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1 <html>
2 <head>
3 <title>pcrepattern specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepattern man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18 <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19 <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20 <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21 <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
22 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</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>
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="pcreunicode.html"><b>pcreunicode</b></a>
76 page.
77 </P>
78 <P>
79 Another special sequence that may appear at the start of a pattern or in
80 combination with (*UTF8) is:
81 <pre>
82 (*UCP)
83 </pre>
84 This has the same effect as setting the PCRE_UCP option: it causes sequences
85 such as \d and \w to use Unicode properties to determine character types,
86 instead of recognizing only characters with codes less than 128 via a lookup
87 table.
88 </P>
89 <P>
90 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
91 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
92 also some more of these special sequences that are concerned with the handling
93 of newlines; they are described below.
94 </P>
95 <P>
96 The remainder of this document discusses the patterns that are supported by
97 PCRE when its main matching function, <b>pcre_exec()</b>, is used.
98 From release 6.0, PCRE offers a second matching function,
99 <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
100 Perl-compatible. Some of the features discussed below are not available when
101 <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
102 alternative function, and how it differs from the normal function, are
103 discussed in the
104 <a href="pcrematching.html"><b>pcrematching</b></a>
105 page.
106 <a name="newlines"></a></P>
107 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
108 <P>
109 PCRE supports five different conventions for indicating line breaks in
110 strings: a single CR (carriage return) character, a single LF (linefeed)
111 character, the two-character sequence CRLF, any of the three preceding, or any
112 Unicode newline sequence. The
113 <a href="pcreapi.html"><b>pcreapi</b></a>
114 page has
115 <a href="pcreapi.html#newlines">further discussion</a>
116 about newlines, and shows how to set the newline convention in the
117 <i>options</i> arguments for the compiling and matching functions.
118 </P>
119 <P>
120 It is also possible to specify a newline convention by starting a pattern
121 string with one of the following five sequences:
122 <pre>
123 (*CR) carriage return
124 (*LF) linefeed
125 (*CRLF) carriage return, followed by linefeed
126 (*ANYCRLF) any of the three above
127 (*ANY) all Unicode newline sequences
128 </pre>
129 These override the default and the options given to <b>pcre_compile()</b> or
130 <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default
131 newline sequence, the pattern
132 <pre>
133 (*CR)a.b
134 </pre>
135 changes the convention to CR. That pattern matches "a\nb" because LF is no
136 longer a newline. Note that these special settings, which are not
137 Perl-compatible, are recognized only at the very start of a pattern, and that
138 they must be in upper case. If more than one of them is present, the last one
139 is used.
140 </P>
141 <P>
142 The newline convention affects the interpretation of the dot metacharacter when
143 PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
144 affect what the \R escape sequence matches. By default, this is any Unicode
145 newline sequence, for Perl compatibility. However, this can be changed; see the
146 description of \R in the section entitled
147 <a href="#newlineseq">"Newline sequences"</a>
148 below. A change of \R setting can be combined with a change of newline
149 convention.
150 </P>
151 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
152 <P>
153 A regular expression is a pattern that is matched against a subject string from
154 left to right. Most characters stand for themselves in a pattern, and match the
155 corresponding characters in the subject. As a trivial example, the pattern
156 <pre>
157 The quick brown fox
158 </pre>
159 matches a portion of a subject string that is identical to itself. When
160 caseless matching is specified (the PCRE_CASELESS option), letters are matched
161 independently of case. In UTF-8 mode, PCRE always understands the concept of
162 case for characters whose values are less than 128, so caseless matching is
163 always possible. For characters with higher values, the concept of case is
164 supported if PCRE is compiled with Unicode property support, but not otherwise.
165 If you want to use caseless matching for characters 128 and above, you must
166 ensure that PCRE is compiled with Unicode property support as well as with
167 UTF-8 support.
168 </P>
169 <P>
170 The power of regular expressions comes from the ability to include alternatives
171 and repetitions in the pattern. These are encoded in the pattern by the use of
172 <i>metacharacters</i>, which do not stand for themselves but instead are
173 interpreted in some special way.
174 </P>
175 <P>
176 There are two different sets of metacharacters: those that are recognized
177 anywhere in the pattern except within square brackets, and those that are
178 recognized within square brackets. Outside square brackets, the metacharacters
179 are as follows:
180 <pre>
181 \ general escape character with several uses
182 ^ assert start of string (or line, in multiline mode)
183 $ assert end of string (or line, in multiline mode)
184 . match any character except newline (by default)
185 [ start character class definition
186 | start of alternative branch
187 ( start subpattern
188 ) end subpattern
189 ? extends the meaning of (
190 also 0 or 1 quantifier
191 also quantifier minimizer
192 * 0 or more quantifier
193 + 1 or more quantifier
194 also "possessive quantifier"
195 { start min/max quantifier
196 </pre>
197 Part of a pattern that is in square brackets is called a "character class". In
198 a character class the only metacharacters are:
199 <pre>
200 \ general escape character
201 ^ negate the class, but only if the first character
202 - indicates character range
203 [ POSIX character class (only if followed by POSIX syntax)
204 ] terminates the character class
205 </pre>
206 The following sections describe the use of each of the metacharacters.
207 </P>
208 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
209 <P>
210 The backslash character has several uses. Firstly, if it is followed by a
211 character that is not a number or a letter, it takes away any special meaning
212 that character may have. This use of backslash as an escape character applies
213 both inside and outside character classes.
214 </P>
215 <P>
216 For example, if you want to match a * character, you write \* in the pattern.
217 This escaping action applies whether or not the following character would
218 otherwise be interpreted as a metacharacter, so it is always safe to precede a
219 non-alphanumeric with backslash to specify that it stands for itself. In
220 particular, if you want to match a backslash, you write \\.
221 </P>
222 <P>
223 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
224 backslash. All other characters (in particular, those whose codepoints are
225 greater than 127) are treated as literals.
226 </P>
227 <P>
228 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
229 pattern (other than in a character class) and characters between a # outside
230 a character class and the next newline are ignored. An escaping backslash can
231 be used to include a whitespace or # character as part of the pattern.
232 </P>
233 <P>
234 If you want to remove the special meaning from a sequence of characters, you
235 can do so by putting them between \Q and \E. This is different from Perl in
236 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
237 Perl, $ and @ cause variable interpolation. Note the following examples:
238 <pre>
239 Pattern PCRE matches Perl matches
241 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
242 \Qabc\$xyz\E abc\$xyz abc\$xyz
243 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
244 </pre>
245 The \Q...\E sequence is recognized both inside and outside character classes.
246 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
247 by \E later in the pattern, the literal interpretation continues to the end of
248 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
249 a character class, this causes an error, because the character class is not
250 terminated.
251 <a name="digitsafterbackslash"></a></P>
252 <br><b>
253 Non-printing characters
254 </b><br>
255 <P>
256 A second use of backslash provides a way of encoding non-printing characters
257 in patterns in a visible manner. There is no restriction on the appearance of
258 non-printing characters, apart from the binary zero that terminates a pattern,
259 but when a pattern is being prepared by text editing, it is often easier to use
260 one of the following escape sequences than the binary character it represents:
261 <pre>
262 \a alarm, that is, the BEL character (hex 07)
263 \cx "control-x", where x is any ASCII character
264 \e escape (hex 1B)
265 \f formfeed (hex 0C)
266 \n linefeed (hex 0A)
267 \r carriage return (hex 0D)
268 \t tab (hex 09)
269 \ddd character with octal code ddd, or back reference
270 \xhh character with hex code hh
271 \x{hhh..} character with hex code hhh..
272 </pre>
273 The precise effect of \cx is as follows: if x is a lower case letter, it
274 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
275 Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
276 \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
277 than 127, a compile-time error occurs. This locks out non-ASCII characters in
278 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
279 values are valid. A lower case letter is converted to upper case, and then the
280 0xc0 bits are flipped.)
281 </P>
282 <P>
283 After \x, from zero to two hexadecimal digits are read (letters can be in
284 upper or lower case). Any number of hexadecimal digits may appear between \x{
285 and }, but the value of the character code must be less than 256 in non-UTF-8
286 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
287 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
288 point, which is 10FFFF.
289 </P>
290 <P>
291 If characters other than hexadecimal digits appear between \x{ and }, or if
292 there is no terminating }, this form of escape is not recognized. Instead, the
293 initial \x will be interpreted as a basic hexadecimal escape, with no
294 following digits, giving a character whose value is zero.
295 </P>
296 <P>
297 Characters whose value is less than 256 can be defined by either of the two
298 syntaxes for \x. There is no difference in the way they are handled. For
299 example, \xdc is exactly the same as \x{dc}.
300 </P>
301 <P>
302 After \0 up to two further octal digits are read. If there are fewer than two
303 digits, just those that are present are used. Thus the sequence \0\x\07
304 specifies two binary zeros followed by a BEL character (code value 7). Make
305 sure you supply two digits after the initial zero if the pattern character that
306 follows is itself an octal digit.
307 </P>
308 <P>
309 The handling of a backslash followed by a digit other than 0 is complicated.
310 Outside a character class, PCRE reads it and any following digits as a decimal
311 number. If the number is less than 10, or if there have been at least that many
312 previous capturing left parentheses in the expression, the entire sequence is
313 taken as a <i>back reference</i>. A description of how this works is given
314 <a href="#backreferences">later,</a>
315 following the discussion of
316 <a href="#subpattern">parenthesized subpatterns.</a>
317 </P>
318 <P>
319 Inside a character class, or if the decimal number is greater than 9 and there
320 have not been that many capturing subpatterns, PCRE re-reads up to three octal
321 digits following the backslash, and uses them to generate a data character. Any
322 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
323 character specified in octal must be less than \400. In UTF-8 mode, values up
324 to \777 are permitted. For example:
325 <pre>
326 \040 is another way of writing a space
327 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
328 \7 is always a back reference
329 \11 might be a back reference, or another way of writing a tab
330 \011 is always a tab
331 \0113 is a tab followed by the character "3"
332 \113 might be a back reference, otherwise the character with octal code 113
333 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
334 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
335 </pre>
336 Note that octal values of 100 or greater must not be introduced by a leading
337 zero, because no more than three octal digits are ever read.
338 </P>
339 <P>
340 All the sequences that define a single character value can be used both inside
341 and outside character classes. In addition, inside a character class, the
342 sequence \b is interpreted as the backspace character (hex 08). The sequences
343 \B, \N, \R, and \X are not special inside a character class. Like any other
344 unrecognized escape sequences, they are treated as the literal characters "B",
345 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
346 set. Outside a character class, these sequences have different meanings.
