/[pcre]/code/trunk/doc/html/pcrepattern.html
ViewVC logotype

Contents of /code/trunk/doc/html/pcrepattern.html

Parent Directory Parent Directory | Revision Log Revision Log


Revision 654 - (show annotations)
Tue Aug 2 11:00:40 2011 UTC (8 years ago) by ph10
File MIME type: text/html
File size: 119062 byte(s)
Documentation and general text tidies in preparation for test release.
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) AND \N</a>
22 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
23 <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24 <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25 <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26 <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27 <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28 <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29 <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30 <li><a name="TOC15" href="#SEC15">REPETITION</a>
31 <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
32 <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33 <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34 <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35 <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36 <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37 <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38 <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39 <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40 <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41 <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42 <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43 <li><a name="TOC28" href="#SEC28">REVISION</a>
44 </ul>
45 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46 <P>
47 The syntax and semantics of the regular expressions that are supported by PCRE
48 are described in detail below. There is a quick-reference syntax summary in the
49 <a href="pcresyntax.html"><b>pcresyntax</b></a>
50 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51 also supports some alternative regular expression syntax (which does not
52 conflict with the Perl syntax) in order to provide some compatibility with
53 regular expressions in Python, .NET, and Oniguruma.
54 </P>
55 <P>
56 Perl's regular expressions are described in its own documentation, and
57 regular expressions in general are covered in a number of books, some of which
58 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59 published by O'Reilly, covers regular expressions in great detail. This
60 description of PCRE's regular expressions is intended as reference material.
61 </P>
62 <P>
63 The original operation of PCRE was on strings of one-byte characters. However,
64 there is now also support for UTF-8 character strings. To use this,
65 PCRE must be built to include UTF-8 support, and you must call
66 <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There
67 is also a special sequence that can be given at the start of a pattern:
68 <pre>
69 (*UTF8)
70 </pre>
71 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
72 option. This feature is not Perl-compatible. How setting UTF-8 mode affects
73 pattern matching is mentioned in several places below. There is also a summary
74 of UTF-8 features in the
75 <a href="pcre.html#utf8support">section on UTF-8 support</a>
76 in the main
77 <a href="pcre.html"><b>pcre</b></a>
78 page.
79 </P>
80 <P>
81 Another special sequence that may appear at the start of a pattern or in
82 combination with (*UTF8) is:
83 <pre>
84 (*UCP)
85 </pre>
86 This has the same effect as setting the PCRE_UCP option: it causes sequences
87 such as \d and \w to use Unicode properties to determine character types,
88 instead of recognizing only characters with codes less than 128 via a lookup
89 table.
90 </P>
91 <P>
92 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
93 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
94 also some more of these special sequences that are concerned with the handling
95 of newlines; they are described below.
96 </P>
97 <P>
98 The remainder of this document discusses the patterns that are supported by
99 PCRE when its main matching function, <b>pcre_exec()</b>, is used.
100 From release 6.0, PCRE offers a second matching function,
101 <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
102 Perl-compatible. Some of the features discussed below are not available when
103 <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
104 alternative function, and how it differs from the normal function, are
105 discussed in the
106 <a href="pcrematching.html"><b>pcrematching</b></a>
107 page.
108 <a name="newlines"></a></P>
109 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
110 <P>
111 PCRE supports five different conventions for indicating line breaks in
112 strings: a single CR (carriage return) character, a single LF (linefeed)
113 character, the two-character sequence CRLF, any of the three preceding, or any
114 Unicode newline sequence. The
115 <a href="pcreapi.html"><b>pcreapi</b></a>
116 page has
117 <a href="pcreapi.html#newlines">further discussion</a>
118 about newlines, and shows how to set the newline convention in the
119 <i>options</i> arguments for the compiling and matching functions.
120 </P>
121 <P>
122 It is also possible to specify a newline convention by starting a pattern
123 string with one of the following five sequences:
124 <pre>
125 (*CR) carriage return
126 (*LF) linefeed
127 (*CRLF) carriage return, followed by linefeed
128 (*ANYCRLF) any of the three above
129 (*ANY) all Unicode newline sequences
130 </pre>
131 These override the default and the options given to <b>pcre_compile()</b> or
132 <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default
133 newline sequence, the pattern
134 <pre>
135 (*CR)a.b
136 </pre>
137 changes the convention to CR. That pattern matches "a\nb" because LF is no
138 longer a newline. Note that these special settings, which are not
139 Perl-compatible, are recognized only at the very start of a pattern, and that
140 they must be in upper case. If more than one of them is present, the last one
141 is used.
142 </P>
143 <P>
144 The newline convention affects the interpretation of the dot metacharacter when
145 PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
146 affect what the \R escape sequence matches. By default, this is any Unicode
147 newline sequence, for Perl compatibility. However, this can be changed; see the
148 description of \R in the section entitled
149 <a href="#newlineseq">"Newline sequences"</a>
150 below. A change of \R setting can be combined with a change of newline
151 convention.
152 </P>
153 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
154 <P>
155 A regular expression is a pattern that is matched against a subject string from
156 left to right. Most characters stand for themselves in a pattern, and match the
157 corresponding characters in the subject. As a trivial example, the pattern
158 <pre>
159 The quick brown fox
160 </pre>
161 matches a portion of a subject string that is identical to itself. When
162 caseless matching is specified (the PCRE_CASELESS option), letters are matched
163 independently of case. In UTF-8 mode, PCRE always understands the concept of
164 case for characters whose values are less than 128, so caseless matching is
165 always possible. For characters with higher values, the concept of case is
166 supported if PCRE is compiled with Unicode property support, but not otherwise.
167 If you want to use caseless matching for characters 128 and above, you must
168 ensure that PCRE is compiled with Unicode property support as well as with
169 UTF-8 support.
170 </P>
171 <P>
172 The power of regular expressions comes from the ability to include alternatives
173 and repetitions in the pattern. These are encoded in the pattern by the use of
174 <i>metacharacters</i>, which do not stand for themselves but instead are
175 interpreted in some special way.
176 </P>
177 <P>
178 There are two different sets of metacharacters: those that are recognized
179 anywhere in the pattern except within square brackets, and those that are
180 recognized within square brackets. Outside square brackets, the metacharacters
181 are as follows:
182 <pre>
183 \ general escape character with several uses
184 ^ assert start of string (or line, in multiline mode)
185 $ assert end of string (or line, in multiline mode)
186 . match any character except newline (by default)
187 [ start character class definition
188 | start of alternative branch
189 ( start subpattern
190 ) end subpattern
191 ? extends the meaning of (
192 also 0 or 1 quantifier
193 also quantifier minimizer
194 * 0 or more quantifier
195 + 1 or more quantifier
196 also "possessive quantifier"
197 { start min/max quantifier
198 </pre>
199 Part of a pattern that is in square brackets is called a "character class". In
200 a character class the only metacharacters are:
201 <pre>
202 \ general escape character
203 ^ negate the class, but only if the first character
204 - indicates character range
205 [ POSIX character class (only if followed by POSIX syntax)
206 ] terminates the character class
207 </pre>
208 The following sections describe the use of each of the metacharacters.
209 </P>
210 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
211 <P>
212 The backslash character has several uses. Firstly, if it is followed by a
213 character that is not a number or a letter, it takes away any special meaning
214 that character may have. This use of backslash as an escape character applies
215 both inside and outside character classes.
216 </P>
217 <P>
218 For example, if you want to match a * character, you write \* in the pattern.
219 This escaping action applies whether or not the following character would
220 otherwise be interpreted as a metacharacter, so it is always safe to precede a
221 non-alphanumeric with backslash to specify that it stands for itself. In
222 particular, if you want to match a backslash, you write \\.
223 </P>
224 <P>
225 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
226 backslash. All other characters (in particular, those whose codepoints are
227 greater than 127) are treated as literals.
228 </P>
229 <P>
230 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
231 pattern (other than in a character class) and characters between a # outside
232 a character class and the next newline are ignored. An escaping backslash can
233 be used to include a whitespace or # character as part of the pattern.
234 </P>
235 <P>
236 If you want to remove the special meaning from a sequence of characters, you
237 can do so by putting them between \Q and \E. This is different from Perl in
238 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
239 Perl, $ and @ cause variable interpolation. Note the following examples:
240 <pre>
241 Pattern PCRE matches Perl matches
242
243 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
244 \Qabc\$xyz\E abc\$xyz abc\$xyz
245 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
246 </pre>
247 The \Q...\E sequence is recognized both inside and outside character classes.
248 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
249 by \E later in the pattern, the literal interpretation continues to the end of
250 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
251 a character class, this causes an error, because the character class is not
252 terminated.
253 <a name="digitsafterbackslash"></a></P>
254 <br><b>
255 Non-printing characters
256 </b><br>
257 <P>
258 A second use of backslash provides a way of encoding non-printing characters
259 in patterns in a visible manner. There is no restriction on the appearance of
260 non-printing characters, apart from the binary zero that terminates a pattern,
261 but when a pattern is being prepared by text editing, it is often easier to use
262 one of the following escape sequences than the binary character it represents:
263 <pre>
264 \a alarm, that is, the BEL character (hex 07)
265 \cx "control-x", where x is any ASCII character
266 \e escape (hex 1B)
267 \f formfeed (hex 0C)
268 \n linefeed (hex 0A)
269 \r carriage return (hex 0D)
270 \t tab (hex 09)
271 \ddd character with octal code ddd, or back reference
272 \xhh character with hex code hh
273 \x{hhh..} character with hex code hhh..
274 </pre>
275 The precise effect of \cx is as follows: if x is a lower case letter, it
276 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
277 Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
278 \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
279 than 127, a compile-time error occurs. This locks out non-ASCII characters in
280 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
281 values are valid. A lower case letter is converted to upper case, and then the
282 0xc0 bits are flipped.)
283 </P>
284 <P>
285 After \x, from zero to two hexadecimal digits are read (letters can be in
286 upper or lower case). Any number of hexadecimal digits may appear between \x{
287 and }, but the value of the character code must be less than 256 in non-UTF-8
288 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
289 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
290 point, which is 10FFFF.
291 </P>
292 <P>
293 If characters other than hexadecimal digits appear between \x{ and }, or if
294 there is no terminating }, this form of escape is not recognized. Instead, the
295 initial \x will be interpreted as a basic hexadecimal escape, with no
296 following digits, giving a character whose value is zero.
297 </P>
298 <P>
299 Characters whose value is less than 256 can be defined by either of the two
300 syntaxes for \x. There is no difference in the way they are handled. For
301 example, \xdc is exactly the same as \x{dc}.
302 </P>
303 <P>
304 After \0 up to two further octal digits are read. If there are fewer than two
305 digits, just those that are present are used. Thus the sequence \0\x\07
306 specifies two binary zeros followed by a BEL character (code value 7). Make
307 sure you supply two digits after the initial zero if the pattern character that
308 follows is itself an octal digit.
309 </P>
310 <P>
311 The handling of a backslash followed by a digit other than 0 is complicated.
312 Outside a character class, PCRE reads it and any following digits as a decimal
313 number. If the number is less than 10, or if there have been at least that many
314 previous capturing left parentheses in the expression, the entire sequence is
315 taken as a <i>back reference</i>. A description of how this works is given
316 <a href="#backreferences">later,</a>
317 following the discussion of
318 <a href="#subpattern">parenthesized subpatterns.</a>
319 </P>
320 <P>
321 Inside a character class, or if the decimal number is greater than 9 and there
322 have not been that many capturing subpatterns, PCRE re-reads up to three octal
323 digits following the backslash, and uses them to generate a data character. Any
324 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
325 character specified in octal must be less than \400. In UTF-8 mode, values up
326 to \777 are permitted. For example:
327 <pre>
328 \040 is another way of writing a space
329 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
330 \7 is always a back reference
331 \11 might be a back reference, or another way of writing a tab
332 \011 is always a tab
333 \0113 is a tab followed by the character "3"
334 \113 might be a back reference, otherwise the character with octal code 113
335 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
336 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
337 </pre>
338 Note that octal values of 100 or greater must not be introduced by a leading
339 zero, because no more than three octal digits are ever read.
340 </P>
341 <P>
342 All the sequences that define a single character value can be used both inside
343 and outside character classes. In addition, inside a character class, the
344 sequence \b is interpreted as the backspace character (hex 08). The sequences
345 \B, \N, \R, and \X are not special inside a character class. Like any other
346 unrecognized escape sequences, they are treated as the literal characters "B",
347 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
348 set. Outside a character class, these sequences have different meanings.
