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

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