347 </P>
348 <br><b>
349 Absolute and relative back references
350 </b><br>
351 <P>
352 The sequence \g followed by an unsigned or a negative number, optionally
353 enclosed in braces, is an absolute or relative back reference. A named back
354 reference can be coded as \g{name}. Back references are discussed
355 <a href="#backreferences">later,</a>
356 following the discussion of
357 <a href="#subpattern">parenthesized subpatterns.</a>
358 </P>
359 <br><b>
360 Absolute and relative subroutine calls
361 </b><br>
362 <P>
363 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
364 a number enclosed either in angle brackets or single quotes, is an alternative
365 syntax for referencing a subpattern as a "subroutine". Details are discussed
366 <a href="#onigurumasubroutines">later.</a>
367 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
368 synonymous. The former is a back reference; the latter is a
369 <a href="#subpatternsassubroutines">subroutine</a>
370 call.
371 <a name="genericchartypes"></a></P>
372 <br><b>
373 Generic character types
374 </b><br>
375 <P>
376 Another use of backslash is for specifying generic character types:
377 <pre>
378 \d any decimal digit
379 \D any character that is not a decimal digit
380 \h any horizontal whitespace character
381 \H any character that is not a horizontal whitespace character
382 \s any whitespace character
383 \S any character that is not a whitespace character
384 \v any vertical whitespace character
385 \V any character that is not a vertical whitespace character
386 \w any "word" character
387 \W any "non-word" character
388 </pre>
389 There is also the single sequence \N, which matches a non-newline character.
390 This is the same as
391 <a href="#fullstopdot">the "." metacharacter</a>
392 when PCRE_DOTALL is not set.
393 </P>
394 <P>
395 Each pair of lower and upper case escape sequences partitions the complete set
396 of characters into two disjoint sets. Any given character matches one, and only
397 one, of each pair. The sequences can appear both inside and outside character
398 classes. They each match one character of the appropriate type. If the current
399 matching point is at the end of the subject string, all of them fail, because
400 there is no character to match.
401 </P>
402 <P>
403 For compatibility with Perl, \s does not match the VT character (code 11).
404 This makes it different from the the POSIX "space" class. The \s characters
405 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
406 included in a Perl script, \s may match the VT character. In PCRE, it never
407 does.
408 </P>
409 <P>
410 A "word" character is an underscore or any character that is a letter or digit.
411 By default, the definition of letters and digits is controlled by PCRE's
412 low-valued character tables, and may vary if locale-specific matching is taking
413 place (see
414 <a href="pcreapi.html#localesupport">"Locale support"</a>
415 in the
416 <a href="pcreapi.html"><b>pcreapi</b></a>
417 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
418 or "french" in Windows, some character codes greater than 128 are used for
419 accented letters, and these are then matched by \w. The use of locales with
420 Unicode is discouraged.
421 </P>
422 <P>
423 By default, in UTF-8 mode, characters with values greater than 128 never match
424 \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
425 their original meanings from before UTF-8 support was available, mainly for
426 efficiency reasons. However, if PCRE is compiled with Unicode property support,
427 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
428 properties are used to determine character types, as follows:
429 <pre>
430 \d any character that \p{Nd} matches (decimal digit)
431 \s any character that \p{Z} matches, plus HT, LF, FF, CR
432 \w any character that \p{L} or \p{N} matches, plus underscore
433 </pre>
434 The upper case escapes match the inverse sets of characters. Note that \d
435 matches only decimal digits, whereas \w matches any Unicode digit, as well as
436 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
437 \B because they are defined in terms of \w and \W. Matching these sequences
438 is noticeably slower when PCRE_UCP is set.
439 </P>
440 <P>
441 The sequences \h, \H, \v, and \V are features that were added to Perl at
442 release 5.10. In contrast to the other sequences, which match only ASCII
443 characters by default, these always match certain high-valued codepoints in
444 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
445 are:
446 <pre>
447 U+0009 Horizontal tab
448 U+0020 Space
449 U+00A0 Non-break space
450 U+1680 Ogham space mark
451 U+180E Mongolian vowel separator
452 U+2000 En quad
453 U+2001 Em quad
454 U+2002 En space
455 U+2003 Em space
456 U+2004 Three-per-em space
457 U+2005 Four-per-em space
458 U+2006 Six-per-em space
459 U+2007 Figure space
460 U+2008 Punctuation space
461 U+2009 Thin space
462 U+200A Hair space
463 U+202F Narrow no-break space
464 U+205F Medium mathematical space
465 U+3000 Ideographic space
466 </pre>
467 The vertical space characters are:
468 <pre>
469 U+000A Linefeed
470 U+000B Vertical tab
471 U+000C Formfeed
472 U+000D Carriage return
473 U+0085 Next line
474 U+2028 Line separator
475 U+2029 Paragraph separator
476 <a name="newlineseq"></a></PRE>
477 </P>
478 <br><b>
479 Newline sequences
480 </b><br>
481 <P>
482 Outside a character class, by default, the escape sequence \R matches any
483 Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following:
484 <pre>
485 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
486 </pre>
487 This is an example of an "atomic group", details of which are given
488 <a href="#atomicgroup">below.</a>
489 This particular group matches either the two-character sequence CR followed by
490 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
491 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
492 line, U+0085). The two-character sequence is treated as a single unit that
493 cannot be split.
494 </P>
495 <P>
496 In UTF-8 mode, two additional characters whose codepoints are greater than 255
497 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
498 Unicode character property support is not needed for these characters to be
499 recognized.
500 </P>
501 <P>
502 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
503 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
504 either at compile time or when the pattern is matched. (BSR is an abbrevation
505 for "backslash R".) This can be made the default when PCRE is built; if this is
506 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
507 It is also possible to specify these settings by starting a pattern string with
508 one of the following sequences:
509 <pre>
510 (*BSR_ANYCRLF) CR, LF, or CRLF only
511 (*BSR_UNICODE) any Unicode newline sequence
512 </pre>
513 These override the default and the options given to <b>pcre_compile()</b> or
514 <b>pcre_compile2()</b>, but they can be overridden by options given to
515 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,
516 which are not Perl-compatible, are recognized only at the very start of a
517 pattern, and that they must be in upper case. If more than one of them is
518 present, the last one is used. They can be combined with a change of newline
519 convention; for example, a pattern can start with:
520 <pre>
522 </pre>
523 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
524 a character class, \R is treated as an unrecognized escape sequence, and so
525 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
526 <a name="uniextseq"></a></P>
527 <br><b>
528 Unicode character properties
529 </b><br>
530 <P>
531 When PCRE is built with Unicode character property support, three additional
532 escape sequences that match characters with specific properties are available.
533 When not in UTF-8 mode, these sequences are of course limited to testing
534 characters whose codepoints are less than 256, but they do work in this mode.
535 The extra escape sequences are:
536 <pre>
537 \p{<i>xx</i>} a character with the <i>xx</i> property
538 \P{<i>xx</i>} a character without the <i>xx</i> property
539 \X an extended Unicode sequence
540 </pre>
541 The property names represented by <i>xx</i> above are limited to the Unicode
542 script names, the general category properties, "Any", which matches any
543 character (including newline), and some special PCRE properties (described
544 in the
545 <a href="#extraprops">next section).</a>
546 Other Perl properties such as "InMusicalSymbols" are not currently supported by
547 PCRE. Note that \P{Any} does not match any characters, so always causes a
548 match failure.
549 </P>
550 <P>
551 Sets of Unicode characters are defined as belonging to certain scripts. A
552 character from one of these sets can be matched using a script name. For
553 example:
554 <pre>
555 \p{Greek}
556 \P{Han}
557 </pre>
558 Those that are not part of an identified script are lumped together as
559 "Common". The current list of scripts is:
560 </P>
561 <P>
562 Arabic,
563 Armenian,
564 Avestan,
565 Balinese,
566 Bamum,
567 Bengali,
568 Bopomofo,
569 Braille,
570 Buginese,
571 Buhid,
572 Canadian_Aboriginal,
573 Carian,
574 Cham,
575 Cherokee,
576 Common,
577 Coptic,
578 Cuneiform,
579 Cypriot,
580 Cyrillic,
581 Deseret,
582 Devanagari,
583 Egyptian_Hieroglyphs,
584 Ethiopic,
585 Georgian,
586 Glagolitic,
587 Gothic,
588 Greek,
589 Gujarati,
590 Gurmukhi,
591 Han,
592 Hangul,
593 Hanunoo,
594 Hebrew,
595 Hiragana,
596 Imperial_Aramaic,
597 Inherited,
598 Inscriptional_Pahlavi,
599 Inscriptional_Parthian,
600 Javanese,
601 Kaithi,
602 Kannada,
603 Katakana,
604 Kayah_Li,
605 Kharoshthi,
606 Khmer,
607 Lao,
608 Latin,
609 Lepcha,
610 Limbu,
611 Linear_B,
612 Lisu,
613 Lycian,
614 Lydian,
615 Malayalam,
616 Meetei_Mayek,
617 Mongolian,
618 Myanmar,
619 New_Tai_Lue,
620 Nko,
621 Ogham,
622 Old_Italic,
623 Old_Persian,
624 Old_South_Arabian,
625 Old_Turkic,
626 Ol_Chiki,
627 Oriya,
628 Osmanya,
629 Phags_Pa,
630 Phoenician,
631 Rejang,
632 Runic,
633 Samaritan,
634 Saurashtra,
635 Shavian,
636 Sinhala,
637 Sundanese,
638 Syloti_Nagri,
639 Syriac,
640 Tagalog,
641 Tagbanwa,
642 Tai_Le,
643 Tai_Tham,
644 Tai_Viet,
645 Tamil,
646 Telugu,
647 Thaana,
648 Thai,
649 Tibetan,
650 Tifinagh,
651 Ugaritic,
652 Vai,
653 Yi.
654 </P>
655 <P>
656 Each character has exactly one Unicode general category property, specified by
657 a two-letter abbreviation. For compatibility with Perl, negation can be
658 specified by including a circumflex between the opening brace and the property
659 name. For example, \p{^Lu} is the same as \P{Lu}.
660 </P>
661 <P>
662 If only one letter is specified with \p or \P, it includes all the general
663 category properties that start with that letter. In this case, in the absence
664 of negation, the curly brackets in the escape sequence are optional; these two
665 examples have the same effect:
666 <pre>
667 \p{L}
668 \pL
669 </pre>
670 The following general category property codes are supported:
671 <pre>
672 C Other
673 Cc Control
674 Cf Format
675 Cn Unassigned
676 Co Private use
677 Cs Surrogate
679 L Letter
680 Ll Lower case letter
681 Lm Modifier letter
682 Lo Other letter
683 Lt Title case letter
684 Lu Upper case letter
686 M Mark
687 Mc Spacing mark
688 Me Enclosing mark
689 Mn Non-spacing mark
691 N Number
692 Nd Decimal number
693 Nl Letter number
694 No Other number
696 P Punctuation
697 Pc Connector punctuation
698 Pd Dash punctuation
699 Pe Close punctuation
700 Pf Final punctuation
701 Pi Initial punctuation
702 Po Other punctuation
703 Ps Open punctuation
705 S Symbol
706 Sc Currency symbol
707 Sk Modifier symbol
708 Sm Mathematical symbol
709 So Other symbol
711 Z Separator
712 Zl Line separator
713 Zp Paragraph separator
714 Zs Space separator
715 </pre>
716 The special property L& is also supported: it matches a character that has
717 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
718 a modifier or "other".