349 </P>
350 <br><b>
351 Absolute and relative back references
352 </b><br>
353 <P>
354 The sequence \g followed by an unsigned or a negative number, optionally
355 enclosed in braces, is an absolute or relative back reference. A named back
356 reference can be coded as \g{name}. Back references are discussed
357 <a href="#backreferences">later,</a>
358 following the discussion of
359 <a href="#subpattern">parenthesized subpatterns.</a>
360 </P>
361 <br><b>
362 Absolute and relative subroutine calls
363 </b><br>
364 <P>
365 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
366 a number enclosed either in angle brackets or single quotes, is an alternative
367 syntax for referencing a subpattern as a "subroutine". Details are discussed
368 <a href="#onigurumasubroutines">later.</a>
369 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
370 synonymous. The former is a back reference; the latter is a
371 <a href="#subpatternsassubroutines">subroutine</a>
372 call.
373 <a name="genericchartypes"></a></P>
374 <br><b>
375 Generic character types
376 </b><br>
377 <P>
378 Another use of backslash is for specifying generic character types:
379 <pre>
380 \d any decimal digit
381 \D any character that is not a decimal digit
382 \h any horizontal whitespace character
383 \H any character that is not a horizontal whitespace character
384 \s any whitespace character
385 \S any character that is not a whitespace character
386 \v any vertical whitespace character
387 \V any character that is not a vertical whitespace character
388 \w any "word" character
389 \W any "non-word" character
390 </pre>
391 There is also the single sequence \N, which matches a non-newline character.
392 This is the same as
393 <a href="#fullstopdot">the "." metacharacter</a>
394 when PCRE_DOTALL is not set.
395 </P>
396 <P>
397 Each pair of lower and upper case escape sequences partitions the complete set
398 of characters into two disjoint sets. Any given character matches one, and only
399 one, of each pair. The sequences can appear both inside and outside character
400 classes. They each match one character of the appropriate type. If the current
401 matching point is at the end of the subject string, all of them fail, because
402 there is no character to match.
403 </P>
404 <P>
405 For compatibility with Perl, \s does not match the VT character (code 11).
406 This makes it different from the the POSIX "space" class. The \s characters
407 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
408 included in a Perl script, \s may match the VT character. In PCRE, it never
409 does.
410 </P>
411 <P>
412 A "word" character is an underscore or any character that is a letter or digit.
413 By default, the definition of letters and digits is controlled by PCRE's
414 low-valued character tables, and may vary if locale-specific matching is taking
415 place (see
416 <a href="pcreapi.html#localesupport">"Locale support"</a>
417 in the
418 <a href="pcreapi.html"><b>pcreapi</b></a>
419 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
420 or "french" in Windows, some character codes greater than 128 are used for
421 accented letters, and these are then matched by \w. The use of locales with
422 Unicode is discouraged.
423 </P>
424 <P>
425 By default, in UTF-8 mode, characters with values greater than 128 never match
426 \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
427 their original meanings from before UTF-8 support was available, mainly for
428 efficiency reasons. However, if PCRE is compiled with Unicode property support,
429 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
430 properties are used to determine character types, as follows:
431 <pre>
432 \d any character that \p{Nd} matches (decimal digit)
433 \s any character that \p{Z} matches, plus HT, LF, FF, CR
434 \w any character that \p{L} or \p{N} matches, plus underscore
435 </pre>
436 The upper case escapes match the inverse sets of characters. Note that \d
437 matches only decimal digits, whereas \w matches any Unicode digit, as well as
438 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
439 \B because they are defined in terms of \w and \W. Matching these sequences
440 is noticeably slower when PCRE_UCP is set.
441 </P>
442 <P>
443 The sequences \h, \H, \v, and \V are features that were added to Perl at
444 release 5.10. In contrast to the other sequences, which match only ASCII
445 characters by default, these always match certain high-valued codepoints in
446 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
447 are:
448 <pre>
449 U+0009 Horizontal tab
450 U+0020 Space
451 U+00A0 Non-break space
452 U+1680 Ogham space mark
453 U+180E Mongolian vowel separator
454 U+2000 En quad
455 U+2001 Em quad
456 U+2002 En space
457 U+2003 Em space
458 U+2004 Three-per-em space
459 U+2005 Four-per-em space
460 U+2006 Six-per-em space
461 U+2007 Figure space
462 U+2008 Punctuation space
463 U+2009 Thin space
464 U+200A Hair space
465 U+202F Narrow no-break space
466 U+205F Medium mathematical space
467 U+3000 Ideographic space
468 </pre>
469 The vertical space characters are:
470 <pre>
471 U+000A Linefeed
472 U+000B Vertical tab
473 U+000C Formfeed
474 U+000D Carriage return
475 U+0085 Next line
476 U+2028 Line separator
477 U+2029 Paragraph separator
478 <a name="newlineseq"></a></PRE>
479 </P>
480 <br><b>
481 Newline sequences
482 </b><br>
483 <P>
484 Outside a character class, by default, the escape sequence \R matches any
485 Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following:
486 <pre>
487 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
488 </pre>
489 This is an example of an "atomic group", details of which are given
490 <a href="#atomicgroup">below.</a>
491 This particular group matches either the two-character sequence CR followed by
492 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
493 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
494 line, U+0085). The two-character sequence is treated as a single unit that
495 cannot be split.
496 </P>
497 <P>
498 In UTF-8 mode, two additional characters whose codepoints are greater than 255
499 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
500 Unicode character property support is not needed for these characters to be
501 recognized.
502 </P>
503 <P>
504 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
505 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
506 either at compile time or when the pattern is matched. (BSR is an abbrevation
507 for "backslash R".) This can be made the default when PCRE is built; if this is
508 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
509 It is also possible to specify these settings by starting a pattern string with
510 one of the following sequences:
511 <pre>
512 (*BSR_ANYCRLF) CR, LF, or CRLF only
513 (*BSR_UNICODE) any Unicode newline sequence
514 </pre>
515 These override the default and the options given to <b>pcre_compile()</b> or
516 <b>pcre_compile2()</b>, but they can be overridden by options given to
517 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,
518 which are not Perl-compatible, are recognized only at the very start of a
519 pattern, and that they must be in upper case. If more than one of them is
520 present, the last one is used. They can be combined with a change of newline
521 convention; for example, a pattern can start with:
522 <pre>
523 (*ANY)(*BSR_ANYCRLF)
524 </pre>
525 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
526 a character class, \R is treated as an unrecognized escape sequence, and so
527 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
528 <a name="uniextseq"></a></P>
529 <br><b>
530 Unicode character properties
531 </b><br>
532 <P>
533 When PCRE is built with Unicode character property support, three additional
534 escape sequences that match characters with specific properties are available.
535 When not in UTF-8 mode, these sequences are of course limited to testing
536 characters whose codepoints are less than 256, but they do work in this mode.
537 The extra escape sequences are:
538 <pre>
539 \p{<i>xx</i>} a character with the <i>xx</i> property
540 \P{<i>xx</i>} a character without the <i>xx</i> property
541 \X an extended Unicode sequence
542 </pre>
543 The property names represented by <i>xx</i> above are limited to the Unicode
544 script names, the general category properties, "Any", which matches any
545 character (including newline), and some special PCRE properties (described
546 in the
547 <a href="#extraprops">next section).</a>
548 Other Perl properties such as "InMusicalSymbols" are not currently supported by
549 PCRE. Note that \P{Any} does not match any characters, so always causes a
550 match failure.
551 </P>
552 <P>
553 Sets of Unicode characters are defined as belonging to certain scripts. A
554 character from one of these sets can be matched using a script name. For
555 example:
556 <pre>
557 \p{Greek}
558 \P{Han}
559 </pre>
560 Those that are not part of an identified script are lumped together as
561 "Common". The current list of scripts is:
562 </P>
563 <P>
564 Arabic,
565 Armenian,
566 Avestan,
567 Balinese,
568 Bamum,
569 Bengali,
570 Bopomofo,
571 Braille,
572 Buginese,
573 Buhid,
574 Canadian_Aboriginal,
575 Carian,
576 Cham,
577 Cherokee,
578 Common,
579 Coptic,
580 Cuneiform,
581 Cypriot,
582 Cyrillic,
583 Deseret,
584 Devanagari,
585 Egyptian_Hieroglyphs,
586 Ethiopic,
587 Georgian,
588 Glagolitic,
589 Gothic,
590 Greek,
591 Gujarati,
592 Gurmukhi,
593 Han,
594 Hangul,
595 Hanunoo,
596 Hebrew,
597 Hiragana,
598 Imperial_Aramaic,
599 Inherited,
600 Inscriptional_Pahlavi,
601 Inscriptional_Parthian,
602 Javanese,
603 Kaithi,
604 Kannada,
605 Katakana,
606 Kayah_Li,
607 Kharoshthi,
608 Khmer,
609 Lao,
610 Latin,
611 Lepcha,
612 Limbu,
613 Linear_B,
614 Lisu,
615 Lycian,
616 Lydian,
617 Malayalam,
618 Meetei_Mayek,
619 Mongolian,
620 Myanmar,
621 New_Tai_Lue,
622 Nko,
623 Ogham,
624 Old_Italic,
625 Old_Persian,
626 Old_South_Arabian,
627 Old_Turkic,
628 Ol_Chiki,
629 Oriya,
630 Osmanya,
631 Phags_Pa,
632 Phoenician,
633 Rejang,
634 Runic,
635 Samaritan,
636 Saurashtra,
637 Shavian,
638 Sinhala,
639 Sundanese,
640 Syloti_Nagri,
641 Syriac,
642 Tagalog,
643 Tagbanwa,
644 Tai_Le,
645 Tai_Tham,
646 Tai_Viet,
647 Tamil,
648 Telugu,
649 Thaana,
650 Thai,
651 Tibetan,
652 Tifinagh,
653 Ugaritic,
654 Vai,
655 Yi.
656 </P>
657 <P>
658 Each character has exactly one Unicode general category property, specified by
659 a two-letter abbreviation. For compatibility with Perl, negation can be
660 specified by including a circumflex between the opening brace and the property
661 name. For example, \p{^Lu} is the same as \P{Lu}.
662 </P>
663 <P>
664 If only one letter is specified with \p or \P, it includes all the general
665 category properties that start with that letter. In this case, in the absence
666 of negation, the curly brackets in the escape sequence are optional; these two
667 examples have the same effect:
668 <pre>
669 \p{L}
670 \pL
671 </pre>
672 The following general category property codes are supported:
673 <pre>
674 C Other
675 Cc Control
676 Cf Format
677 Cn Unassigned
678 Co Private use
679 Cs Surrogate
680
681 L Letter
682 Ll Lower case letter
683 Lm Modifier letter
684 Lo Other letter
685 Lt Title case letter
686 Lu Upper case letter
687
688 M Mark
689 Mc Spacing mark
690 Me Enclosing mark
691 Mn Non-spacing mark
692
693 N Number
694 Nd Decimal number
695 Nl Letter number
696 No Other number
697
698 P Punctuation
699 Pc Connector punctuation
700 Pd Dash punctuation
701 Pe Close punctuation
702 Pf Final punctuation
703 Pi Initial punctuation
704 Po Other punctuation
705 Ps Open punctuation
706
707 S Symbol
708 Sc Currency symbol
709 Sk Modifier symbol
710 Sm Mathematical symbol
711 So Other symbol
712
713 Z Separator
714 Zl Line separator
715 Zp Paragraph separator
716 Zs Space separator
717 </pre>
718 The special property L& is also supported: it matches a character that has
719 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
720 a modifier or "other".
721 </P>
722 <P>
723 The Cs (Surrogate) property applies only to characters in the range U+D800 to
724 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
725 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
726 (see the discussion of PCRE_NO_UTF8_CHECK in the
727 <a href="pcreapi.html"><b>pcreapi</b></a>
728 page). Perl does not support the Cs property.
729 </P>
730 <P>
731 The long synonyms for property names that Perl supports (such as \p{Letter})
732 are not supported by PCRE, nor is it permitted to prefix any of these
733 properties with "Is".
734 </P>
735 <P>
736 No character that is in the Unicode table has the Cn (unassigned) property.
737 Instead, this property is assumed for any code point that is not in the
738 Unicode table.
739 </P>
740 <P>
741 Specifying caseless matching does not affect these escape sequences. For
742 example, \p{Lu} always matches only upper case letters.
743 </P>
744 <P>
745 The \X escape matches any number of Unicode characters that form an extended
746 Unicode sequence. \X is equivalent to
747 <pre>
748 (?&#62;\PM\pM*)
749 </pre>
750 That is, it matches a character without the "mark" property, followed by zero
751 or more characters with the "mark" property, and treats the sequence as an
752 atomic group
753 <a href="#atomicgroup">(see below).</a>
754 Characters with the "mark" property are typically accents that affect the
755 preceding character. None of them have codepoints less than 256, so in
756 non-UTF-8 mode \X matches any one character.
757 </P>
758 <P>
759 Note that recent versions of Perl have changed \X to match what Unicode calls
760 an "extended grapheme cluster", which has a more complicated definition.
761 </P>
762 <P>
763 Matching characters by Unicode property is not fast, because PCRE has to search
764 a structure that contains data for over fifteen thousand characters. That is
765 why the traditional escape sequences such as \d and \w do not use Unicode
766 properties in PCRE by default, though you can make them do so by setting the
767 PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with
768 (*UCP).
