065 references such as (?+2). However, these cannot be recursive because the
2066 reference is not inside the parentheses that are referenced. They are always
2067 <a href="#subpatternsassubroutines">"subroutine"</a>
2068 calls, as described in the next section.
2069 </P>
2070 <P>
2071 An alternative approach is to use named parentheses instead. The Perl syntax
2072 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2073 could rewrite the above example as follows:
2074 <pre>
2075 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2076 </pre>
2077 If there is more than one subpattern with the same name, the earliest one is
2078 used.
2079 </P>
2080 <P>
2081 This particular example pattern that we have been looking at contains nested
2082 unlimited repeats, and so the use of a possessive quantifier for matching
2083 strings of non-parentheses is important when applying the pattern to strings
2084 that do not match. For example, when this pattern is applied to
2085 <pre>
2086 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2087 </pre>
2088 it yields "no match" quickly. However, if a possessive quantifier is not used,
2089 the match runs for a very long time indeed because there are so many different
2090 ways the + and * repeats can carve up the subject, and all have to be tested
2091 before failure can be reported.
2092 </P>
2093 <P>
2094 At the end of a match, the values of capturing parentheses are those from
2095 the outermost level. If you want to obtain intermediate values, a callout
2096 function can be used (see below and the
2097 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2098 documentation). If the pattern above is matched against
2099 <pre>
2100 (ab(cd)ef)
2101 </pre>
2102 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2103 the last value taken on at the top level. If a capturing subpattern is not
2104 matched at the top level, its final value is unset, even if it is (temporarily)
2105 set at a deeper level.
2106 </P>
2107 <P>
2108 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2109 obtain extra memory to store data during a recursion, which it does by using
2110 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2111 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2112 </P>
2113 <P>
2114 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2115 Consider this pattern, which matches text in angle brackets, allowing for
2116 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2117 recursing), whereas any characters are permitted at the outer level.
2118 <pre>
2119 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2120 </pre>
2121 In this pattern, (?(R) is the start of a conditional subpattern, with two
2122 different alternatives for the recursive and non-recursive cases. The (?R) item
2123 is the actual recursive call.
2124 <a name="recursiondifference"></a></P>
2125 <br><b>
2126 Recursion difference from Perl
2127 </b><br>
2128 <P>
2129 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2130 treated as an atomic group. That is, once it has matched some of the subject
2131 string, it is never re-entered, even if it contains untried alternatives and
2132 there is a subsequent matching failure. This can be illustrated by the
2133 following pattern, which purports to match a palindromic string that contains
2134 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2135 <pre>
2136 ^(.|(.)(?1)\2)$
2137 </pre>
2138 The idea is that it either matches a single character, or two identical
2139 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2140 it does not if the pattern is longer than three characters. Consider the
2141 subject string "abcba":
2142 </P>
2143 <P>
2144 At the top level, the first character is matched, but as it is not at the end
2145 of the string, the first alternative fails; the second alternative is taken
2146 and the recursion kicks in. The recursive call to subpattern 1 successfully
2147 matches the next character ("b"). (Note that the beginning and end of line
2148 tests are not part of the recursion).
2149 </P>
2150 <P>
2151 Back at the top level, the next character ("c") is compared with what
2152 subpattern 2 matched, which was "a". This fails. Because the recursion is
2153 treated as an atomic group, there are now no backtracking points, and so the
2154 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2155 try the second alternative.) However, if the pattern is written with the
2156 alternatives in the other order, things are different:
2157 <pre>
2158 ^((.)(?1)\2|.)$
2159 </pre>
2160 This time, the recursing alternative is tried first, and continues to recurse
2161 until it runs out of characters, at which point the recursion fails. But this
2162 time we do have another alternative to try at the higher level. That is the big
2163 difference: in the previous case the remaining alternative is at a deeper
2164 recursion level, which PCRE cannot use.
2165 </P>
2166 <P>
2167 To change the pattern so that matches all palindromic strings, not just those
2168 with an odd number of characters, it is tempting to change the pattern to this:
2169 <pre>
2170 ^((.)(?1)\2|.?)$
2171 </pre>
2172 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2173 deeper recursion has matched a single character, it cannot be entered again in
2174 order to match an empty string. The solution is to separate the two cases, and
2175 write out the odd and even cases as alternatives at the higher level:
2176 <pre>
2177 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2178 </pre>
2179 If you want to match typical palindromic phrases, the pattern has to ignore all
2180 non-word characters, which can be done like this:
2181 <pre>
2182 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2183 </pre>
2184 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2185 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2186 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2187 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2188 more) to match typical phrases, and Perl takes so long that you think it has
2189 gone into a loop.