719 </P>
720 <P>
721 The Cs (Surrogate) property applies only to characters in the range U+D800 to
722 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
723 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
724 (see the discussion of PCRE_NO_UTF8_CHECK in the
725 <a href="pcreapi.html"><b>pcreapi</b></a>
726 page). Perl does not support the Cs property.
727 </P>
728 <P>
729 The long synonyms for property names that Perl supports (such as \p{Letter})
730 are not supported by PCRE, nor is it permitted to prefix any of these
731 properties with "Is".
732 </P>
733 <P>
734 No character that is in the Unicode table has the Cn (unassigned) property.
735 Instead, this property is assumed for any code point that is not in the
736 Unicode table.
737 </P>
738 <P>
739 Specifying caseless matching does not affect these escape sequences. For
740 example, \p{Lu} always matches only upper case letters.
741 </P>
742 <P>
743 The \X escape matches any number of Unicode characters that form an extended
744 Unicode sequence. \X is equivalent to
745 <pre>
746 (?&#62;\PM\pM*)
747 </pre>
748 That is, it matches a character without the "mark" property, followed by zero
749 or more characters with the "mark" property, and treats the sequence as an
750 atomic group
751 <a href="#atomicgroup">(see below).</a>
752 Characters with the "mark" property are typically accents that affect the
753 preceding character. None of them have codepoints less than 256, so in
754 non-UTF-8 mode \X matches any one character.
755 </P>
756 <P>
757 Note that recent versions of Perl have changed \X to match what Unicode calls
758 an "extended grapheme cluster", which has a more complicated definition.
759 </P>
760 <P>
761 Matching characters by Unicode property is not fast, because PCRE has to search
762 a structure that contains data for over fifteen thousand characters. That is
763 why the traditional escape sequences such as \d and \w do not use Unicode
764 properties in PCRE by default, though you can make them do so by setting the
765 PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with
766 (*UCP).
767 <a name="extraprops"></a></P>
768 <br><b>
769 PCRE's additional properties
770 </b><br>
771 <P>
772 As well as the standard Unicode properties described in the previous
773 section, PCRE supports four more that make it possible to convert traditional
774 escape sequences such as \w and \s and POSIX character classes to use Unicode
775 properties. PCRE uses these non-standard, non-Perl properties internally when
776 PCRE_UCP is set. They are:
777 <pre>
778 Xan Any alphanumeric character
779 Xps Any POSIX space character
780 Xsp Any Perl space character
781 Xwd Any Perl "word" character
782 </pre>
783 Xan matches characters that have either the L (letter) or the N (number)
784 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
785 carriage return, and any other character that has the Z (separator) property.
786 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
787 same characters as Xan, plus underscore.
788 <a name="resetmatchstart"></a></P>
789 <br><b>
790 Resetting the match start
791 </b><br>
792 <P>
793 The escape sequence \K causes any previously matched characters not to be
794 included in the final matched sequence. For example, the pattern:
795 <pre>
796 foo\Kbar
797 </pre>
798 matches "foobar", but reports that it has matched "bar". This feature is
799 similar to a lookbehind assertion
800 <a href="#lookbehind">(described below).</a>
801 However, in this case, the part of the subject before the real match does not
802 have to be of fixed length, as lookbehind assertions do. The use of \K does
803 not interfere with the setting of
804 <a href="#subpattern">captured substrings.</a>
805 For example, when the pattern
806 <pre>
807 (foo)\Kbar
808 </pre>
809 matches "foobar", the first substring is still set to "foo".
810 </P>
811 <P>
812 Perl documents that the use of \K within assertions is "not well defined". In
813 PCRE, \K is acted upon when it occurs inside positive assertions, but is
814 ignored in negative assertions.
815 <a name="smallassertions"></a></P>
816 <br><b>
817 Simple assertions
818 </b><br>
819 <P>
820 The final use of backslash is for certain simple assertions. An assertion
821 specifies a condition that has to be met at a particular point in a match,
822 without consuming any characters from the subject string. The use of
823 subpatterns for more complicated assertions is described
824 <a href="#bigassertions">below.</a>
825 The backslashed assertions are:
826 <pre>
827 \b matches at a word boundary
828 \B matches when not at a word boundary
829 \A matches at the start of the subject
830 \Z matches at the end of the subject
831 also matches before a newline at the end of the subject
832 \z matches only at the end of the subject
833 \G matches at the first matching position in the subject
834 </pre>
835 Inside a character class, \b has a different meaning; it matches the backspace
836 character. If any other of these assertions appears in a character class, by
837 default it matches the corresponding literal character (for example, \B
838 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
839 escape sequence" error is generated instead.
840 </P>
841 <P>
842 A word boundary is a position in the subject string where the current character
843 and the previous character do not both match \w or \W (i.e. one matches
844 \w and the other matches \W), or the start or end of the string if the
845 first or last character matches \w, respectively. In UTF-8 mode, the meanings
846 of \w and \W can be changed by setting the PCRE_UCP option. When this is
847 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
848 of word" or "end of word" metasequence. However, whatever follows \b normally
849 determines which it is. For example, the fragment \ba matches "a" at the start
850 of a word.
851 </P>
852 <P>
853 The \A, \Z, and \z assertions differ from the traditional circumflex and
854 dollar (described in the next section) in that they only ever match at the very
855 start and end of the subject string, whatever options are set. Thus, they are
856 independent of multiline mode. These three assertions are not affected by the
857 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
858 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
859 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
860 at a point other than the beginning of the subject, \A can never match. The
861 difference between \Z and \z is that \Z matches before a newline at the end
862 of the string as well as at the very end, whereas \z matches only at the end.
863 </P>
864 <P>
865 The \G assertion is true only when the current matching position is at the
866 start point of the match, as specified by the <i>startoffset</i> argument of
867 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
868 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
869 arguments, you can mimic Perl's /g option, and it is in this kind of
870 implementation where \G can be useful.
871 </P>
872 <P>
873 Note, however, that PCRE's interpretation of \G, as the start of the current
874 match, is subtly different from Perl's, which defines it as the end of the
875 previous match. In Perl, these can be different when the previously matched
876 string was empty. Because PCRE does just one match at a time, it cannot
877 reproduce this behaviour.
878 </P>
879 <P>
880 If all the alternatives of a pattern begin with \G, the expression is anchored
881 to the starting match position, and the "anchored" flag is set in the compiled
882 regular expression.
883 </P>
884 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
885 <P>
886 Outside a character class, in the default matching mode, the circumflex
887 character is an assertion that is true only if the current matching point is
888 at the start of the subject string. If the <i>startoffset</i> argument of
889 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
890 option is unset. Inside a character class, circumflex has an entirely different
891 meaning
892 <a href="#characterclass">(see below).</a>
893 </P>
894 <P>
895 Circumflex need not be the first character of the pattern if a number of
896 alternatives are involved, but it should be the first thing in each alternative
897 in which it appears if the pattern is ever to match that branch. If all
898 possible alternatives start with a circumflex, that is, if the pattern is
899 constrained to match only at the start of the subject, it is said to be an
900 "anchored" pattern. (There are also other constructs that can cause a pattern
901 to be anchored.)
902 </P>
903 <P>
904 A dollar character is an assertion that is true only if the current matching
905 point is at the end of the subject string, or immediately before a newline
906 at the end of the string (by default). Dollar need not be the last character of
907 the pattern if a number of alternatives are involved, but it should be the last
908 item in any branch in which it appears. Dollar has no special meaning in a
909 character class.
910 </P>
911 <P>
912 The meaning of dollar can be changed so that it matches only at the very end of
913 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
914 does not affect the \Z assertion.
915 </P>
916 <P>
917 The meanings of the circumflex and dollar characters are changed if the
918 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
919 immediately after internal newlines as well as at the start of the subject
920 string. It does not match after a newline that ends the string. A dollar
921 matches before any newlines in the string, as well as at the very end, when
922 PCRE_MULTILINE is set. When newline is specified as the two-character
923 sequence CRLF, isolated CR and LF characters do not indicate newlines.
924 </P>
925 <P>
926 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
927 \n represents a newline) in multiline mode, but not otherwise. Consequently,
928 patterns that are anchored in single line mode because all branches start with
929 ^ are not anchored in multiline mode, and a match for circumflex is possible
930 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
931 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
932 </P>
933 <P>
934 Note that the sequences \A, \Z, and \z can be used to match the start and
935 end of the subject in both modes, and if all branches of a pattern start with
936 \A it is always anchored, whether or not PCRE_MULTILINE is set.
937 <a name="fullstopdot"></a></P>
938 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
939 <P>
940 Outside a character class, a dot in the pattern matches any one character in
941 the subject string except (by default) a character that signifies the end of a
942 line. In UTF-8 mode, the matched character may be more than one byte long.
943 </P>
944 <P>
945 When a line ending is defined as a single character, dot never matches that
946 character; when the two-character sequence CRLF is used, dot does not match CR
947 if it is immediately followed by LF, but otherwise it matches all characters
948 (including isolated CRs and LFs). When any Unicode line endings are being
949 recognized, dot does not match CR or LF or any of the other line ending
950 characters.
951 </P>
952 <P>
953 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
954 option is set, a dot matches any one character, without exception. If the
955 two-character sequence CRLF is present in the subject string, it takes two dots
956 to match it.
957 </P>
958 <P>
959 The handling of dot is entirely independent of the handling of circumflex and
960 dollar, the only relationship being that they both involve newlines. Dot has no
961 special meaning in a character class.
962 </P>
963 <P>
964 The escape sequence \N behaves like a dot, except that it is not affected by
965 the PCRE_DOTALL option. In other words, it matches any character except one
966 that signifies the end of a line.
967 </P>
968 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
969 <P>
970 Outside a character class, the escape sequence \C matches any one byte, both
971 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
972 characters. The feature is provided in Perl in order to match individual bytes
973 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
974 rest of the string may start with a malformed UTF-8 character. For this reason,
975 the \C escape sequence is best avoided.
976 </P>
977 <P>
978 PCRE does not allow \C to appear in lookbehind assertions
979 <a href="#lookbehind">(described below),</a>
980 because in UTF-8 mode this would make it impossible to calculate the length of
981 the lookbehind.
982 <a name="characterclass"></a></P>
983 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
984 <P>
985 An opening square bracket introduces a character class, terminated by a closing
986 square bracket. A closing square bracket on its own is not special by default.