769 <a name="extraprops"></a></P>
770 <br><b>
771 PCRE's additional properties
772 </b><br>
773 <P>
774 As well as the standard Unicode properties described in the previous
775 section, PCRE supports four more that make it possible to convert traditional
776 escape sequences such as \w and \s and POSIX character classes to use Unicode
777 properties. PCRE uses these non-standard, non-Perl properties internally when
778 PCRE_UCP is set. They are:
779 <pre>
780 Xan Any alphanumeric character
781 Xps Any POSIX space character
782 Xsp Any Perl space character
783 Xwd Any Perl "word" character
784 </pre>
785 Xan matches characters that have either the L (letter) or the N (number)
786 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
787 carriage return, and any other character that has the Z (separator) property.
788 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
789 same characters as Xan, plus underscore.
790 <a name="resetmatchstart"></a></P>
791 <br><b>
792 Resetting the match start
793 </b><br>
794 <P>
795 The escape sequence \K causes any previously matched characters not to be
796 included in the final matched sequence. For example, the pattern:
797 <pre>
798 foo\Kbar
799 </pre>
800 matches "foobar", but reports that it has matched "bar". This feature is
801 similar to a lookbehind assertion
802 <a href="#lookbehind">(described below).</a>
803 However, in this case, the part of the subject before the real match does not
804 have to be of fixed length, as lookbehind assertions do. The use of \K does
805 not interfere with the setting of
806 <a href="#subpattern">captured substrings.</a>
807 For example, when the pattern
808 <pre>
809 (foo)\Kbar
810 </pre>
811 matches "foobar", the first substring is still set to "foo".
812 </P>
813 <P>
814 Perl documents that the use of \K within assertions is "not well defined". In
815 PCRE, \K is acted upon when it occurs inside positive assertions, but is
816 ignored in negative assertions.
817 <a name="smallassertions"></a></P>
818 <br><b>
819 Simple assertions
820 </b><br>
821 <P>
822 The final use of backslash is for certain simple assertions. An assertion
823 specifies a condition that has to be met at a particular point in a match,
824 without consuming any characters from the subject string. The use of
825 subpatterns for more complicated assertions is described
826 <a href="#bigassertions">below.</a>
827 The backslashed assertions are:
828 <pre>
829 \b matches at a word boundary
830 \B matches when not at a word boundary
831 \A matches at the start of the subject
832 \Z matches at the end of the subject
833 also matches before a newline at the end of the subject
834 \z matches only at the end of the subject
835 \G matches at the first matching position in the subject
836 </pre>
837 Inside a character class, \b has a different meaning; it matches the backspace
838 character. If any other of these assertions appears in a character class, by
839 default it matches the corresponding literal character (for example, \B
840 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
841 escape sequence" error is generated instead.
842 </P>
843 <P>
844 A word boundary is a position in the subject string where the current character
845 and the previous character do not both match \w or \W (i.e. one matches
846 \w and the other matches \W), or the start or end of the string if the
847 first or last character matches \w, respectively. In UTF-8 mode, the meanings
848 of \w and \W can be changed by setting the PCRE_UCP option. When this is
849 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
850 of word" or "end of word" metasequence. However, whatever follows \b normally
851 determines which it is. For example, the fragment \ba matches "a" at the start
852 of a word.
853 </P>
854 <P>
855 The \A, \Z, and \z assertions differ from the traditional circumflex and
856 dollar (described in the next section) in that they only ever match at the very
857 start and end of the subject string, whatever options are set. Thus, they are
858 independent of multiline mode. These three assertions are not affected by the
859 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
860 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
861 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
862 at a point other than the beginning of the subject, \A can never match. The
863 difference between \Z and \z is that \Z matches before a newline at the end
864 of the string as well as at the very end, whereas \z matches only at the end.
865 </P>
866 <P>
867 The \G assertion is true only when the current matching position is at the
868 start point of the match, as specified by the <i>startoffset</i> argument of
869 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
870 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
871 arguments, you can mimic Perl's /g option, and it is in this kind of
872 implementation where \G can be useful.
873 </P>
874 <P>
875 Note, however, that PCRE's interpretation of \G, as the start of the current
876 match, is subtly different from Perl's, which defines it as the end of the
877 previous match. In Perl, these can be different when the previously matched
878 string was empty. Because PCRE does just one match at a time, it cannot
879 reproduce this behaviour.
880 </P>
881 <P>
882 If all the alternatives of a pattern begin with \G, the expression is anchored
883 to the starting match position, and the "anchored" flag is set in the compiled
884 regular expression.
885 </P>
886 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
887 <P>
888 Outside a character class, in the default matching mode, the circumflex
889 character is an assertion that is true only if the current matching point is
890 at the start of the subject string. If the <i>startoffset</i> argument of
891 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
892 option is unset. Inside a character class, circumflex has an entirely different
893 meaning
894 <a href="#characterclass">(see below).</a>
895 </P>
896 <P>
897 Circumflex need not be the first character of the pattern if a number of
898 alternatives are involved, but it should be the first thing in each alternative
899 in which it appears if the pattern is ever to match that branch. If all
900 possible alternatives start with a circumflex, that is, if the pattern is
901 constrained to match only at the start of the subject, it is said to be an
902 "anchored" pattern. (There are also other constructs that can cause a pattern
903 to be anchored.)
904 </P>
905 <P>
906 A dollar character is an assertion that is true only if the current matching
907 point is at the end of the subject string, or immediately before a newline
908 at the end of the string (by default). Dollar need not be the last character of
909 the pattern if a number of alternatives are involved, but it should be the last
910 item in any branch in which it appears. Dollar has no special meaning in a
911 character class.
912 </P>
913 <P>
914 The meaning of dollar can be changed so that it matches only at the very end of
915 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
916 does not affect the \Z assertion.
917 </P>
918 <P>
919 The meanings of the circumflex and dollar characters are changed if the
920 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
921 immediately after internal newlines as well as at the start of the subject
922 string. It does not match after a newline that ends the string. A dollar
923 matches before any newlines in the string, as well as at the very end, when
924 PCRE_MULTILINE is set. When newline is specified as the two-character
925 sequence CRLF, isolated CR and LF characters do not indicate newlines.
926 </P>
927 <P>
928 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
929 \n represents a newline) in multiline mode, but not otherwise. Consequently,
930 patterns that are anchored in single line mode because all branches start with
931 ^ are not anchored in multiline mode, and a match for circumflex is possible
932 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
933 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
934 </P>
935 <P>
936 Note that the sequences \A, \Z, and \z can be used to match the start and
937 end of the subject in both modes, and if all branches of a pattern start with
938 \A it is always anchored, whether or not PCRE_MULTILINE is set.
939 <a name="fullstopdot"></a></P>
940 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
941 <P>
942 Outside a character class, a dot in the pattern matches any one character in
943 the subject string except (by default) a character that signifies the end of a
944 line. In UTF-8 mode, the matched character may be more than one byte long.
945 </P>
946 <P>
947 When a line ending is defined as a single character, dot never matches that
948 character; when the two-character sequence CRLF is used, dot does not match CR
949 if it is immediately followed by LF, but otherwise it matches all characters
950 (including isolated CRs and LFs). When any Unicode line endings are being
951 recognized, dot does not match CR or LF or any of the other line ending
952 characters.
953 </P>
954 <P>
955 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
956 option is set, a dot matches any one character, without exception. If the
957 two-character sequence CRLF is present in the subject string, it takes two dots
958 to match it.
959 </P>
960 <P>
961 The handling of dot is entirely independent of the handling of circumflex and
962 dollar, the only relationship being that they both involve newlines. Dot has no
963 special meaning in a character class.
964 </P>
965 <P>
966 The escape sequence \N behaves like a dot, except that it is not affected by
967 the PCRE_DOTALL option. In other words, it matches any character except one
968 that signifies the end of a line.
969 </P>
970 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
971 <P>
972 Outside a character class, the escape sequence \C matches any one byte, both
973 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
974 characters. The feature is provided in Perl in order to match individual bytes
975 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
976 rest of the string may start with a malformed UTF-8 character. For this reason,
977 the \C escape sequence is best avoided.
978 </P>
979 <P>
980 PCRE does not allow \C to appear in lookbehind assertions
981 <a href="#lookbehind">(described below),</a>
982 because in UTF-8 mode this would make it impossible to calculate the length of
983 the lookbehind.
984 <a name="characterclass"></a></P>
985 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
986 <P>
987 An opening square bracket introduces a character class, terminated by a closing
988 square bracket. A closing square bracket on its own is not special by default.
989 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
990 bracket causes a compile-time error. If a closing square bracket is required as
991 a member of the class, it should be the first data character in the class
992 (after an initial circumflex, if present) or escaped with a backslash.
993 </P>
994 <P>
995 A character class matches a single character in the subject. In UTF-8 mode, the
996 character may be more than one byte long. A matched character must be in the
997 set of characters defined by the class, unless the first character in the class
998 definition is a circumflex, in which case the subject character must not be in
999 the set defined by the class. If a circumflex is actually required as a member
1000 of the class, ensure it is not the first character, or escape it with a
1001 backslash.
1002 </P>
1003 <P>
1004 For example, the character class [aeiou] matches any lower case vowel, while
1005 [^aeiou] matches any character that is not a lower case vowel. Note that a
1006 circumflex is just a convenient notation for specifying the characters that
1007 are in the class by enumerating those that are not. A class that starts with a
1008 circumflex is not an assertion; it still consumes a character from the subject
1009 string, and therefore it fails if the current pointer is at the end of the
1010 string.
1011 </P>
1012 <P>
1013 In UTF-8 mode, characters with values greater than 255 can be included in a
1014 class as a literal string of bytes, or by using the \x{ escaping mechanism.
1015 </P>
1016 <P>
1017 When caseless matching is set, any letters in a class represent both their
1018 upper case and lower case versions, so for example, a caseless [aeiou] matches
1019 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1020 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1021 case for characters whose values are less than 128, so caseless matching is
1022 always possible. For characters with higher values, the concept of case is
1023 supported if PCRE is compiled with Unicode property support, but not otherwise.
1024 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1025 you must ensure that PCRE is compiled with Unicode property support as well as
1026 with UTF-8 support.
1027 </P>
1028 <P>
1029 Characters that might indicate line breaks are never treated in any special way
1030 when matching character classes, whatever line-ending sequence is in use, and
1031 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1032 such as [^a] always matches one of these characters.
1033 </P>
1034 <P>
1035 The minus (hyphen) character can be used to specify a range of characters in a
1036 character class. For example, [d-m] matches any letter between d and m,
1037 inclusive. If a minus character is required in a class, it must be escaped with
1038 a backslash or appear in a position where it cannot be interpreted as
1039 indicating a range, typically as the first or last character in the class.
1040 </P>
1041 <P>
1042 It is not possible to have the literal character "]" as the end character of a
1043 range. A pattern such as [W-]46] is interpreted as a class of two characters
1044 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1045 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1046 the end of range, so [W-\]46] is interpreted as a class containing a range
1047 followed by two other characters. The octal or hexadecimal representation of
1048 "]" can also be used to end a range.
1049 </P>
1050 <P>
1051 Ranges operate in the collating sequence of character values. They can also be
1052 used for characters specified numerically, for example [\000-\037]. In UTF-8
1053 mode, ranges can include characters whose values are greater than 255, for
1054 example [\x{100}-\x{2ff}].
1055 </P>
1056 <P>
1057 If a range that includes letters is used when caseless matching is set, it
1058 matches the letters in either case. For example, [W-c] is equivalent to
1059 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1060 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1061 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1062 characters with values greater than 128 only when it is compiled with Unicode
1063 property support.
1064 </P>
1065 <P>
1066 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1067 \V, \w, and \W may appear in a character class, and add the characters that
1068 they match to the class. For example, [\dABCDEF] matches any hexadecimal
1069 digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w
1070 and their upper case partners, just as it does when they appear outside a
1071 character class, as described in the section entitled
1072 <a href="#genericchartypes">"Generic character types"</a>
1073 above. The escape sequence \b has a different meaning inside a character
1074 class; it matches the backspace character. The sequences \B, \N, \R, and \X
1075 are not special inside a character class. Like any other unrecognized escape
1076 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1077 default, but cause an error if the PCRE_EXTRA option is set.
1078 </P>
1079 <P>
1080 A circumflex can conveniently be used with the upper case character types to
1081 specify a more restricted set of characters than the matching lower case type.
1082 For example, the class [^\W_] matches any letter or digit, but not underscore,
1083 whereas [\w] includes underscore. A positive character class should be read as
1084 "something OR something OR ..." and a negative class as "NOT something AND NOT
1085 something AND NOT ...".