2190 </P>
2191 <P>
2192 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2193 string does not start with a palindrome that is shorter than the entire string.
2194 For example, although "abcba" is correctly matched, if the subject is "ababa",
2195 PCRE finds the palindrome "aba" at the start, then fails at top level because
2196 the end of the string does not follow. Once again, it cannot jump back into the
2197 recursion to try other alternatives, so the entire match fails.
2198 <a name="subpatternsassubroutines"></a></P>
2199 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2200 <P>
2201 If the syntax for a recursive subpattern reference (either by number or by
2202 name) is used outside the parentheses to which it refers, it operates like a
2203 subroutine in a programming language. The "called" subpattern may be defined
2204 before or after the reference. A numbered reference can be absolute or
2205 relative, as in these examples:
2206 <pre>
2207 (...(absolute)...)...(?2)...
2208 (...(relative)...)...(?-1)...
2209 (...(?+1)...(relative)...
2210 </pre>
2211 An earlier example pointed out that the pattern
2212 <pre>
2213 (sens|respons)e and \1ibility
2214 </pre>
2215 matches "sense and sensibility" and "response and responsibility", but not
2216 "sense and responsibility". If instead the pattern
2217 <pre>
2218 (sens|respons)e and (?1)ibility
2219 </pre>
2220 is used, it does match "sense and responsibility" as well as the other two
2221 strings. Another example is given in the discussion of DEFINE above.
2222 </P>
2223 <P>
2224 Like recursive subpatterns, a subroutine call is always treated as an atomic
2225 group. That is, once it has matched some of the subject string, it is never
2226 re-entered, even if it contains untried alternatives and there is a subsequent
2227 matching failure. Any capturing parentheses that are set during the subroutine
2228 call revert to their previous values afterwards.
2229 </P>
2230 <P>
2231 When a subpattern is used as a subroutine, processing options such as
2232 case-independence are fixed when the subpattern is defined. They cannot be
2233 changed for different calls. For example, consider this pattern:
2234 <pre>
2235 (abc)(?i:(?-1))
2236 </pre>
2237 It matches "abcabc". It does not match "abcABC" because the change of
2238 processing option does not affect the called subpattern.
2239 <a name="onigurumasubroutines"></a></P>
2240 <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2241 <P>
2242 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2243 a number enclosed either in angle brackets or single quotes, is an alternative
2244 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2245 are two of the examples used above, rewritten using this syntax:
2246 <pre>
2247 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2248 (sens|respons)e and \g'1'ibility
2249 </pre>
2250 PCRE supports an extension to Oniguruma: if a number is preceded by a
2251 plus or a minus sign it is taken as a relative reference. For example:
2252 <pre>
2253 (abc)(?i:\g&#60;-1&#62;)
2254 </pre>
2255 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2256 synonymous. The former is a back reference; the latter is a subroutine call.
2257 </P>
2258 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2259 <P>
2260 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2261 code to be obeyed in the middle of matching a regular expression. This makes it
2262 possible, amongst other things, to extract different substrings that match the
2263 same pair of parentheses when there is a repetition.
2264 </P>
2265 <P>
2266 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2267 code. The feature is called "callout". The caller of PCRE provides an external
2268 function by putting its entry point in the global variable <i>pcre_callout</i>.
2269 By default, this variable contains NULL, which disables all calling out.
2270 </P>
2271 <P>
2272 Within a regular expression, (?C) indicates the points at which the external
2273 function is to be called. If you want to identify different callout points, you
2274 can put a number less than 256 after the letter C. The default value is zero.
2275 For example, this pattern has two callout points:
2276 <pre>
2277 (?C1)abc(?C2)def
2278 </pre>
2279 If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2280 automatically installed before each item in the pattern. They are all numbered
2281 255.
2282 </P>
2283 <P>
2284 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2285 set), the external function is called. It is provided with the number of the
2286 callout, the position in the pattern, and, optionally, one item of data
2287 originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2288 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2289 description of the interface to the callout function is given in the
2290 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2291 documentation.
2292 </P>
2293 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2294 <P>
2295 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2296 are described in the Perl documentation as "experimental and subject to change
2297 or removal in a future version of Perl". It goes on to say: "Their usage in
2298 production code should be noted to avoid problems during upgrades." The same
2299 remarks apply to the PCRE features described in this section.
2300 </P>
2301 <P>
2302 Since these verbs are specifically related to backtracking, most of them can be
2303 used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2304 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2305 failing negative assertion, they cause an error if encountered by
2306 <b>pcre_dfa_exec()</b>.