987 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
988 bracket causes a compile-time error. If a closing square bracket is required as
989 a member of the class, it should be the first data character in the class
990 (after an initial circumflex, if present) or escaped with a backslash.
991 </P>
992 <P>
993 A character class matches a single character in the subject. In UTF-8 mode, the
994 character may be more than one byte long. A matched character must be in the
995 set of characters defined by the class, unless the first character in the class
996 definition is a circumflex, in which case the subject character must not be in
997 the set defined by the class. If a circumflex is actually required as a member
998 of the class, ensure it is not the first character, or escape it with a
999 backslash.
1000 </P>
1001 <P>
1002 For example, the character class [aeiou] matches any lower case vowel, while
1003 [^aeiou] matches any character that is not a lower case vowel. Note that a
1004 circumflex is just a convenient notation for specifying the characters that
1005 are in the class by enumerating those that are not. A class that starts with a
1006 circumflex is not an assertion; it still consumes a character from the subject
1007 string, and therefore it fails if the current pointer is at the end of the
1008 string.
1009 </P>
1010 <P>
1011 In UTF-8 mode, characters with values greater than 255 can be included in a
1012 class as a literal string of bytes, or by using the \x{ escaping mechanism.
1013 </P>
1014 <P>
1015 When caseless matching is set, any letters in a class represent both their
1016 upper case and lower case versions, so for example, a caseless [aeiou] matches
1017 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1018 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1019 case for characters whose values are less than 128, so caseless matching is
1020 always possible. For characters with higher values, the concept of case is
1021 supported if PCRE is compiled with Unicode property support, but not otherwise.
1022 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1023 you must ensure that PCRE is compiled with Unicode property support as well as
1024 with UTF-8 support.
1025 </P>
1026 <P>
1027 Characters that might indicate line breaks are never treated in any special way
1028 when matching character classes, whatever line-ending sequence is in use, and
1029 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1030 such as [^a] always matches one of these characters.
1031 </P>
1032 <P>
1033 The minus (hyphen) character can be used to specify a range of characters in a
1034 character class. For example, [d-m] matches any letter between d and m,
1035 inclusive. If a minus character is required in a class, it must be escaped with
1036 a backslash or appear in a position where it cannot be interpreted as
1037 indicating a range, typically as the first or last character in the class.
1038 </P>
1039 <P>
1040 It is not possible to have the literal character "]" as the end character of a
1041 range. A pattern such as [W-]46] is interpreted as a class of two characters
1042 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1043 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1044 the end of range, so [W-\]46] is interpreted as a class containing a range
1045 followed by two other characters. The octal or hexadecimal representation of
1046 "]" can also be used to end a range.
1047 </P>
1048 <P>
1049 Ranges operate in the collating sequence of character values. They can also be
1050 used for characters specified numerically, for example [\000-\037]. In UTF-8
1051 mode, ranges can include characters whose values are greater than 255, for
1052 example [\x{100}-\x{2ff}].
1053 </P>
1054 <P>
1055 If a range that includes letters is used when caseless matching is set, it
1056 matches the letters in either case. For example, [W-c] is equivalent to
1057 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1058 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1059 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1060 characters with values greater than 128 only when it is compiled with Unicode
1061 property support.
1062 </P>
1063 <P>
1064 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1065 \V, \w, and \W may appear in a character class, and add the characters that
1066 they match to the class. For example, [\dABCDEF] matches any hexadecimal
1067 digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w
1068 and their upper case partners, just as it does when they appear outside a
1069 character class, as described in the section entitled
1070 <a href="#genericchartypes">"Generic character types"</a>
1071 above. The escape sequence \b has a different meaning inside a character
1072 class; it matches the backspace character. The sequences \B, \N, \R, and \X
1073 are not special inside a character class. Like any other unrecognized escape
1074 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1075 default, but cause an error if the PCRE_EXTRA option is set.
1076 </P>
1077 <P>
1078 A circumflex can conveniently be used with the upper case character types to
1079 specify a more restricted set of characters than the matching lower case type.
1080 For example, the class [^\W_] matches any letter or digit, but not underscore,
1081 whereas [\w] includes underscore. A positive character class should be read as
1082 "something OR something OR ..." and a negative class as "NOT something AND NOT
1083 something AND NOT ...".
1084 </P>
1085 <P>
1086 The only metacharacters that are recognized in character classes are backslash,
1087 hyphen (only where it can be interpreted as specifying a range), circumflex
1088 (only at the start), opening square bracket (only when it can be interpreted as
1089 introducing a POSIX class name - see the next section), and the terminating
1090 closing square bracket. However, escaping other non-alphanumeric characters
1091 does no harm.
1092 </P>
1093 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1094 <P>
1095 Perl supports the POSIX notation for character classes. This uses names
1096 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1097 this notation. For example,
1098 <pre>
1099 [01[:alpha:]%]
1100 </pre>
1101 matches "0", "1", any alphabetic character, or "%". The supported class names
1102 are:
1103 <pre>
1104 alnum letters and digits
1105 alpha letters
1106 ascii character codes 0 - 127
1107 blank space or tab only
1108 cntrl control characters
1109 digit decimal digits (same as \d)
1110 graph printing characters, excluding space
1111 lower lower case letters
1112 print printing characters, including space
1113 punct printing characters, excluding letters and digits and space
1114 space white space (not quite the same as \s)
1115 upper upper case letters
1116 word "word" characters (same as \w)
1117 xdigit hexadecimal digits
1118 </pre>
1119 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1120 space (32). Notice that this list includes the VT character (code 11). This
1121 makes "space" different to \s, which does not include VT (for Perl
1122 compatibility).
1123 </P>
1124 <P>
1125 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1126 5.8. Another Perl extension is negation, which is indicated by a ^ character
1127 after the colon. For example,
1128 <pre>
1129 [12[:^digit:]]
1130 </pre>
1131 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1132 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1133 supported, and an error is given if they are encountered.
1134 </P>
1135 <P>
1136 By default, in UTF-8 mode, characters with values greater than 128 do not match
1137 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1138 to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1139 character properties are used. This is achieved by replacing the POSIX classes
1140 by other sequences, as follows:
1141 <pre>
1142 [:alnum:] becomes \p{Xan}
1143 [:alpha:] becomes \p{L}
1144 [:blank:] becomes \h
1145 [:digit:] becomes \p{Nd}
1146 [:lower:] becomes \p{Ll}
1147 [:space:] becomes \p{Xps}
1148 [:upper:] becomes \p{Lu}
1149 [:word:] becomes \p{Xwd}
1150 </pre>
1151 Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1152 classes are unchanged, and match only characters with code points less than
1153 128.
1154 </P>
1155 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1156 <P>
1157 Vertical bar characters are used to separate alternative patterns. For example,
1158 the pattern
1159 <pre>
1160 gilbert|sullivan
1161 </pre>
1162 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1163 and an empty alternative is permitted (matching the empty string). The matching
1164 process tries each alternative in turn, from left to right, and the first one
1165 that succeeds is used. If the alternatives are within a subpattern
1166 <a href="#subpattern">(defined below),</a>
1167 "succeeds" means matching the rest of the main pattern as well as the
1168 alternative in the subpattern.
1169 </P>
1170 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1171 <P>
1172 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1173 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1174 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1175 The option letters are
1176 <pre>
1177 i for PCRE_CASELESS
1179 s for PCRE_DOTALL
1180 x for PCRE_EXTENDED
1181 </pre>
1182 For example, (?im) sets caseless, multiline matching. It is also possible to
1183 unset these options by preceding the letter with a hyphen, and a combined
1184 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1185 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1186 permitted. If a letter appears both before and after the hyphen, the option is
1187 unset.
1188 </P>
1189 <P>
1190 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1191 changed in the same way as the Perl-compatible options by using the characters
1192 J, U and X respectively.
1193 </P>
1194 <P>
1195 When one of these option changes occurs at top level (that is, not inside
1196 subpattern parentheses), the change applies to the remainder of the pattern
1197 that follows. If the change is placed right at the start of a pattern, PCRE
1198 extracts it into the global options (and it will therefore show up in data
1199 extracted by the <b>pcre_fullinfo()</b> function).
1200 </P>
1201 <P>
1202 An option change within a subpattern (see below for a description of
1203 subpatterns) affects only that part of the subpattern that follows it, so
1204 <pre>
1205 (a(?i)b)c
1206 </pre>
1207 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1208 By this means, options can be made to have different settings in different
1209 parts of the pattern. Any changes made in one alternative do carry on
1210 into subsequent branches within the same subpattern. For example,
1211 <pre>
1212 (a(?i)b|c)
1213 </pre>
1214 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1215 branch is abandoned before the option setting. This is because the effects of
1216 option settings happen at compile time. There would be some very weird
1217 behaviour otherwise.
1218 </P>
1219 <P>
1220 <b>Note:</b> There are other PCRE-specific options that can be set by the
1221 application when the compile or match functions are called. In some cases the
1222 pattern can contain special leading sequences such as (*CRLF) to override what
1223 the application has set or what has been defaulted. Details are given in the
1224 section entitled
1225 <a href="#newlineseq">"Newline sequences"</a>
1226 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1227 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1228 PCRE_UTF8 and the PCRE_UCP options, respectively.
1229 <a name="subpattern"></a></P>
1230 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1231 <P>
1232 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1233 Turning part of a pattern into a subpattern does two things:
1234 <br>
1235 <br>
1236 1. It localizes a set of alternatives. For example, the pattern
1237 <pre>
1238 cat(aract|erpillar|)
1239 </pre>
1240 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1241 match "cataract", "erpillar" or an empty string.
1242 <br>
1243 <br>
1244 2. It sets up the subpattern as a capturing subpattern. This means that, when
1245 the whole pattern matches, that portion of the subject string that matched the
1246 subpattern is passed back to the caller via the <i>ovector</i> argument of
1247 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1248 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1249 string "the red king" is matched against the pattern
1250 <pre>
1251 the ((red|white) (king|queen))
1252 </pre>
1253 the captured substrings are "red king", "red", and "king", and are numbered 1,
1254 2, and 3, respectively.
1255 </P>
1256 <P>
1257 The fact that plain parentheses fulfil two functions is not always helpful.
1258 There are often times when a grouping subpattern is required without a
1259 capturing requirement. If an opening parenthesis is followed by a question mark
1260 and a colon, the subpattern does not do any capturing, and is not counted when
1261 computing the number of any subsequent capturing subpatterns. For example, if
1262 the string "the white queen" is matched against the pattern
1263 <pre>
1264 the ((?:red|white) (king|queen))
1265 </pre>
1266 the captured substrings are "white queen" and "queen", and are numbered 1 and
1267 2. The maximum number of capturing subpatterns is 65535.