1086 </P>
1087 <P>
1088 The only metacharacters that are recognized in character classes are backslash,
1089 hyphen (only where it can be interpreted as specifying a range), circumflex
1090 (only at the start), opening square bracket (only when it can be interpreted as
1091 introducing a POSIX class name - see the next section), and the terminating
1092 closing square bracket. However, escaping other non-alphanumeric characters
1093 does no harm.
1094 </P>
1095 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1096 <P>
1097 Perl supports the POSIX notation for character classes. This uses names
1098 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1099 this notation. For example,
1100 <pre>
1101 [01[:alpha:]%]
1102 </pre>
1103 matches "0", "1", any alphabetic character, or "%". The supported class names
1104 are:
1105 <pre>
1106 alnum letters and digits
1107 alpha letters
1108 ascii character codes 0 - 127
1109 blank space or tab only
1110 cntrl control characters
1111 digit decimal digits (same as \d)
1112 graph printing characters, excluding space
1113 lower lower case letters
1114 print printing characters, including space
1115 punct printing characters, excluding letters and digits and space
1116 space white space (not quite the same as \s)
1117 upper upper case letters
1118 word "word" characters (same as \w)
1119 xdigit hexadecimal digits
1120 </pre>
1121 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1122 space (32). Notice that this list includes the VT character (code 11). This
1123 makes "space" different to \s, which does not include VT (for Perl
1124 compatibility).
1125 </P>
1126 <P>
1127 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1128 5.8. Another Perl extension is negation, which is indicated by a ^ character
1129 after the colon. For example,
1130 <pre>
1131 [12[:^digit:]]
1132 </pre>
1133 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1134 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1135 supported, and an error is given if they are encountered.
1136 </P>
1137 <P>
1138 By default, in UTF-8 mode, characters with values greater than 128 do not match
1139 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1140 to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1141 character properties are used. This is achieved by replacing the POSIX classes
1142 by other sequences, as follows:
1143 <pre>
1144 [:alnum:] becomes \p{Xan}
1145 [:alpha:] becomes \p{L}
1146 [:blank:] becomes \h
1147 [:digit:] becomes \p{Nd}
1148 [:lower:] becomes \p{Ll}
1149 [:space:] becomes \p{Xps}
1150 [:upper:] becomes \p{Lu}
1151 [:word:] becomes \p{Xwd}
1152 </pre>
1153 Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1154 classes are unchanged, and match only characters with code points less than
1155 128.
1156 </P>
1157 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1158 <P>
1159 Vertical bar characters are used to separate alternative patterns. For example,
1160 the pattern
1161 <pre>
1162 gilbert|sullivan
1163 </pre>
1164 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1165 and an empty alternative is permitted (matching the empty string). The matching
1166 process tries each alternative in turn, from left to right, and the first one
1167 that succeeds is used. If the alternatives are within a subpattern
1168 <a href="#subpattern">(defined below),</a>
1169 "succeeds" means matching the rest of the main pattern as well as the
1170 alternative in the subpattern.
1171 </P>
1172 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1173 <P>
1174 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1175 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1176 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1177 The option letters are
1178 <pre>
1179 i for PCRE_CASELESS
1180 m for PCRE_MULTILINE
1181 s for PCRE_DOTALL
1182 x for PCRE_EXTENDED
1183 </pre>
1184 For example, (?im) sets caseless, multiline matching. It is also possible to
1185 unset these options by preceding the letter with a hyphen, and a combined
1186 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1187 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1188 permitted. If a letter appears both before and after the hyphen, the option is
1189 unset.
1190 </P>
1191 <P>
1192 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1193 changed in the same way as the Perl-compatible options by using the characters
1194 J, U and X respectively.
1195 </P>
1196 <P>
1197 When one of these option changes occurs at top level (that is, not inside
1198 subpattern parentheses), the change applies to the remainder of the pattern
1199 that follows. If the change is placed right at the start of a pattern, PCRE
1200 extracts it into the global options (and it will therefore show up in data
1201 extracted by the <b>pcre_fullinfo()</b> function).
1202 </P>
1203 <P>
1204 An option change within a subpattern (see below for a description of
1205 subpatterns) affects only that part of the subpattern that follows it, so
1206 <pre>
1207 (a(?i)b)c
1208 </pre>
1209 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1210 By this means, options can be made to have different settings in different
1211 parts of the pattern. Any changes made in one alternative do carry on
1212 into subsequent branches within the same subpattern. For example,
1213 <pre>
1214 (a(?i)b|c)
1215 </pre>
1216 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1217 branch is abandoned before the option setting. This is because the effects of
1218 option settings happen at compile time. There would be some very weird
1219 behaviour otherwise.
1220 </P>
1221 <P>
1222 <b>Note:</b> There are other PCRE-specific options that can be set by the
1223 application when the compile or match functions are called. In some cases the
1224 pattern can contain special leading sequences such as (*CRLF) to override what
1225 the application has set or what has been defaulted. Details are given in the
1226 section entitled
1227 <a href="#newlineseq">"Newline sequences"</a>
1228 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1229 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1230 PCRE_UTF8 and the PCRE_UCP options, respectively.
1231 <a name="subpattern"></a></P>
1232 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1233 <P>
1234 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1235 Turning part of a pattern into a subpattern does two things:
1236 <br>
1237 <br>
1238 1. It localizes a set of alternatives. For example, the pattern
1239 <pre>
1240 cat(aract|erpillar|)
1241 </pre>
1242 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1243 match "cataract", "erpillar" or an empty string.
1244 <br>
1245 <br>
1246 2. It sets up the subpattern as a capturing subpattern. This means that, when
1247 the whole pattern matches, that portion of the subject string that matched the
1248 subpattern is passed back to the caller via the <i>ovector</i> argument of
1249 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1250 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1251 string "the red king" is matched against the pattern
1252 <pre>
1253 the ((red|white) (king|queen))
1254 </pre>
1255 the captured substrings are "red king", "red", and "king", and are numbered 1,
1256 2, and 3, respectively.
1257 </P>
1258 <P>
1259 The fact that plain parentheses fulfil two functions is not always helpful.
1260 There are often times when a grouping subpattern is required without a
1261 capturing requirement. If an opening parenthesis is followed by a question mark
1262 and a colon, the subpattern does not do any capturing, and is not counted when
1263 computing the number of any subsequent capturing subpatterns. For example, if
1264 the string "the white queen" is matched against the pattern
1265 <pre>
1266 the ((?:red|white) (king|queen))
1267 </pre>
1268 the captured substrings are "white queen" and "queen", and are numbered 1 and
1269 2. The maximum number of capturing subpatterns is 65535.
1270 </P>
1271 <P>
1272 As a convenient shorthand, if any option settings are required at the start of
1273 a non-capturing subpattern, the option letters may appear between the "?" and
1274 the ":". Thus the two patterns
1275 <pre>
1276 (?i:saturday|sunday)
1277 (?:(?i)saturday|sunday)
1278 </pre>
1279 match exactly the same set of strings. Because alternative branches are tried
1280 from left to right, and options are not reset until the end of the subpattern
1281 is reached, an option setting in one branch does affect subsequent branches, so
1282 the above patterns match "SUNDAY" as well as "Saturday".
1283 <a name="dupsubpatternnumber"></a></P>
1284 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1285 <P>
1286 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1287 the same numbers for its capturing parentheses. Such a subpattern starts with
1288 (?| and is itself a non-capturing subpattern. For example, consider this
1289 pattern:
1290 <pre>
1291 (?|(Sat)ur|(Sun))day
1292 </pre>
1293 Because the two alternatives are inside a (?| group, both sets of capturing
1294 parentheses are numbered one. Thus, when the pattern matches, you can look
1295 at captured substring number one, whichever alternative matched. This construct
1296 is useful when you want to capture part, but not all, of one of a number of
1297 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1298 number is reset at the start of each branch. The numbers of any capturing
1299 parentheses that follow the subpattern start after the highest number used in
1300 any branch. The following example is taken from the Perl documentation. The
1301 numbers underneath show in which buffer the captured content will be stored.
1302 <pre>
1303 # before ---------------branch-reset----------- after
1304 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1305 # 1 2 2 3 2 3 4
1306 </pre>
1307 A back reference to a numbered subpattern uses the most recent value that is
1308 set for that number by any subpattern. The following pattern matches "abcabc"
1309 or "defdef":
1310 <pre>
1311 /(?|(abc)|(def))\1/
1312 </pre>
1313 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1314 refers to the first one in the pattern with the given number. The following
1315 pattern matches "abcabc" or "defabc":
1316 <pre>
1317 /(?|(abc)|(def))(?1)/
1318 </pre>
1319 If a
1320 <a href="#conditions">condition test</a>
1321 for a subpattern's having matched refers to a non-unique number, the test is
1322 true if any of the subpatterns of that number have matched.
1323 </P>
1324 <P>
1325 An alternative approach to using this "branch reset" feature is to use
1326 duplicate named subpatterns, as described in the next section.
1327 </P>
1328 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1329 <P>
1330 Identifying capturing parentheses by number is simple, but it can be very hard
1331 to keep track of the numbers in complicated regular expressions. Furthermore,
1332 if an expression is modified, the numbers may change. To help with this
1333 difficulty, PCRE supports the naming of subpatterns. This feature was not
1334 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1335 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1336 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1337 have different names, but PCRE does not.
1338 </P>
1339 <P>
1340 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1341 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1342 parentheses from other parts of the pattern, such as
1343 <a href="#backreferences">back references,</a>
1344 <a href="#recursion">recursion,</a>
1345 and
1346 <a href="#conditions">conditions,</a>
1347 can be made by name as well as by number.
1348 </P>
1349 <P>
1350 Names consist of up to 32 alphanumeric characters and underscores. Named
1351 capturing parentheses are still allocated numbers as well as names, exactly as
1352 if the names were not present. The PCRE API provides function calls for
1353 extracting the name-to-number translation table from a compiled pattern. There
1354 is also a convenience function for extracting a captured substring by name.
1355 </P>
1356 <P>
1357 By default, a name must be unique within a pattern, but it is possible to relax
1358 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1359 names are also always permitted for subpatterns with the same number, set up as
1360 described in the previous section.) Duplicate names can be useful for patterns
1361 where only one instance of the named parentheses can match. Suppose you want to
1362 match the name of a weekday, either as a 3-letter abbreviation or as the full
1363 name, and in both cases you want to extract the abbreviation. This pattern
1364 (ignoring the line breaks) does the job:
1365 <pre>
1366 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1367 (?&#60;DN&#62;Tue)(?:sday)?|
1368 (?&#60;DN&#62;Wed)(?:nesday)?|
1369 (?&#60;DN&#62;Thu)(?:rsday)?|
1370 (?&#60;DN&#62;Sat)(?:urday)?
1371 </pre>
1372 There are five capturing substrings, but only one is ever set after a match.
1373 (An alternative way of solving this problem is to use a "branch reset"
1374 subpattern, as described in the previous section.)
1375 </P>
1376 <P>
1377 The convenience function for extracting the data by name returns the substring
1378 for the first (and in this example, the only) subpattern of that name that
1379 matched. This saves searching to find which numbered subpattern it was.
1380 </P>
1381 <P>
1382 If you make a back reference to a non-unique named subpattern from elsewhere in
1383 the pattern, the one that corresponds to the first occurrence of the name is
1384 used. In the absence of duplicate numbers (see the previous section) this is
1385 the one with the lowest number. If you use a named reference in a condition
1386 test (see the
1387 <a href="#conditions">section about conditions</a>
1388 below), either to check whether a subpattern has matched, or to check for
1389 recursion, all subpatterns with the same name are tested. If the condition is
1390 true for any one of them, the overall condition is true. This is the same
1391 behaviour as testing by number. For further details of the interfaces for
1392 handling named subpatterns, see the
1393 <a href="pcreapi.html"><b>pcreapi</b></a>
1394 documentation.
1395 </P>
1396 <P>
1397 <b>Warning:</b> You cannot use different names to distinguish between two
1398 subpatterns with the same number because PCRE uses only the numbers when
1399 matching. For this reason, an error is given at compile time if different names
1400 are given to subpatterns with the same number. However, you can give the same
1401 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1402 </P>
1403 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1404 <P>
1405 Repetition is specified by quantifiers, which can follow any of the following
1406 items:
1407 <pre>
1408 a literal data character
1409 the dot metacharacter
1410 the \C escape sequence
1411 the \X escape sequence (in UTF-8 mode with Unicode properties)
1412 the \R escape sequence
1413 an escape such as \d or \pL that matches a single character
1414 a character class
1415 a back reference (see next section)
1416 a parenthesized subpattern (including assertions)
1417 a recursive or "subroutine" call to a subpattern
1418 </pre>
1419 The general repetition quantifier specifies a minimum and maximum number of
1420 permitted matches, by giving the two numbers in curly brackets (braces),
1421 separated by a comma. The numbers must be less than 65536, and the first must
1422 be less than or equal to the second. For example:
1423 <pre>
1424 z{2,4}
1425 </pre>
1426 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1427 character. If the second number is omitted, but the comma is present, there is
1428 no upper limit; if the second number and the comma are both omitted, the
1429 quantifier specifies an exact number of required matches. Thus
1430 <pre>
1431 [aeiou]{3,}
1432 </pre>
1433 matches at least 3 successive vowels, but may match many more, while
1434 <pre>
1435 \d{8}
1436 </pre>
1437 matches exactly 8 digits. An opening curly bracket that appears in a position
1438 where a quantifier is not allowed, or one that does not match the syntax of a
1439 quantifier, is taken as a literal character. For example, {,6} is not a
1440 quantifier, but a literal string of four characters.
