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

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