1268 </P>
1269 <P>
1270 As a convenient shorthand, if any option settings are required at the start of
1271 a non-capturing subpattern, the option letters may appear between the "?" and
1272 the ":". Thus the two patterns
1273 <pre>
1274 (?i:saturday|sunday)
1275 (?:(?i)saturday|sunday)
1276 </pre>
1277 match exactly the same set of strings. Because alternative branches are tried
1278 from left to right, and options are not reset until the end of the subpattern
1279 is reached, an option setting in one branch does affect subsequent branches, so
1280 the above patterns match "SUNDAY" as well as "Saturday".
1281 <a name="dupsubpatternnumber"></a></P>
1282 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1283 <P>
1284 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1285 the same numbers for its capturing parentheses. Such a subpattern starts with
1286 (?| and is itself a non-capturing subpattern. For example, consider this
1287 pattern:
1288 <pre>
1289 (?|(Sat)ur|(Sun))day
1290 </pre>
1291 Because the two alternatives are inside a (?| group, both sets of capturing
1292 parentheses are numbered one. Thus, when the pattern matches, you can look
1293 at captured substring number one, whichever alternative matched. This construct
1294 is useful when you want to capture part, but not all, of one of a number of
1295 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1296 number is reset at the start of each branch. The numbers of any capturing
1297 parentheses that follow the subpattern start after the highest number used in
1298 any branch. The following example is taken from the Perl documentation. The
1299 numbers underneath show in which buffer the captured content will be stored.
1300 <pre>
1301 # before ---------------branch-reset----------- after
1302 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1303 # 1 2 2 3 2 3 4
1304 </pre>
1305 A back reference to a numbered subpattern uses the most recent value that is
1306 set for that number by any subpattern. The following pattern matches "abcabc"
1307 or "defdef":
1308 <pre>
1309 /(?|(abc)|(def))\1/
1310 </pre>
1311 In contrast, a subroutine call to a numbered subpattern always refers to the
1312 first one in the pattern with the given number. The following pattern matches
1313 "abcabc" or "defabc":
1314 <pre>
1315 /(?|(abc)|(def))(?1)/
1316 </pre>
1317 If a
1318 <a href="#conditions">condition test</a>
1319 for a subpattern's having matched refers to a non-unique number, the test is
1320 true if any of the subpatterns of that number have matched.
1321 </P>
1322 <P>
1323 An alternative approach to using this "branch reset" feature is to use
1324 duplicate named subpatterns, as described in the next section.
1325 </P>
1326 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1327 <P>
1328 Identifying capturing parentheses by number is simple, but it can be very hard
1329 to keep track of the numbers in complicated regular expressions. Furthermore,
1330 if an expression is modified, the numbers may change. To help with this
1331 difficulty, PCRE supports the naming of subpatterns. This feature was not
1332 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1333 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1334 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1335 have different names, but PCRE does not.
1336 </P>
1337 <P>
1338 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1339 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1340 parentheses from other parts of the pattern, such as
1341 <a href="#backreferences">back references,</a>
1342 <a href="#recursion">recursion,</a>
1343 and
1344 <a href="#conditions">conditions,</a>
1345 can be made by name as well as by number.
1346 </P>
1347 <P>
1348 Names consist of up to 32 alphanumeric characters and underscores. Named
1349 capturing parentheses are still allocated numbers as well as names, exactly as
1350 if the names were not present. The PCRE API provides function calls for
1351 extracting the name-to-number translation table from a compiled pattern. There
1352 is also a convenience function for extracting a captured substring by name.
1353 </P>
1354 <P>
1355 By default, a name must be unique within a pattern, but it is possible to relax
1356 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1357 names are also always permitted for subpatterns with the same number, set up as
1358 described in the previous section.) Duplicate names can be useful for patterns
1359 where only one instance of the named parentheses can match. Suppose you want to
1360 match the name of a weekday, either as a 3-letter abbreviation or as the full
1361 name, and in both cases you want to extract the abbreviation. This pattern
1362 (ignoring the line breaks) does the job:
1363 <pre>
1364 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1365 (?&#60;DN&#62;Tue)(?:sday)?|
1366 (?&#60;DN&#62;Wed)(?:nesday)?|
1367 (?&#60;DN&#62;Thu)(?:rsday)?|
1368 (?&#60;DN&#62;Sat)(?:urday)?
1369 </pre>
1370 There are five capturing substrings, but only one is ever set after a match.
1371 (An alternative way of solving this problem is to use a "branch reset"
1372 subpattern, as described in the previous section.)
1373 </P>
1374 <P>
1375 The convenience function for extracting the data by name returns the substring
1376 for the first (and in this example, the only) subpattern of that name that
1377 matched. This saves searching to find which numbered subpattern it was.
1378 </P>
1379 <P>
1380 If you make a back reference to a non-unique named subpattern from elsewhere in
1381 the pattern, the one that corresponds to the first occurrence of the name is
1382 used. In the absence of duplicate numbers (see the previous section) this is
1383 the one with the lowest number. If you use a named reference in a condition
1384 test (see the
1385 <a href="#conditions">section about conditions</a>
1386 below), either to check whether a subpattern has matched, or to check for
1387 recursion, all subpatterns with the same name are tested. If the condition is
1388 true for any one of them, the overall condition is true. This is the same
1389 behaviour as testing by number. For further details of the interfaces for
1390 handling named subpatterns, see the
1391 <a href="pcreapi.html"><b>pcreapi</b></a>
1392 documentation.
1393 </P>
1394 <P>
1395 <b>Warning:</b> You cannot use different names to distinguish between two
1396 subpatterns with the same number because PCRE uses only the numbers when
1397 matching. For this reason, an error is given at compile time if different names
1398 are given to subpatterns with the same number. However, you can give the same
1399 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1400 </P>
1401 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1402 <P>
1403 Repetition is specified by quantifiers, which can follow any of the following
1404 items:
1405 <pre>
1406 a literal data character
1407 the dot metacharacter
1408 the \C escape sequence
1409 the \X escape sequence (in UTF-8 mode with Unicode properties)
1410 the \R escape sequence
1411 an escape such as \d or \pL that matches a single character
1412 a character class
1413 a back reference (see next section)
1414 a parenthesized subpattern (including assertions)
1415 a subroutine call to a subpattern (recursive or otherwise)
1416 </pre>
1417 The general repetition quantifier specifies a minimum and maximum number of
1418 permitted matches, by giving the two numbers in curly brackets (braces),
1419 separated by a comma. The numbers must be less than 65536, and the first must
1420 be less than or equal to the second. For example:
1421 <pre>
1422 z{2,4}
1423 </pre>
1424 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1425 character. If the second number is omitted, but the comma is present, there is
1426 no upper limit; if the second number and the comma are both omitted, the
1427 quantifier specifies an exact number of required matches. Thus
1428 <pre>
1429 [aeiou]{3,}
1430 </pre>
1431 matches at least 3 successive vowels, but may match many more, while
1432 <pre>
1433 \d{8}
1434 </pre>
1435 matches exactly 8 digits. An opening curly bracket that appears in a position
1436 where a quantifier is not allowed, or one that does not match the syntax of a
1437 quantifier, is taken as a literal character. For example, {,6} is not a
1438 quantifier, but a literal string of four characters.
1439 </P>
1440 <P>
1441 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1442 bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1443 which is represented by a two-byte sequence. Similarly, when Unicode property
1444 support is available, \X{3} matches three Unicode extended sequences, each of
1445 which may be several bytes long (and they may be of different lengths).
1446 </P>
1447 <P>
1448 The quantifier {0} is permitted, causing the expression to behave as if the
1449 previous item and the quantifier were not present. This may be useful for
1450 subpatterns that are referenced as
1451 <a href="#subpatternsassubroutines">subroutines</a>
1452 from elsewhere in the pattern (but see also the section entitled
1453 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1454 below). Items other than subpatterns that have a {0} quantifier are omitted
1455 from the compiled pattern.
1456 </P>
1457 <P>
1458 For convenience, the three most common quantifiers have single-character
1459 abbreviations:
1460 <pre>
1461 * is equivalent to {0,}
1462 + is equivalent to {1,}
1463 ? is equivalent to {0,1}
1464 </pre>
1465 It is possible to construct infinite loops by following a subpattern that can
1466 match no characters with a quantifier that has no upper limit, for example:
1467 <pre>
1468 (a?)*
1469 </pre>
1470 Earlier versions of Perl and PCRE used to give an error at compile time for
1471 such patterns. However, because there are cases where this can be useful, such
1472 patterns are now accepted, but if any repetition of the subpattern does in fact
1473 match no characters, the loop is forcibly broken.
1474 </P>
1475 <P>
1476 By default, the quantifiers are "greedy", that is, they match as much as
1477 possible (up to the maximum number of permitted times), without causing the
1478 rest of the pattern to fail. The classic example of where this gives problems
1479 is in trying to match comments in C programs. These appear between /* and */
1480 and within the comment, individual * and / characters may appear. An attempt to
1481 match C comments by applying the pattern
1482 <pre>
1483 /\*.*\*/
1484 </pre>
1485 to the string
1486 <pre>
1487 /* first comment */ not comment /* second comment */
1488 </pre>
1489 fails, because it matches the entire string owing to the greediness of the .*
1490 item.
1491 </P>
1492 <P>
1493 However, if a quantifier is followed by a question mark, it ceases to be
1494 greedy, and instead matches the minimum number of times possible, so the
1495 pattern
1496 <pre>
1497 /\*.*?\*/
1498 </pre>
1499 does the right thing with the C comments. The meaning of the various
1500 quantifiers is not otherwise changed, just the preferred number of matches.
1501 Do not confuse this use of question mark with its use as a quantifier in its
1502 own right. Because it has two uses, it can sometimes appear doubled, as in
1503 <pre>
1504 \d??\d
1505 </pre>
1506 which matches one digit by preference, but can match two if that is the only
1507 way the rest of the pattern matches.
1508 </P>
1509 <P>
1510 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1511 the quantifiers are not greedy by default, but individual ones can be made
1512 greedy by following them with a question mark. In other words, it inverts the
1513 default behaviour.
1514 </P>
1515 <P>
1516 When a parenthesized subpattern is quantified with a minimum repeat count that
1517 is greater than 1 or with a limited maximum, more memory is required for the
1518 compiled pattern, in proportion to the size of the minimum or maximum.
1519 </P>
1520 <P>
1521 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1522 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1523 implicitly anchored, because whatever follows will be tried against every
1524 character position in the subject string, so there is no point in retrying the
1525 overall match at any position after the first. PCRE normally treats such a
1526 pattern as though it were preceded by \A.
1527 </P>
1528 <P>
1529 In cases where it is known that the subject string contains no newlines, it is
1530 worth setting PCRE_DOTALL in order to obtain this optimization, or
1531 alternatively using ^ to indicate anchoring explicitly.