1441 </P>
1442 <P>
1443 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1444 bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1445 which is represented by a two-byte sequence. Similarly, when Unicode property
1446 support is available, \X{3} matches three Unicode extended sequences, each of
1447 which may be several bytes long (and they may be of different lengths).
1448 </P>
1449 <P>
1450 The quantifier {0} is permitted, causing the expression to behave as if the
1451 previous item and the quantifier were not present. This may be useful for
1452 subpatterns that are referenced as
1453 <a href="#subpatternsassubroutines">subroutines</a>
1454 from elsewhere in the pattern (but see also the section entitled
1455 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1456 below). Items other than subpatterns that have a {0} quantifier are omitted
1457 from the compiled pattern.
1458 </P>
1459 <P>
1460 For convenience, the three most common quantifiers have single-character
1461 abbreviations:
1462 <pre>
1463 * is equivalent to {0,}
1464 + is equivalent to {1,}
1465 ? is equivalent to {0,1}
1466 </pre>
1467 It is possible to construct infinite loops by following a subpattern that can
1468 match no characters with a quantifier that has no upper limit, for example:
1469 <pre>
1470 (a?)*
1471 </pre>
1472 Earlier versions of Perl and PCRE used to give an error at compile time for
1473 such patterns. However, because there are cases where this can be useful, such
1474 patterns are now accepted, but if any repetition of the subpattern does in fact
1475 match no characters, the loop is forcibly broken.
1476 </P>
1477 <P>
1478 By default, the quantifiers are "greedy", that is, they match as much as
1479 possible (up to the maximum number of permitted times), without causing the
1480 rest of the pattern to fail. The classic example of where this gives problems
1481 is in trying to match comments in C programs. These appear between /* and */
1482 and within the comment, individual * and / characters may appear. An attempt to
1483 match C comments by applying the pattern
1484 <pre>
1485 /\*.*\*/
1486 </pre>
1487 to the string
1488 <pre>
1489 /* first comment */ not comment /* second comment */
1490 </pre>
1491 fails, because it matches the entire string owing to the greediness of the .*
1492 item.
1493 </P>
1494 <P>
1495 However, if a quantifier is followed by a question mark, it ceases to be
1496 greedy, and instead matches the minimum number of times possible, so the
1497 pattern
1498 <pre>
1499 /\*.*?\*/
1500 </pre>
1501 does the right thing with the C comments. The meaning of the various
1502 quantifiers is not otherwise changed, just the preferred number of matches.
1503 Do not confuse this use of question mark with its use as a quantifier in its
1504 own right. Because it has two uses, it can sometimes appear doubled, as in
1505 <pre>
1506 \d??\d
1507 </pre>
1508 which matches one digit by preference, but can match two if that is the only
1509 way the rest of the pattern matches.
1510 </P>
1511 <P>
1512 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1513 the quantifiers are not greedy by default, but individual ones can be made
1514 greedy by following them with a question mark. In other words, it inverts the
1515 default behaviour.
1516 </P>
1517 <P>
1518 When a parenthesized subpattern is quantified with a minimum repeat count that
1519 is greater than 1 or with a limited maximum, more memory is required for the
1520 compiled pattern, in proportion to the size of the minimum or maximum.
1521 </P>
1522 <P>
1523 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1524 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1525 implicitly anchored, because whatever follows will be tried against every
1526 character position in the subject string, so there is no point in retrying the
1527 overall match at any position after the first. PCRE normally treats such a
1528 pattern as though it were preceded by \A.
1529 </P>
1530 <P>
1531 In cases where it is known that the subject string contains no newlines, it is
1532 worth setting PCRE_DOTALL in order to obtain this optimization, or
1533 alternatively using ^ to indicate anchoring explicitly.
1534 </P>
1535 <P>
1536 However, there is one situation where the optimization cannot be used. When .*
1537 is inside capturing parentheses that are the subject of a back reference
1538 elsewhere in the pattern, a match at the start may fail where a later one
1539 succeeds. Consider, for example:
1540 <pre>
1541 (.*)abc\1
1542 </pre>
1543 If the subject is "xyz123abc123" the match point is the fourth character. For
1544 this reason, such a pattern is not implicitly anchored.
1545 </P>
1546 <P>
1547 When a capturing subpattern is repeated, the value captured is the substring
1548 that matched the final iteration. For example, after
1549 <pre>
1550 (tweedle[dume]{3}\s*)+
1551 </pre>
1552 has matched "tweedledum tweedledee" the value of the captured substring is
1553 "tweedledee". However, if there are nested capturing subpatterns, the
1554 corresponding captured values may have been set in previous iterations. For
1555 example, after
1556 <pre>
1557 /(a|(b))+/
1558 </pre>
1559 matches "aba" the value of the second captured substring is "b".
1560 <a name="atomicgroup"></a></P>
1561 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1562 <P>
1563 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1564 repetition, failure of what follows normally causes the repeated item to be
1565 re-evaluated to see if a different number of repeats allows the rest of the
1566 pattern to match. Sometimes it is useful to prevent this, either to change the
1567 nature of the match, or to cause it fail earlier than it otherwise might, when
1568 the author of the pattern knows there is no point in carrying on.
1569 </P>
1570 <P>
1571 Consider, for example, the pattern \d+foo when applied to the subject line
1572 <pre>
1573 123456bar
1574 </pre>
1575 After matching all 6 digits and then failing to match "foo", the normal
1576 action of the matcher is to try again with only 5 digits matching the \d+
1577 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1578 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1579 that once a subpattern has matched, it is not to be re-evaluated in this way.
1580 </P>
1581 <P>
1582 If we use atomic grouping for the previous example, the matcher gives up
1583 immediately on failing to match "foo" the first time. The notation is a kind of
1584 special parenthesis, starting with (?&#62; as in this example:
1585 <pre>
1586 (?&#62;\d+)foo
1587 </pre>
1588 This kind of parenthesis "locks up" the part of the pattern it contains once
1589 it has matched, and a failure further into the pattern is prevented from
1590 backtracking into it. Backtracking past it to previous items, however, works as
1591 normal.
1592 </P>
1593 <P>
1594 An alternative description is that a subpattern of this type matches the string
1595 of characters that an identical standalone pattern would match, if anchored at
1596 the current point in the subject string.
1597 </P>
1598 <P>
1599 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1600 the above example can be thought of as a maximizing repeat that must swallow
1601 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1602 number of digits they match in order to make the rest of the pattern match,
1603 (?&#62;\d+) can only match an entire sequence of digits.
1604 </P>
1605 <P>
1606 Atomic groups in general can of course contain arbitrarily complicated
1607 subpatterns, and can be nested. However, when the subpattern for an atomic
1608 group is just a single repeated item, as in the example above, a simpler
1609 notation, called a "possessive quantifier" can be used. This consists of an
1610 additional + character following a quantifier. Using this notation, the
1611 previous example can be rewritten as
1612 <pre>
1613 \d++foo
1614 </pre>
1615 Note that a possessive quantifier can be used with an entire group, for
1616 example:
1617 <pre>
1618 (abc|xyz){2,3}+
1619 </pre>
1620 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1621 option is ignored. They are a convenient notation for the simpler forms of
1622 atomic group. However, there is no difference in the meaning of a possessive
1623 quantifier and the equivalent atomic group, though there may be a performance
1624 difference; possessive quantifiers should be slightly faster.
1625 </P>
1626 <P>
1627 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1628 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1629 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1630 package, and PCRE copied it from there. It ultimately found its way into Perl
1631 at release 5.10.
1632 </P>
1633 <P>
1634 PCRE has an optimization that automatically "possessifies" certain simple
1635 pattern constructs. For example, the sequence A+B is treated as A++B because
1636 there is no point in backtracking into a sequence of A's when B must follow.
1637 </P>
1638 <P>
1639 When a pattern contains an unlimited repeat inside a subpattern that can itself
1640 be repeated an unlimited number of times, the use of an atomic group is the
1641 only way to avoid some failing matches taking a very long time indeed. The
1642 pattern
1643 <pre>
1644 (\D+|&#60;\d+&#62;)*[!?]
1645 </pre>
1646 matches an unlimited number of substrings that either consist of non-digits, or
1647 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1648 quickly. However, if it is applied to
1649 <pre>
1650 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1651 </pre>
1652 it takes a long time before reporting failure. This is because the string can
1653 be divided between the internal \D+ repeat and the external * repeat in a
1654 large number of ways, and all have to be tried. (The example uses [!?] rather
1655 than a single character at the end, because both PCRE and Perl have an
1656 optimization that allows for fast failure when a single character is used. They
1657 remember the last single character that is required for a match, and fail early
1658 if it is not present in the string.) If the pattern is changed so that it uses
1659 an atomic group, like this:
1660 <pre>
1661 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1662 </pre>
1663 sequences of non-digits cannot be broken, and failure happens quickly.
1664 <a name="backreferences"></a></P>
1665 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1666 <P>
1667 Outside a character class, a backslash followed by a digit greater than 0 (and
1668 possibly further digits) is a back reference to a capturing subpattern earlier
1669 (that is, to its left) in the pattern, provided there have been that many
1670 previous capturing left parentheses.
1671 </P>
1672 <P>
1673 However, if the decimal number following the backslash is less than 10, it is
1674 always taken as a back reference, and causes an error only if there are not
1675 that many capturing left parentheses in the entire pattern. In other words, the
1676 parentheses that are referenced need not be to the left of the reference for
1677 numbers less than 10. A "forward back reference" of this type can make sense
1678 when a repetition is involved and the subpattern to the right has participated
1679 in an earlier iteration.
1680 </P>
1681 <P>
1682 It is not possible to have a numerical "forward back reference" to a subpattern
1683 whose number is 10 or more using this syntax because a sequence such as \50 is
1684 interpreted as a character defined in octal. See the subsection entitled
1685 "Non-printing characters"
1686 <a href="#digitsafterbackslash">above</a>
1687 for further details of the handling of digits following a backslash. There is
1688 no such problem when named parentheses are used. A back reference to any
1689 subpattern is possible using named parentheses (see below).
1690 </P>
1691 <P>
1692 Another way of avoiding the ambiguity inherent in the use of digits following a
1693 backslash is to use the \g escape sequence. This escape must be followed by an
1694 unsigned number or a negative number, optionally enclosed in braces. These
1695 examples are all identical:
1696 <pre>
1697 (ring), \1
1698 (ring), \g1
1699 (ring), \g{1}
1700 </pre>
1701 An unsigned number specifies an absolute reference without the ambiguity that
1702 is present in the older syntax. It is also useful when literal digits follow
1703 the reference. A negative number is a relative reference. Consider this
1704 example:
1705 <pre>
1706 (abc(def)ghi)\g{-1}
1707 </pre>
1708 The sequence \g{-1} is a reference to the most recently started capturing
1709 subpattern before \g, that is, is it equivalent to \2 in this example.
1710 Similarly, \g{-2} would be equivalent to \1. The use of relative references
1711 can be helpful in long patterns, and also in patterns that are created by
1712 joining together fragments that contain references within themselves.
1713 </P>
1714 <P>
1715 A back reference matches whatever actually matched the capturing subpattern in
1716 the current subject string, rather than anything matching the subpattern
1717 itself (see
1718 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1719 below for a way of doing that). So the pattern
1720 <pre>
1721 (sens|respons)e and \1ibility
1722 </pre>
1723 matches "sense and sensibility" and "response and responsibility", but not
1724 "sense and responsibility". If caseful matching is in force at the time of the
1725 back reference, the case of letters is relevant. For example,
1726 <pre>
1727 ((?i)rah)\s+\1
1728 </pre>
1729 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1730 capturing subpattern is matched caselessly.
1731 </P>
1732 <P>
1733 There are several different ways of writing back references to named
1734 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1735 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1736 back reference syntax, in which \g can be used for both numeric and named
1737 references, is also supported. We could rewrite the above example in any of
1738 the following ways:
1739 <pre>
1740 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1741 (?'p1'(?i)rah)\s+\k{p1}
1742 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1743 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1744 </pre>
1745 A subpattern that is referenced by name may appear in the pattern before or
1746 after the reference.
1747 </P>
1748 <P>
1749 There may be more than one back reference to the same subpattern. If a
1750 subpattern has not actually been used in a particular match, any back
1751 references to it always fail by default. For example, the pattern
1752 <pre>
1753 (a|(bc))\2
1754 </pre>
1755 always fails if it starts to match "a" rather than "bc". However, if the
1756 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1757 unset value matches an empty string.