2307 </P>
2308 <P>
2309 If any of these verbs are used in an assertion or subroutine subpattern
2310 (including recursive subpatterns), their effect is confined to that subpattern;
2311 it does not extend to the surrounding pattern. Note that such subpatterns are
2312 processed as anchored at the point where they are tested.
2313 </P>
2314 <P>
2315 The new verbs make use of what was previously invalid syntax: an opening
2316 parenthesis followed by an asterisk. In Perl, they are generally of the form
2317 (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2318 form is just (*VERB). Any number of these verbs may occur in a pattern. There
2319 are two kinds:
2320 </P>
2321 <br><b>
2322 Verbs that act immediately
2323 </b><br>
2324 <P>
2325 The following verbs act as soon as they are encountered:
2326 <pre>
2327 (*ACCEPT)
2328 </pre>
2329 This verb causes the match to end successfully, skipping the remainder of the
2330 pattern. When inside a recursion, only the innermost pattern is ended
2331 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2332 captured. (This feature was added to PCRE at release 8.00.) For example:
2333 <pre>
2334 A((?:A|B(*ACCEPT)|C)D)
2335 </pre>
2336 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2337 the outer parentheses.
2338 <pre>
2339 (*FAIL) or (*F)
2340 </pre>
2341 This verb causes the match to fail, forcing backtracking to occur. It is
2342 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2343 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2344 Perl features that are not present in PCRE. The nearest equivalent is the
2345 callout feature, as for example in this pattern:
2346 <pre>
2347 a+(?C)(*FAIL)
2348 </pre>
2349 A match with the string "aaaa" always fails, but the callout is taken before
2350 each backtrack happens (in this example, 10 times).
2351 </P>
2352 <br><b>
2353 Verbs that act after backtracking
2354 </b><br>
2355 <P>
2356 The following verbs do nothing when they are encountered. Matching continues
2357 with what follows, but if there is no subsequent match, a failure is forced.
2358 The verbs differ in exactly what kind of failure occurs.
2359 <pre>
2360 (*COMMIT)
2361 </pre>
2362 This verb causes the whole match to fail outright if the rest of the pattern
2363 does not match. Even if the pattern is unanchored, no further attempts to find
2364 a match by advancing the starting point take place. Once (*COMMIT) has been
2365 passed, <b>pcre_exec()</b> is committed to finding a match at the current
2366 starting point, or not at all. For example:
2367 <pre>
2368 a+(*COMMIT)b
2369 </pre>
2370 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2371 dynamic anchor, or "I've started, so I must finish."
2372 <pre>
2373 (*PRUNE)
2374 </pre>
2375 This verb causes the match to fail at the current position if the rest of the
2376 pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2377 advance to the next starting character then happens. Backtracking can occur as
2378 usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2379 if there is no match to the right, backtracking cannot cross (*PRUNE).
2380 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2381 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2382 be expressed in any other way.
2383 <pre>
2384 (*SKIP)
2385 </pre>
2386 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2387 "bumpalong" advance is not to the next character, but to the position in the
2388 subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2389 was matched leading up to it cannot be part of a successful match. Consider:
2390 <pre>
2391 a+(*SKIP)b
2392 </pre>
2393 If the subject is "aaaac...", after the first match attempt fails (starting at
2394 the first character in the string), the starting point skips on to start the
2395 next attempt at "c". Note that a possessive quantifer does not have the same
2396 effect as this example; although it would suppress backtracking during the
2397 first match attempt, the second attempt would start at the second character
2398 instead of skipping on to "c".
2399 <pre>
2400 (*THEN)
2401 </pre>
2402 This verb causes a skip to the next alternation if the rest of the pattern does
2403 not match. That is, it cancels pending backtracking, but only within the
2404 current alternation. Its name comes from the observation that it can be used
2405 for a pattern-based if-then-else block:
2406 <pre>
2407 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2408 </pre>
2409 If the COND1 pattern matches, FOO is tried (and possibly further items after
2410 the end of the group if FOO succeeds); on failure the matcher skips to the
2411 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2412 is used outside of any alternation, it acts exactly like (*PRUNE).
2413 </P>
2414 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2415 <P>
2416 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2417 <b>pcresyntax</b>(3), <b>pcre</b>(3).
2418 </P>
2419 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2420 <P>
2421 Philip Hazel
2422 <br>
2423 University Computing Service
2424 <br>
2425 Cambridge CB2 3QH, England.
2426 <br>
2427 </P>
2428 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2429 <P>
2430 Last updated: 06 March 2010
2431 <br>
2432 Copyright &copy; 1997-2010 University of Cambridge.
2433 <br>
2434 <p>
2435 Return to the <a href="index.html">PCRE index page</a>.
2436 </p>

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