1532 </P>
1533 <P>
1534 However, there is one situation where the optimization cannot be used. When .*
1535 is inside capturing parentheses that are the subject of a back reference
1536 elsewhere in the pattern, a match at the start may fail where a later one
1537 succeeds. Consider, for example:
1538 <pre>
1539 (.*)abc\1
1540 </pre>
1541 If the subject is "xyz123abc123" the match point is the fourth character. For
1542 this reason, such a pattern is not implicitly anchored.
1543 </P>
1544 <P>
1545 When a capturing subpattern is repeated, the value captured is the substring
1546 that matched the final iteration. For example, after
1547 <pre>
1548 (tweedle[dume]{3}\s*)+
1549 </pre>
1550 has matched "tweedledum tweedledee" the value of the captured substring is
1551 "tweedledee". However, if there are nested capturing subpatterns, the
1552 corresponding captured values may have been set in previous iterations. For
1553 example, after
1554 <pre>
1555 /(a|(b))+/
1556 </pre>
1557 matches "aba" the value of the second captured substring is "b".
1558 <a name="atomicgroup"></a></P>
1559 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1560 <P>
1561 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1562 repetition, failure of what follows normally causes the repeated item to be
1563 re-evaluated to see if a different number of repeats allows the rest of the
1564 pattern to match. Sometimes it is useful to prevent this, either to change the
1565 nature of the match, or to cause it fail earlier than it otherwise might, when
1566 the author of the pattern knows there is no point in carrying on.
1567 </P>
1568 <P>
1569 Consider, for example, the pattern \d+foo when applied to the subject line
1570 <pre>
1571 123456bar
1572 </pre>
1573 After matching all 6 digits and then failing to match "foo", the normal
1574 action of the matcher is to try again with only 5 digits matching the \d+
1575 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1576 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1577 that once a subpattern has matched, it is not to be re-evaluated in this way.
1578 </P>
1579 <P>
1580 If we use atomic grouping for the previous example, the matcher gives up
1581 immediately on failing to match "foo" the first time. The notation is a kind of
1582 special parenthesis, starting with (?&#62; as in this example:
1583 <pre>
1584 (?&#62;\d+)foo
1585 </pre>
1586 This kind of parenthesis "locks up" the part of the pattern it contains once
1587 it has matched, and a failure further into the pattern is prevented from
1588 backtracking into it. Backtracking past it to previous items, however, works as
1589 normal.
1590 </P>
1591 <P>
1592 An alternative description is that a subpattern of this type matches the string
1593 of characters that an identical standalone pattern would match, if anchored at
1594 the current point in the subject string.
1595 </P>
1596 <P>
1597 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1598 the above example can be thought of as a maximizing repeat that must swallow
1599 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1600 number of digits they match in order to make the rest of the pattern match,
1601 (?&#62;\d+) can only match an entire sequence of digits.
1602 </P>
1603 <P>
1604 Atomic groups in general can of course contain arbitrarily complicated
1605 subpatterns, and can be nested. However, when the subpattern for an atomic
1606 group is just a single repeated item, as in the example above, a simpler
1607 notation, called a "possessive quantifier" can be used. This consists of an
1608 additional + character following a quantifier. Using this notation, the
1609 previous example can be rewritten as
1610 <pre>
1611 \d++foo
1612 </pre>
1613 Note that a possessive quantifier can be used with an entire group, for
1614 example:
1615 <pre>
1616 (abc|xyz){2,3}+
1617 </pre>
1618 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1619 option is ignored. They are a convenient notation for the simpler forms of
1620 atomic group. However, there is no difference in the meaning of a possessive
1621 quantifier and the equivalent atomic group, though there may be a performance
1622 difference; possessive quantifiers should be slightly faster.
1623 </P>
1624 <P>
1625 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1626 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1627 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1628 package, and PCRE copied it from there. It ultimately found its way into Perl
1629 at release 5.10.
1630 </P>
1631 <P>
1632 PCRE has an optimization that automatically "possessifies" certain simple
1633 pattern constructs. For example, the sequence A+B is treated as A++B because
1634 there is no point in backtracking into a sequence of A's when B must follow.
1635 </P>
1636 <P>
1637 When a pattern contains an unlimited repeat inside a subpattern that can itself
1638 be repeated an unlimited number of times, the use of an atomic group is the
1639 only way to avoid some failing matches taking a very long time indeed. The
1640 pattern
1641 <pre>
1642 (\D+|&#60;\d+&#62;)*[!?]
1643 </pre>
1644 matches an unlimited number of substrings that either consist of non-digits, or
1645 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1646 quickly. However, if it is applied to
1647 <pre>
1648 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1649 </pre>
1650 it takes a long time before reporting failure. This is because the string can
1651 be divided between the internal \D+ repeat and the external * repeat in a
1652 large number of ways, and all have to be tried. (The example uses [!?] rather
1653 than a single character at the end, because both PCRE and Perl have an
1654 optimization that allows for fast failure when a single character is used. They
1655 remember the last single character that is required for a match, and fail early
1656 if it is not present in the string.) If the pattern is changed so that it uses
1657 an atomic group, like this:
1658 <pre>
1659 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1660 </pre>
1661 sequences of non-digits cannot be broken, and failure happens quickly.
1662 <a name="backreferences"></a></P>
1663 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1664 <P>
1665 Outside a character class, a backslash followed by a digit greater than 0 (and
1666 possibly further digits) is a back reference to a capturing subpattern earlier
1667 (that is, to its left) in the pattern, provided there have been that many
1668 previous capturing left parentheses.
1669 </P>
1670 <P>
1671 However, if the decimal number following the backslash is less than 10, it is
1672 always taken as a back reference, and causes an error only if there are not
1673 that many capturing left parentheses in the entire pattern. In other words, the
1674 parentheses that are referenced need not be to the left of the reference for
1675 numbers less than 10. A "forward back reference" of this type can make sense
1676 when a repetition is involved and the subpattern to the right has participated
1677 in an earlier iteration.
1678 </P>
1679 <P>
1680 It is not possible to have a numerical "forward back reference" to a subpattern
1681 whose number is 10 or more using this syntax because a sequence such as \50 is
1682 interpreted as a character defined in octal. See the subsection entitled
1683 "Non-printing characters"
1684 <a href="#digitsafterbackslash">above</a>
1685 for further details of the handling of digits following a backslash. There is
1686 no such problem when named parentheses are used. A back reference to any
1687 subpattern is possible using named parentheses (see below).
1688 </P>
1689 <P>
1690 Another way of avoiding the ambiguity inherent in the use of digits following a
1691 backslash is to use the \g escape sequence. This escape must be followed by an
1692 unsigned number or a negative number, optionally enclosed in braces. These
1693 examples are all identical:
1694 <pre>
1695 (ring), \1
1696 (ring), \g1
1697 (ring), \g{1}
1698 </pre>
1699 An unsigned number specifies an absolute reference without the ambiguity that
1700 is present in the older syntax. It is also useful when literal digits follow
1701 the reference. A negative number is a relative reference. Consider this
1702 example:
1703 <pre>
1704 (abc(def)ghi)\g{-1}
1705 </pre>
1706 The sequence \g{-1} is a reference to the most recently started capturing
1707 subpattern before \g, that is, is it equivalent to \2 in this example.
1708 Similarly, \g{-2} would be equivalent to \1. The use of relative references
1709 can be helpful in long patterns, and also in patterns that are created by
1710 joining together fragments that contain references within themselves.
1711 </P>
1712 <P>
1713 A back reference matches whatever actually matched the capturing subpattern in
1714 the current subject string, rather than anything matching the subpattern
1715 itself (see
1716 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1717 below for a way of doing that). So the pattern
1718 <pre>
1719 (sens|respons)e and \1ibility
1720 </pre>
1721 matches "sense and sensibility" and "response and responsibility", but not
1722 "sense and responsibility". If caseful matching is in force at the time of the
1723 back reference, the case of letters is relevant. For example,
1724 <pre>
1725 ((?i)rah)\s+\1
1726 </pre>
1727 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1728 capturing subpattern is matched caselessly.
1729 </P>
1730 <P>
1731 There are several different ways of writing back references to named
1732 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1733 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1734 back reference syntax, in which \g can be used for both numeric and named
1735 references, is also supported. We could rewrite the above example in any of
1736 the following ways:
1737 <pre>
1738 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1739 (?'p1'(?i)rah)\s+\k{p1}
1740 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1741 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1742 </pre>
1743 A subpattern that is referenced by name may appear in the pattern before or
1744 after the reference.
1745 </P>
1746 <P>
1747 There may be more than one back reference to the same subpattern. If a
1748 subpattern has not actually been used in a particular match, any back
1749 references to it always fail by default. For example, the pattern
1750 <pre>
1751 (a|(bc))\2
1752 </pre>
1753 always fails if it starts to match "a" rather than "bc". However, if the
1754 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1755 unset value matches an empty string.
1756 </P>
1757 <P>
1758 Because there may be many capturing parentheses in a pattern, all digits
1759 following a backslash are taken as part of a potential back reference number.
1760 If the pattern continues with a digit character, some delimiter must be used to
1761 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1762 whitespace. Otherwise, the \g{ syntax or an empty comment (see
1763 <a href="#comments">"Comments"</a>
1764 below) can be used.
1765 </P>
1766 <br><b>
1767 Recursive back references
1768 </b><br>
1769 <P>
1770 A back reference that occurs inside the parentheses to which it refers fails
1771 when the subpattern is first used, so, for example, (a\1) never matches.
1772 However, such references can be useful inside repeated subpatterns. For
1773 example, the pattern
1774 <pre>
1775 (a|b\1)+
1776 </pre>
1777 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1778 the subpattern, the back reference matches the character string corresponding
1779 to the previous iteration. In order for this to work, the pattern must be such
1780 that the first iteration does not need to match the back reference. This can be
1781 done using alternation, as in the example above, or by a quantifier with a
1782 minimum of zero.
1783 </P>
1784 <P>
1785 Back references of this type cause the group that they reference to be treated
1786 as an
1787 <a href="#atomicgroup">atomic group.</a>
1788 Once the whole group has been matched, a subsequent matching failure cannot
1789 cause backtracking into the middle of the group.
1790 <a name="bigassertions"></a></P>
1791 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1792 <P>
1793 An assertion is a test on the characters following or preceding the current
1794 matching point that does not actually consume any characters. The simple
1795 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1796 <a href="#smallassertions">above.</a>
1797 </P>
1798 <P>
1799 More complicated assertions are coded as subpatterns. There are two kinds:
1800 those that look ahead of the current position in the subject string, and those
1801 that look behind it. An assertion subpattern is matched in the normal way,
1802 except that it does not cause the current matching position to be changed.