1758 </P>
1759 <P>
1760 Because there may be many capturing parentheses in a pattern, all digits
1761 following a backslash are taken as part of a potential back reference number.
1762 If the pattern continues with a digit character, some delimiter must be used to
1763 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1764 whitespace. Otherwise, the \g{ syntax or an empty comment (see
1765 <a href="#comments">"Comments"</a>
1766 below) can be used.
1767 </P>
1768 <br><b>
1769 Recursive back references
1770 </b><br>
1771 <P>
1772 A back reference that occurs inside the parentheses to which it refers fails
1773 when the subpattern is first used, so, for example, (a\1) never matches.
1774 However, such references can be useful inside repeated subpatterns. For
1775 example, the pattern
1776 <pre>
1777 (a|b\1)+
1778 </pre>
1779 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1780 the subpattern, the back reference matches the character string corresponding
1781 to the previous iteration. In order for this to work, the pattern must be such
1782 that the first iteration does not need to match the back reference. This can be
1783 done using alternation, as in the example above, or by a quantifier with a
1784 minimum of zero.
1785 </P>
1786 <P>
1787 Back references of this type cause the group that they reference to be treated
1788 as an
1789 <a href="#atomicgroup">atomic group.</a>
1790 Once the whole group has been matched, a subsequent matching failure cannot
1791 cause backtracking into the middle of the group.
1792 <a name="bigassertions"></a></P>
1793 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1794 <P>
1795 An assertion is a test on the characters following or preceding the current
1796 matching point that does not actually consume any characters. The simple
1797 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1798 <a href="#smallassertions">above.</a>
1799 </P>
1800 <P>
1801 More complicated assertions are coded as subpatterns. There are two kinds:
1802 those that look ahead of the current position in the subject string, and those
1803 that look behind it. An assertion subpattern is matched in the normal way,
1804 except that it does not cause the current matching position to be changed.
1805 </P>
1806 <P>
1807 Assertion subpatterns are not capturing subpatterns. If such an assertion
1808 contains capturing subpatterns within it, these are counted for the purposes of
1809 numbering the capturing subpatterns in the whole pattern. However, substring
1810 capturing is carried out only for positive assertions, because it does not make
1811 sense for negative assertions.
1812 </P>
1813 <P>
1814 For compatibility with Perl, assertion subpatterns may be repeated; though
1815 it makes no sense to assert the same thing several times, the side effect of
1816 capturing parentheses may occasionally be useful. In practice, there only three
1817 cases:
1818 <br>
1819 <br>
1820 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1821 However, it may contain internal capturing parenthesized groups that are called
1822 from elsewhere via the
1823 <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1824 <br>
1825 <br>
1826 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1827 were {0,1}. At run time, the rest of the pattern match is tried with and
1828 without the assertion, the order depending on the greediness of the quantifier.
1829 <br>
1830 <br>
1831 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1832 The assertion is obeyed just once when encountered during matching.
1833 </P>
1834 <br><b>
1835 Lookahead assertions
1836 </b><br>
1837 <P>
1838 Lookahead assertions start with (?= for positive assertions and (?! for
1839 negative assertions. For example,
1840 <pre>
1841 \w+(?=;)
1842 </pre>
1843 matches a word followed by a semicolon, but does not include the semicolon in
1844 the match, and
1845 <pre>
1846 foo(?!bar)
1847 </pre>
1848 matches any occurrence of "foo" that is not followed by "bar". Note that the
1849 apparently similar pattern
1850 <pre>
1851 (?!foo)bar
1852 </pre>
1853 does not find an occurrence of "bar" that is preceded by something other than
1854 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1855 (?!foo) is always true when the next three characters are "bar". A
1856 lookbehind assertion is needed to achieve the other effect.
1857 </P>
1858 <P>
1859 If you want to force a matching failure at some point in a pattern, the most
1860 convenient way to do it is with (?!) because an empty string always matches, so
1861 an assertion that requires there not to be an empty string must always fail.
1862 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1863 <a name="lookbehind"></a></P>
1864 <br><b>
1865 Lookbehind assertions
1866 </b><br>
1867 <P>
1868 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1869 negative assertions. For example,
1870 <pre>
1871 (?&#60;!foo)bar
1872 </pre>
1873 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1874 a lookbehind assertion are restricted such that all the strings it matches must
1875 have a fixed length. However, if there are several top-level alternatives, they
1876 do not all have to have the same fixed length. Thus
1877 <pre>
1878 (?&#60;=bullock|donkey)
1879 </pre>
1880 is permitted, but
1881 <pre>
1882 (?&#60;!dogs?|cats?)
1883 </pre>
1884 causes an error at compile time. Branches that match different length strings
1885 are permitted only at the top level of a lookbehind assertion. This is an
1886 extension compared with Perl, which requires all branches to match the same
1887 length of string. An assertion such as
1888 <pre>
1889 (?&#60;=ab(c|de))
1890 </pre>
1891 is not permitted, because its single top-level branch can match two different
1892 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1893 branches:
1894 <pre>
1895 (?&#60;=abc|abde)
1896 </pre>
1897 In some cases, the escape sequence \K
1898 <a href="#resetmatchstart">(see above)</a>
1899 can be used instead of a lookbehind assertion to get round the fixed-length
1900 restriction.
1901 </P>
1902 <P>
1903 The implementation of lookbehind assertions is, for each alternative, to
1904 temporarily move the current position back by the fixed length and then try to
1905 match. If there are insufficient characters before the current position, the
1906 assertion fails.
1907 </P>
1908 <P>
1909 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1910 to appear in lookbehind assertions, because it makes it impossible to calculate
1911 the length of the lookbehind. The \X and \R escapes, which can match
1912 different numbers of bytes, are also not permitted.
1913 </P>
1914 <P>
1915 <a href="#subpatternsassubroutines">"Subroutine"</a>
1916 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1917 as the subpattern matches a fixed-length string.
1918 <a href="#recursion">Recursion,</a>
1919 however, is not supported.
1920 </P>
1921 <P>
1922 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1923 specify efficient matching of fixed-length strings at the end of subject
1924 strings. Consider a simple pattern such as
1925 <pre>
1926 abcd$
1927 </pre>
1928 when applied to a long string that does not match. Because matching proceeds
1929 from left to right, PCRE will look for each "a" in the subject and then see if
1930 what follows matches the rest of the pattern. If the pattern is specified as
1931 <pre>
1932 ^.*abcd$
1933 </pre>
1934 the initial .* matches the entire string at first, but when this fails (because
1935 there is no following "a"), it backtracks to match all but the last character,
1936 then all but the last two characters, and so on. Once again the search for "a"
1937 covers the entire string, from right to left, so we are no better off. However,
1938 if the pattern is written as
1939 <pre>
1940 ^.*+(?&#60;=abcd)
1941 </pre>
1942 there can be no backtracking for the .*+ item; it can match only the entire
1943 string. The subsequent lookbehind assertion does a single test on the last four
1944 characters. If it fails, the match fails immediately. For long strings, this
1945 approach makes a significant difference to the processing time.
1946 </P>
1947 <br><b>
1948 Using multiple assertions
1949 </b><br>
1950 <P>
1951 Several assertions (of any sort) may occur in succession. For example,
1952 <pre>
1953 (?&#60;=\d{3})(?&#60;!999)foo
1954 </pre>
1955 matches "foo" preceded by three digits that are not "999". Notice that each of
1956 the assertions is applied independently at the same point in the subject
1957 string. First there is a check that the previous three characters are all
1958 digits, and then there is a check that the same three characters are not "999".
1959 This pattern does <i>not</i> match "foo" preceded by six characters, the first
1960 of which are digits and the last three of which are not "999". For example, it
1961 doesn't match "123abcfoo". A pattern to do that is
1962 <pre>
1963 (?&#60;=\d{3}...)(?&#60;!999)foo
1964 </pre>
1965 This time the first assertion looks at the preceding six characters, checking
1966 that the first three are digits, and then the second assertion checks that the
1967 preceding three characters are not "999".
1968 </P>
1969 <P>
1970 Assertions can be nested in any combination. For example,
1971 <pre>
1972 (?&#60;=(?&#60;!foo)bar)baz
1973 </pre>
1974 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1975 preceded by "foo", while
1976 <pre>
1977 (?&#60;=\d{3}(?!999)...)foo
1978 </pre>
1979 is another pattern that matches "foo" preceded by three digits and any three
1980 characters that are not "999".
1981 <a name="conditions"></a></P>
1982 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1983 <P>
1984 It is possible to cause the matching process to obey a subpattern
1985 conditionally or to choose between two alternative subpatterns, depending on
1986 the result of an assertion, or whether a specific capturing subpattern has
1987 already been matched. The two possible forms of conditional subpattern are:
1988 <pre>
1989 (?(condition)yes-pattern)
1990 (?(condition)yes-pattern|no-pattern)
1991 </pre>
1992 If the condition is satisfied, the yes-pattern is used; otherwise the
1993 no-pattern (if present) is used. If there are more than two alternatives in the
1994 subpattern, a compile-time error occurs. Each of the two alternatives may
1995 itself contain nested subpatterns of any form, including conditional
1996 subpatterns; the restriction to two alternatives applies only at the level of
1997 the condition. This pattern fragment is an example where the alternatives are
1998 complex:
1999 <pre>
2000 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2001
2002 </PRE>
2003 </P>
2004 <P>
2005 There are four kinds of condition: references to subpatterns, references to
2006 recursion, a pseudo-condition called DEFINE, and assertions.
2007 </P>
2008 <br><b>
2009 Checking for a used subpattern by number
2010 </b><br>
2011 <P>
2012 If the text between the parentheses consists of a sequence of digits, the
2013 condition is true if a capturing subpattern of that number has previously
2014 matched. If there is more than one capturing subpattern with the same number
2015 (see the earlier
2016 <a href="#recursion">section about duplicate subpattern numbers),</a>
2017 the condition is true if any of them have matched. An alternative notation is
2018 to precede the digits with a plus or minus sign. In this case, the subpattern
2019 number is relative rather than absolute. The most recently opened parentheses
2020 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2021 loops it can also make sense to refer to subsequent groups. The next
2022 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2023 zero in any of these forms is not used; it provokes a compile-time error.)
2024 </P>
2025 <P>
2026 Consider the following pattern, which contains non-significant white space to
2027 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2028 three parts for ease of discussion:
2029 <pre>
2030 ( \( )? [^()]+ (?(1) \) )
2031 </pre>
2032 The first part matches an optional opening parenthesis, and if that
2033 character is present, sets it as the first captured substring. The second part
2034 matches one or more characters that are not parentheses. The third part is a
2035 conditional subpattern that tests whether or not the first set of parentheses
2036 matched. If they did, that is, if subject started with an opening parenthesis,
2037 the condition is true, and so the yes-pattern is executed and a closing
2038 parenthesis is required. Otherwise, since no-pattern is not present, the
2039 subpattern matches nothing. In other words, this pattern matches a sequence of
2040 non-parentheses, optionally enclosed in parentheses.
2041 </P>
2042 <P>
2043 If you were embedding this pattern in a larger one, you could use a relative
2044 reference:
2045 <pre>
2046 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2047 </pre>
2048 This makes the fragment independent of the parentheses in the larger pattern.
2049 </P>
2050 <br><b>
2051 Checking for a used subpattern by name
2052 </b><br>
2053 <P>
2054 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2055 subpattern by name. For compatibility with earlier versions of PCRE, which had
2056 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2057 there is a possible ambiguity with this syntax, because subpattern names may
2058 consist entirely of digits. PCRE looks first for a named subpattern; if it
2059 cannot find one and the name consists entirely of digits, PCRE looks for a
2060 subpattern of that number, which must be greater than zero. Using subpattern
2061 names that consist entirely of digits is not recommended.
2062 </P>
2063 <P>
2064 Rewriting the above example to use a named subpattern gives this:
2065 <pre>
2066 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
2067 </pre>
2068 If the name used in a condition of this kind is a duplicate, the test is
2069 applied to all subpatterns of the same name, and is true if any one of them has
2070 matched.
2071 </P>
2072 <br><b>
2073 Checking for pattern recursion
2074 </b><br>
2075 <P>
2076 If the condition is the string (R), and there is no subpattern with the name R,
2077 the condition is true if a recursive call to the whole pattern or any
2078 subpattern has been made. If digits or a name preceded by ampersand follow the
2079 letter R, for example:
2080 <pre>
2081 (?(R3)...) or (?(R&name)...)
2082 </pre>
2083 the condition is true if the most recent recursion is into a subpattern whose
2084 number or name is given. This condition does not check the entire recursion
2085 stack. If the name used in a condition of this kind is a duplicate, the test is
2086 applied to all subpatterns of the same name, and is true if any one of them is
2087 the most recent recursion.
