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

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