1803 </P>
1804 <P>
1805 Assertion subpatterns are not capturing subpatterns. If such an assertion
1806 contains capturing subpatterns within it, these are counted for the purposes of
1807 numbering the capturing subpatterns in the whole pattern. However, substring
1808 capturing is carried out only for positive assertions, because it does not make
1809 sense for negative assertions.
1810 </P>
1811 <P>
1812 For compatibility with Perl, assertion subpatterns may be repeated; though
1813 it makes no sense to assert the same thing several times, the side effect of
1814 capturing parentheses may occasionally be useful. In practice, there only three
1815 cases:
1816 <br>
1817 <br>
1818 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1819 However, it may contain internal capturing parenthesized groups that are called
1820 from elsewhere via the
1821 <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1822 <br>
1823 <br>
1824 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1825 were {0,1}. At run time, the rest of the pattern match is tried with and
1826 without the assertion, the order depending on the greediness of the quantifier.
1827 <br>
1828 <br>
1829 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1830 The assertion is obeyed just once when encountered during matching.
1831 </P>
1832 <br><b>
1833 Lookahead assertions
1834 </b><br>
1835 <P>
1836 Lookahead assertions start with (?= for positive assertions and (?! for
1837 negative assertions. For example,
1838 <pre>
1839 \w+(?=;)
1840 </pre>
1841 matches a word followed by a semicolon, but does not include the semicolon in
1842 the match, and
1843 <pre>
1844 foo(?!bar)
1845 </pre>
1846 matches any occurrence of "foo" that is not followed by "bar". Note that the
1847 apparently similar pattern
1848 <pre>
1849 (?!foo)bar
1850 </pre>
1851 does not find an occurrence of "bar" that is preceded by something other than
1852 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1853 (?!foo) is always true when the next three characters are "bar". A
1854 lookbehind assertion is needed to achieve the other effect.
1855 </P>
1856 <P>
1857 If you want to force a matching failure at some point in a pattern, the most
1858 convenient way to do it is with (?!) because an empty string always matches, so
1859 an assertion that requires there not to be an empty string must always fail.
1860 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1861 <a name="lookbehind"></a></P>
1862 <br><b>
1863 Lookbehind assertions
1864 </b><br>
1865 <P>
1866 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1867 negative assertions. For example,
1868 <pre>
1869 (?&#60;!foo)bar
1870 </pre>
1871 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1872 a lookbehind assertion are restricted such that all the strings it matches must
1873 have a fixed length. However, if there are several top-level alternatives, they
1874 do not all have to have the same fixed length. Thus
1875 <pre>
1876 (?&#60;=bullock|donkey)
1877 </pre>
1878 is permitted, but
1879 <pre>
1880 (?&#60;!dogs?|cats?)
1881 </pre>
1882 causes an error at compile time. Branches that match different length strings
1883 are permitted only at the top level of a lookbehind assertion. This is an
1884 extension compared with Perl, which requires all branches to match the same
1885 length of string. An assertion such as
1886 <pre>
1887 (?&#60;=ab(c|de))
1888 </pre>
1889 is not permitted, because its single top-level branch can match two different
1890 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1891 branches:
1892 <pre>
1893 (?&#60;=abc|abde)
1894 </pre>
1895 In some cases, the escape sequence \K
1896 <a href="#resetmatchstart">(see above)</a>
1897 can be used instead of a lookbehind assertion to get round the fixed-length
1898 restriction.
1899 </P>
1900 <P>
1901 The implementation of lookbehind assertions is, for each alternative, to
1902 temporarily move the current position back by the fixed length and then try to
1903 match. If there are insufficient characters before the current position, the
1904 assertion fails.
1905 </P>
1906 <P>
1907 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1908 to appear in lookbehind assertions, because it makes it impossible to calculate
1909 the length of the lookbehind. The \X and \R escapes, which can match
1910 different numbers of bytes, are also not permitted.
1911 </P>
1912 <P>
1913 <a href="#subpatternsassubroutines">"Subroutine"</a>
1914 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1915 as the subpattern matches a fixed-length string.
1916 <a href="#recursion">Recursion,</a>
1917 however, is not supported.
1918 </P>
1919 <P>
1920 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1921 specify efficient matching of fixed-length strings at the end of subject
1922 strings. Consider a simple pattern such as
1923 <pre>
1924 abcd$
1925 </pre>
1926 when applied to a long string that does not match. Because matching proceeds
1927 from left to right, PCRE will look for each "a" in the subject and then see if
1928 what follows matches the rest of the pattern. If the pattern is specified as
1929 <pre>
1930 ^.*abcd$
1931 </pre>
1932 the initial .* matches the entire string at first, but when this fails (because
1933 there is no following "a"), it backtracks to match all but the last character,
1934 then all but the last two characters, and so on. Once again the search for "a"
1935 covers the entire string, from right to left, so we are no better off. However,
1936 if the pattern is written as
1937 <pre>
1938 ^.*+(?&#60;=abcd)
1939 </pre>
1940 there can be no backtracking for the .*+ item; it can match only the entire
1941 string. The subsequent lookbehind assertion does a single test on the last four
1942 characters. If it fails, the match fails immediately. For long strings, this
1943 approach makes a significant difference to the processing time.
1944 </P>
1945 <br><b>
1946 Using multiple assertions
1947 </b><br>
1948 <P>
1949 Several assertions (of any sort) may occur in succession. For example,
1950 <pre>
1951 (?&#60;=\d{3})(?&#60;!999)foo
1952 </pre>
1953 matches "foo" preceded by three digits that are not "999". Notice that each of
1954 the assertions is applied independently at the same point in the subject
1955 string. First there is a check that the previous three characters are all
1956 digits, and then there is a check that the same three characters are not "999".
1957 This pattern does <i>not</i> match "foo" preceded by six characters, the first
1958 of which are digits and the last three of which are not "999". For example, it
1959 doesn't match "123abcfoo". A pattern to do that is
1960 <pre>
1961 (?&#60;=\d{3}...)(?&#60;!999)foo
1962 </pre>
1963 This time the first assertion looks at the preceding six characters, checking
1964 that the first three are digits, and then the second assertion checks that the
1965 preceding three characters are not "999".
1966 </P>
1967 <P>
1968 Assertions can be nested in any combination. For example,
1969 <pre>
1970 (?&#60;=(?&#60;!foo)bar)baz
1971 </pre>
1972 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1973 preceded by "foo", while
1974 <pre>
1975 (?&#60;=\d{3}(?!999)...)foo
1976 </pre>
1977 is another pattern that matches "foo" preceded by three digits and any three
1978 characters that are not "999".
1979 <a name="conditions"></a></P>
1980 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1981 <P>
1982 It is possible to cause the matching process to obey a subpattern
1983 conditionally or to choose between two alternative subpatterns, depending on
1984 the result of an assertion, or whether a specific capturing subpattern has
1985 already been matched. The two possible forms of conditional subpattern are:
1986 <pre>
1987 (?(condition)yes-pattern)
1988 (?(condition)yes-pattern|no-pattern)
1989 </pre>
1990 If the condition is satisfied, the yes-pattern is used; otherwise the
1991 no-pattern (if present) is used. If there are more than two alternatives in the
1992 subpattern, a compile-time error occurs. Each of the two alternatives may
1993 itself contain nested subpatterns of any form, including conditional
1994 subpatterns; the restriction to two alternatives applies only at the level of
1995 the condition. This pattern fragment is an example where the alternatives are
1996 complex:
1997 <pre>
1998 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2000 </PRE>
2001 </P>
2002 <P>
2003 There are four kinds of condition: references to subpatterns, references to
2004 recursion, a pseudo-condition called DEFINE, and assertions.
2005 </P>
2006 <br><b>
2007 Checking for a used subpattern by number
2008 </b><br>
2009 <P>
2010 If the text between the parentheses consists of a sequence of digits, the
2011 condition is true if a capturing subpattern of that number has previously
2012 matched. If there is more than one capturing subpattern with the same number
2013 (see the earlier
2014 <a href="#recursion">section about duplicate subpattern numbers),</a>
2015 the condition is true if any of them have matched. An alternative notation is
2016 to precede the digits with a plus or minus sign. In this case, the subpattern
2017 number is relative rather than absolute. The most recently opened parentheses
2018 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2019 loops it can also make sense to refer to subsequent groups. The next
2020 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2021 zero in any of these forms is not used; it provokes a compile-time error.)
2022 </P>
2023 <P>
2024 Consider the following pattern, which contains non-significant white space to
2025 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2026 three parts for ease of discussion:
2027 <pre>
2028 ( \( )? [^()]+ (?(1) \) )
2029 </pre>
2030 The first part matches an optional opening parenthesis, and if that
2031 character is present, sets it as the first captured substring. The second part
2032 matches one or more characters that are not parentheses. The third part is a
2033 conditional subpattern that tests whether or not the first set of parentheses
2034 matched. If they did, that is, if subject started with an opening parenthesis,
2035 the condition is true, and so the yes-pattern is executed and a closing
2036 parenthesis is required. Otherwise, since no-pattern is not present, the
2037 subpattern matches nothing. In other words, this pattern matches a sequence of
2038 non-parentheses, optionally enclosed in parentheses.
2039 </P>
2040 <P>
2041 If you were embedding this pattern in a larger one, you could use a relative
2042 reference:
2043 <pre>
2044 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2045 </pre>
2046 This makes the fragment independent of the parentheses in the larger pattern.
2047 </P>
2048 <br><b>
2049 Checking for a used subpattern by name
2050 </b><br>
2051 <P>
2052 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2053 subpattern by name. For compatibility with earlier versions of PCRE, which had
2054 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2055 there is a possible ambiguity with this syntax, because subpattern names may
2056 consist entirely of digits. PCRE looks first for a named subpattern; if it
2057 cannot find one and the name consists entirely of digits, PCRE looks for a
2058 subpattern of that number, which must be greater than zero. Using subpattern
2059 names that consist entirely of digits is not recommended.
2060 </P>
2061 <P>
2062 Rewriting the above example to use a named subpattern gives this:
2063 <pre>
2064 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
2065 </pre>
2066 If the name used in a condition of this kind is a duplicate, the test is
2067 applied to all subpatterns of the same name, and is true if any one of them has
2068 matched.
2069 </P>
2070 <br><b>
2071 Checking for pattern recursion
2072 </b><br>
2073 <P>
2074 If the condition is the string (R), and there is no subpattern with the name R,
2075 the condition is true if a recursive call to the whole pattern or any
2076 subpattern has been made. If digits or a name preceded by ampersand follow the
2077 letter R, for example:
2078 <pre>
2079 (?(R3)...) or (?(R&name)...)
2080 </pre>
2081 the condition is true if the most recent recursion is into a subpattern whose
2082 number or name is given. This condition does not check the entire recursion
2083 stack. If the name used in a condition of this kind is a duplicate, the test is
2084 applied to all subpatterns of the same name, and is true if any one of them is
2085 the most recent recursion.