2088 </P>
2089 <P>
2090 At "top level", all these recursion test conditions are false.
2091 <a href="#recursion">The syntax for recursive patterns</a>
2092 is described below.
2093 <a name="subdefine"></a></P>
2094 <br><b>
2095 Defining subpatterns for use by reference only
2096 </b><br>
2097 <P>
2098 If the condition is the string (DEFINE), and there is no subpattern with the
2099 name DEFINE, the condition is always false. In this case, there may be only one
2100 alternative in the subpattern. It is always skipped if control reaches this
2101 point in the pattern; the idea of DEFINE is that it can be used to define
2102 "subroutines" that can be referenced from elsewhere. (The use of
2103 <a href="#subpatternsassubroutines">"subroutines"</a>
2104 is described below.) For example, a pattern to match an IPv4 address such as
2105 "192.168.23.245" could be written like this (ignore whitespace and line
2106 breaks):
2107 <pre>
2108 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2109 \b (?&byte) (\.(?&byte)){3} \b
2110 </pre>
2111 The first part of the pattern is a DEFINE group inside which a another group
2112 named "byte" is defined. This matches an individual component of an IPv4
2113 address (a number less than 256). When matching takes place, this part of the
2114 pattern is skipped because DEFINE acts like a false condition. The rest of the
2115 pattern uses references to the named group to match the four dot-separated
2116 components of an IPv4 address, insisting on a word boundary at each end.
2117 </P>
2118 <br><b>
2119 Assertion conditions
2120 </b><br>
2121 <P>
2122 If the condition is not in any of the above formats, it must be an assertion.
2123 This may be a positive or negative lookahead or lookbehind assertion. Consider
2124 this pattern, again containing non-significant white space, and with the two
2125 alternatives on the second line:
2126 <pre>
2127 (?(?=[^a-z]*[a-z])
2128 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2129 </pre>
2130 The condition is a positive lookahead assertion that matches an optional
2131 sequence of non-letters followed by a letter. In other words, it tests for the
2132 presence of at least one letter in the subject. If a letter is found, the
2133 subject is matched against the first alternative; otherwise it is matched
2134 against the second. This pattern matches strings in one of the two forms
2135 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2136 <a name="comments"></a></P>
2137 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2138 <P>
2139 There are two ways of including comments in patterns that are processed by
2140 PCRE. In both cases, the start of the comment must not be in a character class,
2141 nor in the middle of any other sequence of related characters such as (?: or a
2142 subpattern name or number. The characters that make up a comment play no part
2143 in the pattern matching.
2144 </P>
2145 <P>
2146 The sequence (?# marks the start of a comment that continues up to the next
2147 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2148 option is set, an unescaped # character also introduces a comment, which in
2149 this case continues to immediately after the next newline character or
2150 character sequence in the pattern. Which characters are interpreted as newlines
2151 is controlled by the options passed to <b>pcre_compile()</b> or by a special
2152 sequence at the start of the pattern, as described in the section entitled
2153 <a href="#newlines">"Newline conventions"</a>
2154 above. Note that the end of this type of comment is a literal newline sequence
2155 in the pattern; escape sequences that happen to represent a newline do not
2156 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2157 default newline convention is in force:
2158 <pre>
2159 abc #comment \n still comment
2160 </pre>
2161 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2162 a newline in the pattern. The sequence \n is still literal at this stage, so
2163 it does not terminate the comment. Only an actual character with the code value
2164 0x0a (the default newline) does so.
2165 <a name="recursion"></a></P>
2166 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2167 <P>
2168 Consider the problem of matching a string in parentheses, allowing for
2169 unlimited nested parentheses. Without the use of recursion, the best that can
2170 be done is to use a pattern that matches up to some fixed depth of nesting. It
2171 is not possible to handle an arbitrary nesting depth.
2172 </P>
2173 <P>
2174 For some time, Perl has provided a facility that allows regular expressions to
2175 recurse (amongst other things). It does this by interpolating Perl code in the
2176 expression at run time, and the code can refer to the expression itself. A Perl
2177 pattern using code interpolation to solve the parentheses problem can be
2178 created like this:
2179 <pre>
2180 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2181 </pre>
2182 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2183 recursively to the pattern in which it appears.
2184 </P>
2185 <P>
2186 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2187 supports special syntax for recursion of the entire pattern, and also for
2188 individual subpattern recursion. After its introduction in PCRE and Python,
2189 this kind of recursion was subsequently introduced into Perl at release 5.10.
2190 </P>
2191 <P>
2192 A special item that consists of (? followed by a number greater than zero and a
2193 closing parenthesis is a recursive call of the subpattern of the given number,
2194 provided that it occurs inside that subpattern. (If not, it is a
2195 <a href="#subpatternsassubroutines">"subroutine"</a>
2196 call, which is described in the next section.) The special item (?R) or (?0) is
2197 a recursive call of the entire regular expression.
2198 </P>
2199 <P>
2200 This PCRE pattern solves the nested parentheses problem (assume the
2201 PCRE_EXTENDED option is set so that white space is ignored):
2202 <pre>
2203 \( ( [^()]++ | (?R) )* \)
2204 </pre>
2205 First it matches an opening parenthesis. Then it matches any number of
2206 substrings which can either be a sequence of non-parentheses, or a recursive
2207 match of the pattern itself (that is, a correctly parenthesized substring).
2208 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2209 to avoid backtracking into sequences of non-parentheses.
2210 </P>
2211 <P>
2212 If this were part of a larger pattern, you would not want to recurse the entire
2213 pattern, so instead you could use this:
2214 <pre>
2215 ( \( ( [^()]++ | (?1) )* \) )
2216 </pre>
2217 We have put the pattern into parentheses, and caused the recursion to refer to
2218 them instead of the whole pattern.
2219 </P>
2220 <P>
2221 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2222 is made easier by the use of relative references. Instead of (?1) in the
2223 pattern above you can write (?-2) to refer to the second most recently opened
2224 parentheses preceding the recursion. In other words, a negative number counts
2225 capturing parentheses leftwards from the point at which it is encountered.
2226 </P>
2227 <P>
2228 It is also possible to refer to subsequently opened parentheses, by writing
2229 references such as (?+2). However, these cannot be recursive because the
2230 reference is not inside the parentheses that are referenced. They are always
2231 <a href="#subpatternsassubroutines">"subroutine"</a>
2232 calls, as described in the next section.
2233 </P>
2234 <P>
2235 An alternative approach is to use named parentheses instead. The Perl syntax
2236 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2237 could rewrite the above example as follows:
2238 <pre>
2239 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2240 </pre>
2241 If there is more than one subpattern with the same name, the earliest one is
2242 used.
2243 </P>
2244 <P>
2245 This particular example pattern that we have been looking at contains nested
2246 unlimited repeats, and so the use of a possessive quantifier for matching
2247 strings of non-parentheses is important when applying the pattern to strings
2248 that do not match. For example, when this pattern is applied to
2249 <pre>
2250 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2251 </pre>
2252 it yields "no match" quickly. However, if a possessive quantifier is not used,
2253 the match runs for a very long time indeed because there are so many different
2254 ways the + and * repeats can carve up the subject, and all have to be tested
2255 before failure can be reported.
2256 </P>
2257 <P>
2258 At the end of a match, the values of capturing parentheses are those from
2259 the outermost level. If you want to obtain intermediate values, a callout
2260 function can be used (see below and the
2261 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2262 documentation). If the pattern above is matched against
2263 <pre>
2264 (ab(cd)ef)
2265 </pre>
2266 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2267 the last value taken on at the top level. If a capturing subpattern is not
2268 matched at the top level, its final value is unset, even if it is (temporarily)
2269 set at a deeper level.
2270 </P>
2271 <P>
2272 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2273 obtain extra memory to store data during a recursion, which it does by using
2274 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2275 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2276 </P>
2277 <P>
2278 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2279 Consider this pattern, which matches text in angle brackets, allowing for
2280 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2281 recursing), whereas any characters are permitted at the outer level.
2282 <pre>
2283 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2284 </pre>
2285 In this pattern, (?(R) is the start of a conditional subpattern, with two
2286 different alternatives for the recursive and non-recursive cases. The (?R) item
2287 is the actual recursive call.
2288 <a name="recursiondifference"></a></P>
2289 <br><b>
2290 Recursion difference from Perl
2291 </b><br>
2292 <P>
2293 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2294 treated as an atomic group. That is, once it has matched some of the subject
2295 string, it is never re-entered, even if it contains untried alternatives and
2296 there is a subsequent matching failure. This can be illustrated by the
2297 following pattern, which purports to match a palindromic string that contains
2298 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2299 <pre>
2300 ^(.|(.)(?1)\2)$
2301 </pre>
2302 The idea is that it either matches a single character, or two identical
2303 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2304 it does not if the pattern is longer than three characters. Consider the
2305 subject string "abcba":
2306 </P>
2307 <P>
2308 At the top level, the first character is matched, but as it is not at the end
2309 of the string, the first alternative fails; the second alternative is taken
2310 and the recursion kicks in. The recursive call to subpattern 1 successfully
2311 matches the next character ("b"). (Note that the beginning and end of line
2312 tests are not part of the recursion).
2313 </P>
2314 <P>
2315 Back at the top level, the next character ("c") is compared with what
2316 subpattern 2 matched, which was "a". This fails. Because the recursion is
2317 treated as an atomic group, there are now no backtracking points, and so the
2318 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2319 try the second alternative.) However, if the pattern is written with the
2320 alternatives in the other order, things are different:
2321 <pre>
2322 ^((.)(?1)\2|.)$
2323 </pre>
2324 This time, the recursing alternative is tried first, and continues to recurse
2325 until it runs out of characters, at which point the recursion fails. But this
2326 time we do have another alternative to try at the higher level. That is the big
2327 difference: in the previous case the remaining alternative is at a deeper
2328 recursion level, which PCRE cannot use.
2329 </P>
2330 <P>
2331 To change the pattern so that it matches all palindromic strings, not just
2332 those with an odd number of characters, it is tempting to change the pattern to
2333 this:
2334 <pre>
2335 ^((.)(?1)\2|.?)$
2336 </pre>
2337 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2338 deeper recursion has matched a single character, it cannot be entered again in
2339 order to match an empty string. The solution is to separate the two cases, and
2340 write out the odd and even cases as alternatives at the higher level:
2341 <pre>
2342 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2343 </pre>
2344 If you want to match typical palindromic phrases, the pattern has to ignore all
2345 non-word characters, which can be done like this:
2346 <pre>
2347 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2348 </pre>
2349 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2350 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2351 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2352 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2353 more) to match typical phrases, and Perl takes so long that you think it has
2354 gone into a loop.
2355 </P>
2356 <P>
2357 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2358 string does not start with a palindrome that is shorter than the entire string.
2359 For example, although "abcba" is correctly matched, if the subject is "ababa",
2360 PCRE finds the palindrome "aba" at the start, then fails at top level because
2361 the end of the string does not follow. Once again, it cannot jump back into the
2362 recursion to try other alternatives, so the entire match fails.
2363 <a name="subpatternsassubroutines"></a></P>
2364 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2365 <P>
2366 If the syntax for a recursive subpattern reference (either by number or by
2367 name) is used outside the parentheses to which it refers, it operates like a
2368 subroutine in a programming language. The "called" subpattern may be defined
2369 before or after the reference. A numbered reference can be absolute or
2370 relative, as in these examples:
2371 <pre>
2372 (...(absolute)...)...(?2)...
2373 (...(relative)...)...(?-1)...
2374 (...(?+1)...(relative)...
2375 </pre>
2376 An earlier example pointed out that the pattern
2377 <pre>
2378 (sens|respons)e and \1ibility
2379 </pre>
2380 matches "sense and sensibility" and "response and responsibility", but not
2381 "sense and responsibility". If instead the pattern
2382 <pre>
2383 (sens|respons)e and (?1)ibility
2384 </pre>
2385 is used, it does match "sense and responsibility" as well as the other two
2386 strings. Another example is given in the discussion of DEFINE above.
2387 </P>
2388 <P>
2389 Like recursive subpatterns, a subroutine call is always treated as an atomic
2390 group. That is, once it has matched some of the subject string, it is never
2391 re-entered, even if it contains untried alternatives and there is a subsequent
2392 matching failure. Any capturing parentheses that are set during the subroutine
2393 call revert to their previous values afterwards.
2394 </P>
2395 <P>
2396 When a subpattern is used as a subroutine, processing options such as
2397 case-independence are fixed when the subpattern is defined. They cannot be
2398 changed for different calls. For example, consider this pattern:
2399 <pre>
2400 (abc)(?i:(?-1))
2401 </pre>
2402 It matches "abcabc". It does not match "abcABC" because the change of
2403 processing option does not affect the called subpattern.
2404 <a name="onigurumasubroutines"></a></P>
2405 <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2406 <P>
2407 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2408 a number enclosed either in angle brackets or single quotes, is an alternative
2409 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2410 are two of the examples used above, rewritten using this syntax:
2411 <pre>
2412 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2413 (sens|respons)e and \g'1'ibility
2414 </pre>
2415 PCRE supports an extension to Oniguruma: if a number is preceded by a
2416 plus or a minus sign it is taken as a relative reference. For example:
2417 <pre>
2418 (abc)(?i:\g&#60;-1&#62;)
2419 </pre>
2420 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2421 synonymous. The former is a back reference; the latter is a subroutine call.
2422 </P>
2423 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2424 <P>
2425 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2426 code to be obeyed in the middle of matching a regular expression. This makes it
2427 possible, amongst other things, to extract different substrings that match the
2428 same pair of parentheses when there is a repetition.
2429 </P>
2430 <P>
2431 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2432 code. The feature is called "callout". The caller of PCRE provides an external
2433 function by putting its entry point in the global variable <i>pcre_callout</i>.
2434 By default, this variable contains NULL, which disables all calling out.
2435 </P>
2436 <P>
2437 Within a regular expression, (?C) indicates the points at which the external
2438 function is to be called. If you want to identify different callout points, you
2439 can put a number less than 256 after the letter C. The default value is zero.
2440 For example, this pattern has two callout points:
2441 <pre>
2442 (?C1)abc(?C2)def
2443 </pre>
2444 If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2445 automatically installed before each item in the pattern. They are all numbered
2446 255.
2447 </P>
2448 <P>
2449 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2450 set), the external function is called. It is provided with the number of the
2451 callout, the position in the pattern, and, optionally, one item of data
2452 originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2453 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2454 description of the interface to the callout function is given in the
2455 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2456 documentation.
2457 <a name="backtrackcontrol"></a></P>
2458 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2459 <P>
2460 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2461 are described in the Perl documentation as "experimental and subject to change
2462 or removal in a future version of Perl". It goes on to say: "Their usage in
2463 production code should be noted to avoid problems during upgrades." The same
2464 remarks apply to the PCRE features described in this section.
2465 </P>
2466 <P>
2467 Since these verbs are specifically related to backtracking, most of them can be
2468 used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2469 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2470 failing negative assertion, they cause an error if encountered by
2471 <b>pcre_dfa_exec()</b>.
2472 </P>
2473 <P>
2474 If any of these verbs are used in an assertion or subroutine subpattern
2475 (including recursive subpatterns), their effect is confined to that subpattern;
2476 it does not extend to the surrounding pattern, with one exception: a *MARK that
2477 is encountered in a positive assertion <i>is</i> passed back (compare capturing
2478 parentheses in assertions). Note that such subpatterns are processed as
2479 anchored at the point where they are tested.
2480 </P>
2481 <P>
2482 The new verbs make use of what was previously invalid syntax: an opening
2483 parenthesis followed by an asterisk. They are generally of the form
2484 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2485 depending on whether or not an argument is present. An name is a sequence of
2486 letters, digits, and underscores. If the name is empty, that is, if the closing
2487 parenthesis immediately follows the colon, the effect is as if the colon were
2488 not there. Any number of these verbs may occur in a pattern.
2489 </P>
2490 <P>
2491 PCRE contains some optimizations that are used to speed up matching by running
2492 some checks at the start of each match attempt. For example, it may know the
2493 minimum length of matching subject, or that a particular character must be
2494 present. When one of these optimizations suppresses the running of a match, any
2495 included backtracking verbs will not, of course, be processed. You can suppress
2496 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2497 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2498 pattern with (*NO_START_OPT).
2499 </P>
2500 <br><b>
2501 Verbs that act immediately
2502 </b><br>
2503 <P>
2504 The following verbs act as soon as they are encountered. They may not be
2505 followed by a name.
2506 <pre>
2507 (*ACCEPT)
2508 </pre>
2509 This verb causes the match to end successfully, skipping the remainder of the
2510 pattern. When inside a recursion, only the innermost pattern is ended
2511 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2512 captured. (This feature was added to PCRE at release 8.00.) For example:
2513 <pre>
2514 A((?:A|B(*ACCEPT)|C)D)
2515 </pre>
2516 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2517 the outer parentheses.
2518 <pre>
2519 (*FAIL) or (*F)
2520 </pre>
2521 This verb causes the match to fail, forcing backtracking to occur. It is
2522 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2523 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2524 Perl features that are not present in PCRE. The nearest equivalent is the
2525 callout feature, as for example in this pattern:
2526 <pre>
2527 a+(?C)(*FAIL)
2528 </pre>
2529 A match with the string "aaaa" always fails, but the callout is taken before
2530 each backtrack happens (in this example, 10 times).
2531 </P>
2532 <br><b>
2533 Recording which path was taken
2534 </b><br>
2535 <P>
2536 There is one verb whose main purpose is to track how a match was arrived at,
2537 though it also has a secondary use in conjunction with advancing the match
2538 starting point (see (*SKIP) below).
2539 <pre>
2540 (*MARK:NAME) or (*:NAME)
2541 </pre>
2542 A name is always required with this verb. There may be as many instances of
2543 (*MARK) as you like in a pattern, and their names do not have to be unique.
2544 </P>
2545 <P>
2546 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2547 to the caller via the <i>pcre_extra</i> data structure, as described in the
2548 <a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a>
2549 in the
2550 <a href="pcreapi.html"><b>pcreapi</b></a>
2551 documentation. No data is returned for a partial match. Here is an example of
2552 <b>pcretest</b> output, where the /K modifier requests the retrieval and
2553 outputting of (*MARK) data:
2554 <pre>
2555 /X(*MARK:A)Y|X(*MARK:B)Z/K
2556 XY
2557 0: XY
2558 MK: A
2559 XZ
2560 0: XZ
2561 MK: B
2562 </pre>
2563 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2564 indicates which of the two alternatives matched. This is a more efficient way
2565 of obtaining this information than putting each alternative in its own
2566 capturing parentheses.
2567 </P>
2568 <P>
2569 If (*MARK) is encountered in a positive assertion, its name is recorded and
2570 passed back if it is the last-encountered. This does not happen for negative
2571 assetions.
2572 </P>
2573 <P>
2574 A name may also be returned after a failed match if the final path through the
2575 pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2576 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2577 starting point for matching is advanced, the final check is often with an empty
2578 string, causing a failure before (*MARK) is reached. For example:
2579 <pre>
2580 /X(*MARK:A)Y|X(*MARK:B)Z/K
2581 XP
2582 No match
2583 </pre>
2584 There are three potential starting points for this match (starting with X,
2585 starting with P, and with an empty string). If the pattern is anchored, the
2586 result is different:
2587 <pre>
2588 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2589 XP
2590 No match, mark = B
2591 </pre>
2592 PCRE's start-of-match optimizations can also interfere with this. For example,
2593 if, as a result of a call to <b>pcre_study()</b>, it knows the minimum
2594 subject length for a match, a shorter subject will not be scanned at all.
2595 </P>
2596 <P>
2597 Note that similar anomalies (though different in detail) exist in Perl, no
2598 doubt for the same reasons. The use of (*MARK) data after a failed match of an
2599 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2600 </P>
2601 <br><b>
2602 Verbs that act after backtracking
2603 </b><br>
2604 <P>
2605 The following verbs do nothing when they are encountered. Matching continues
2606 with what follows, but if there is no subsequent match, causing a backtrack to
2607 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2608 the verb. However, when one of these verbs appears inside an atomic group, its
2609 effect is confined to that group, because once the group has been matched,
2610 there is never any backtracking into it. In this situation, backtracking can
2611 "jump back" to the left of the entire atomic group. (Remember also, as stated
2612 above, that this localization also applies in subroutine calls and assertions.)
2613 </P>
2614 <P>
2615 These verbs differ in exactly what kind of failure occurs when backtracking
2616 reaches them.
2617 <pre>
2618 (*COMMIT)
2619 </pre>
2620 This verb, which may not be followed by a name, causes the whole match to fail
2621 outright if the rest of the pattern does not match. Even if the pattern is
2622 unanchored, no further attempts to find a match by advancing the starting point
2623 take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2624 finding a match at the current starting point, or not at all. For example:
2625 <pre>
2626 a+(*COMMIT)b
2627 </pre>
2628 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2629 dynamic anchor, or "I've started, so I must finish." The name of the most
2630 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2631 match failure.
2632 </P>
2633 <P>
2634 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2635 unless PCRE's start-of-match optimizations are turned off, as shown in this
2636 <b>pcretest</b> example:
2637 <pre>
2638 /(*COMMIT)abc/
2639 xyzabc
2640 0: abc
2641 xyzabc\Y
2642 No match
2643 </pre>
2644 PCRE knows that any match must start with "a", so the optimization skips along
2645 the subject to "a" before running the first match attempt, which succeeds. When
2646 the optimization is disabled by the \Y escape in the second subject, the match
2647 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2648 starting points.
2649 <pre>
2650 (*PRUNE) or (*PRUNE:NAME)
2651 </pre>
2652 This verb causes the match to fail at the current starting position in the
2653 subject if the rest of the pattern does not match. If the pattern is
2654 unanchored, the normal "bumpalong" advance to the next starting character then
2655 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2656 reached, or when matching to the right of (*PRUNE), but if there is no match to
2657 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2658 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2659 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2660 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2661 match fails completely; the name is passed back if this is the final attempt.
2662 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2663 pattern (*PRUNE) has the same effect as (*COMMIT).
2664 <pre>
2665 (*SKIP)
2666 </pre>
2667 This verb, when given without a name, is like (*PRUNE), except that if the
2668 pattern is unanchored, the "bumpalong" advance is not to the next character,
2669 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2670 signifies that whatever text was matched leading up to it cannot be part of a
2671 successful match. Consider:
2672 <pre>
2673 a+(*SKIP)b
2674 </pre>
2675 If the subject is "aaaac...", after the first match attempt fails (starting at
2676 the first character in the string), the starting point skips on to start the
2677 next attempt at "c". Note that a possessive quantifer does not have the same
2678 effect as this example; although it would suppress backtracking during the
2679 first match attempt, the second attempt would start at the second character
2680 instead of skipping on to "c".
2681 <pre>
2682 (*SKIP:NAME)
2683 </pre>
2684 When (*SKIP) has an associated name, its behaviour is modified. If the
2685 following pattern fails to match, the previous path through the pattern is
2686 searched for the most recent (*MARK) that has the same name. If one is found,
2687 the "bumpalong" advance is to the subject position that corresponds to that
2688 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2689 matching name is found, normal "bumpalong" of one character happens (the
2690 (*SKIP) is ignored).
2691 <pre>
2692 (*THEN) or (*THEN:NAME)
2693 </pre>
2694 This verb causes a skip to the next alternation in the innermost enclosing
2695 group if the rest of the pattern does not match. That is, it cancels pending
2696 backtracking, but only within the current alternation. Its name comes from the
2697 observation that it can be used for a pattern-based if-then-else block:
2698 <pre>
2699 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2700 </pre>
2701 If the COND1 pattern matches, FOO is tried (and possibly further items after
2702 the end of the group if FOO succeeds); on failure the matcher skips to the
2703 second alternative and tries COND2, without backtracking into COND1. The
2704 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2705 overall match fails. If (*THEN) is not directly inside an alternation, it acts
2706 like (*PRUNE).
2707 </P>
2708 <P>
2709 The above verbs provide four different "strengths" of control when subsequent
2710 matching fails. (*THEN) is the weakest, carrying on the match at the next
2711 alternation. (*PRUNE) comes next, failing the match at the current starting
2712 position, but allowing an advance to the next character (for an unanchored
2713 pattern). (*SKIP) is similar, except that the advance may be more than one
2714 character. (*COMMIT) is the strongest, causing the entire match to fail.
2715 </P>
2716 <P>
2717 If more than one is present in a pattern, the "stongest" one wins. For example,
2718 consider this pattern, where A, B, etc. are complex pattern fragments:
2719 <pre>
2720 (A(*COMMIT)B(*THEN)C|D)
2721 </pre>
2722 Once A has matched, PCRE is committed to this match, at the current starting
2723 position. If subsequently B matches, but C does not, the normal (*THEN) action
2724 of trying the next alternation (that is, D) does not happen because (*COMMIT)
2725 overrides.
2726 </P>
2727 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2728 <P>
2729 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2730 <b>pcresyntax</b>(3), <b>pcre</b>(3).
2731 </P>
2732 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2733 <P>
2734 Philip Hazel
2735 <br>
2736 University Computing Service
2737 <br>
2738 Cambridge CB2 3QH, England.
2739 <br>
2740 </P>
2741 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2742 <P>
2743 Last updated: 24 July 2011
2744 <br>
2745 Copyright &copy; 1997-2011 University of Cambridge.
2746 <br>
2747 <p>
2748 Return to the <a href="index.html">PCRE index page</a>.
2749 </p>

Properties

Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

  ViewVC Help
Powered by ViewVC 1.1.5