2086 </P>
2087 <P>
2088 At "top level", all these recursion test conditions are false.
2089 <a href="#recursion">The syntax for recursive patterns</a>
2090 is described below.
2091 <a name="subdefine"></a></P>
2092 <br><b>
2093 Defining subpatterns for use by reference only
2094 </b><br>
2095 <P>
2096 If the condition is the string (DEFINE), and there is no subpattern with the
2097 name DEFINE, the condition is always false. In this case, there may be only one
2098 alternative in the subpattern. It is always skipped if control reaches this
2099 point in the pattern; the idea of DEFINE is that it can be used to define
2100 subroutines that can be referenced from elsewhere. (The use of
2101 <a href="#subpatternsassubroutines">subroutines</a>
2102 is described below.) For example, a pattern to match an IPv4 address such as
2103 "" could be written like this (ignore whitespace and line
2104 breaks):
2105 <pre>
2106 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2107 \b (?&byte) (\.(?&byte)){3} \b
2108 </pre>
2109 The first part of the pattern is a DEFINE group inside which a another group
2110 named "byte" is defined. This matches an individual component of an IPv4
2111 address (a number less than 256). When matching takes place, this part of the
2112 pattern is skipped because DEFINE acts like a false condition. The rest of the
2113 pattern uses references to the named group to match the four dot-separated
2114 components of an IPv4 address, insisting on a word boundary at each end.
2115 </P>
2116 <br><b>
2117 Assertion conditions
2118 </b><br>
2119 <P>
2120 If the condition is not in any of the above formats, it must be an assertion.
2121 This may be a positive or negative lookahead or lookbehind assertion. Consider
2122 this pattern, again containing non-significant white space, and with the two
2123 alternatives on the second line:
2124 <pre>
2125 (?(?=[^a-z]*[a-z])
2126 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2127 </pre>
2128 The condition is a positive lookahead assertion that matches an optional
2129 sequence of non-letters followed by a letter. In other words, it tests for the
2130 presence of at least one letter in the subject. If a letter is found, the
2131 subject is matched against the first alternative; otherwise it is matched
2132 against the second. This pattern matches strings in one of the two forms
2133 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2134 <a name="comments"></a></P>
2135 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2136 <P>
2137 There are two ways of including comments in patterns that are processed by
2138 PCRE. In both cases, the start of the comment must not be in a character class,
2139 nor in the middle of any other sequence of related characters such as (?: or a
2140 subpattern name or number. The characters that make up a comment play no part
2141 in the pattern matching.
2142 </P>
2143 <P>
2144 The sequence (?# marks the start of a comment that continues up to the next
2145 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2146 option is set, an unescaped # character also introduces a comment, which in
2147 this case continues to immediately after the next newline character or
2148 character sequence in the pattern. Which characters are interpreted as newlines
2149 is controlled by the options passed to <b>pcre_compile()</b> or by a special
2150 sequence at the start of the pattern, as described in the section entitled
2151 <a href="#newlines">"Newline conventions"</a>
2152 above. Note that the end of this type of comment is a literal newline sequence
2153 in the pattern; escape sequences that happen to represent a newline do not
2154 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2155 default newline convention is in force:
2156 <pre>
2157 abc #comment \n still comment
2158 </pre>
2159 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2160 a newline in the pattern. The sequence \n is still literal at this stage, so
2161 it does not terminate the comment. Only an actual character with the code value
2162 0x0a (the default newline) does so.
2163 <a name="recursion"></a></P>
2164 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2165 <P>
2166 Consider the problem of matching a string in parentheses, allowing for
2167 unlimited nested parentheses. Without the use of recursion, the best that can
2168 be done is to use a pattern that matches up to some fixed depth of nesting. It
2169 is not possible to handle an arbitrary nesting depth.
2170 </P>
2171 <P>
2172 For some time, Perl has provided a facility that allows regular expressions to
2173 recurse (amongst other things). It does this by interpolating Perl code in the
2174 expression at run time, and the code can refer to the expression itself. A Perl
2175 pattern using code interpolation to solve the parentheses problem can be
2176 created like this:
2177 <pre>
2178 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2179 </pre>
2180 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2181 recursively to the pattern in which it appears.
2182 </P>
2183 <P>
2184 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2185 supports special syntax for recursion of the entire pattern, and also for
2186 individual subpattern recursion. After its introduction in PCRE and Python,
2187 this kind of recursion was subsequently introduced into Perl at release 5.10.
2188 </P>
2189 <P>
2190 A special item that consists of (? followed by a number greater than zero and a
2191 closing parenthesis is a recursive subroutine call of the subpattern of the
2192 given number, provided that it occurs inside that subpattern. (If not, it is a
2193 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2194 call, which is described in the next section.) The special item (?R) or (?0) is
2195 a recursive call of the entire regular expression.
2196 </P>
2197 <P>
2198 This PCRE pattern solves the nested parentheses problem (assume the
2199 PCRE_EXTENDED option is set so that white space is ignored):
2200 <pre>
2201 \( ( [^()]++ | (?R) )* \)
2202 </pre>
2203 First it matches an opening parenthesis. Then it matches any number of
2204 substrings which can either be a sequence of non-parentheses, or a recursive
2205 match of the pattern itself (that is, a correctly parenthesized substring).
2206 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2207 to avoid backtracking into sequences of non-parentheses.
2208 </P>
2209 <P>
2210 If this were part of a larger pattern, you would not want to recurse the entire
2211 pattern, so instead you could use this:
2212 <pre>
2213 ( \( ( [^()]++ | (?1) )* \) )
2214 </pre>
2215 We have put the pattern into parentheses, and caused the recursion to refer to
2216 them instead of the whole pattern.
2217 </P>
2218 <P>
2219 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2220 is made easier by the use of relative references. Instead of (?1) in the
2221 pattern above you can write (?-2) to refer to the second most recently opened
2222 parentheses preceding the recursion. In other words, a negative number counts
2223 capturing parentheses leftwards from the point at which it is encountered.
2224 </P>
2225 <P>
2226 It is also possible to refer to subsequently opened parentheses, by writing
2227 references such as (?+2). However, these cannot be recursive because the
2228 reference is not inside the parentheses that are referenced. They are always
2229 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2230 calls, as described in the next section.
2231 </P>
2232 <P>
2233 An alternative approach is to use named parentheses instead. The Perl syntax
2234 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2235 could rewrite the above example as follows:
2236 <pre>
2237 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2238 </pre>
2239 If there is more than one subpattern with the same name, the earliest one is
2240 used.
2241 </P>
2242 <P>
2243 This particular example pattern that we have been looking at contains nested
2244 unlimited repeats, and so the use of a possessive quantifier for matching
2245 strings of non-parentheses is important when applying the pattern to strings
2246 that do not match. For example, when this pattern is applied to
2247 <pre>
2248 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2249 </pre>
2250 it yields "no match" quickly. However, if a possessive quantifier is not used,
2251 the match runs for a very long time indeed because there are so many different
2252 ways the + and * repeats can carve up the subject, and all have to be tested
2253 before failure can be reported.
2254 </P>
2255 <P>
2256 At the end of a match, the values of capturing parentheses are those from
2257 the outermost level. If you want to obtain intermediate values, a callout
2258 function can be used (see below and the
2259 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2260 documentation). If the pattern above is matched against
2261 <pre>
2262 (ab(cd)ef)
2263 </pre>
2264 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2265 the last value taken on at the top level. If a capturing subpattern is not
2266 matched at the top level, its final captured value is unset, even if it was
2267 (temporarily) set at a deeper level during the matching process.
2268 </P>
2269 <P>
2270 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2271 obtain extra memory to store data during a recursion, which it does by using
2272 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2273 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2274 </P>
2275 <P>
2276 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2277 Consider this pattern, which matches text in angle brackets, allowing for
2278 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2279 recursing), whereas any characters are permitted at the outer level.
2280 <pre>
2281 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2282 </pre>
2283 In this pattern, (?(R) is the start of a conditional subpattern, with two
2284 different alternatives for the recursive and non-recursive cases. The (?R) item
2285 is the actual recursive call.
2286 <a name="recursiondifference"></a></P>
2287 <br><b>
2288 Differences in recursion processing between PCRE and Perl
2289 </b><br>
2290 <P>
2291 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2292 (like Python, but unlike Perl), a recursive subpattern call is always treated
2293 as an atomic group. That is, once it has matched some of the subject string, it
2294 is never re-entered, even if it contains untried alternatives and there is a
2295 subsequent matching failure. This can be illustrated by the following pattern,
2296 which purports to match a palindromic string that contains an odd number of
2297 characters (for example, "a", "aba", "abcba", "abcdcba"):
2298 <pre>
2299 ^(.|(.)(?1)\2)$
2300 </pre>
2301 The idea is that it either matches a single character, or two identical
2302 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2303 it does not if the pattern is longer than three characters. Consider the
2304 subject string "abcba":
2305 </P>
2306 <P>
2307 At the top level, the first character is matched, but as it is not at the end
2308 of the string, the first alternative fails; the second alternative is taken
2309 and the recursion kicks in. The recursive call to subpattern 1 successfully
2310 matches the next character ("b"). (Note that the beginning and end of line
2311 tests are not part of the recursion).
2312 </P>
2313 <P>
2314 Back at the top level, the next character ("c") is compared with what
2315 subpattern 2 matched, which was "a". This fails. Because the recursion is
2316 treated as an atomic group, there are now no backtracking points, and so the
2317 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2318 try the second alternative.) However, if the pattern is written with the
2319 alternatives in the other order, things are different:
2320 <pre>
2321 ^((.)(?1)\2|.)$
2322 </pre>
2323 This time, the recursing alternative is tried first, and continues to recurse
2324 until it runs out of characters, at which point the recursion fails. But this
2325 time we do have another alternative to try at the higher level. That is the big
2326 difference: in the previous case the remaining alternative is at a deeper
2327 recursion level, which PCRE cannot use.
2328 </P>
2329 <P>
2330 To change the pattern so that it matches all palindromic strings, not just
2331 those with an odd number of characters, it is tempting to change the pattern to
2332 this:
2333 <pre>
2334 ^((.)(?1)\2|.?)$
2335 </pre>
2336 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2337 deeper recursion has matched a single character, it cannot be entered again in
2338 order to match an empty string. The solution is to separate the two cases, and
2339 write out the odd and even cases as alternatives at the higher level:
2340 <pre>
2341 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2342 </pre>
2343 If you want to match typical palindromic phrases, the pattern has to ignore all
2344 non-word characters, which can be done like this:
2345 <pre>
2346 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2347 </pre>
2348 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2349 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2350 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2351 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2352 more) to match typical phrases, and Perl takes so long that you think it has
2353 gone into a loop.
2354 </P>
2355 <P>
2356 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2357 string does not start with a palindrome that is shorter than the entire string.
2358 For example, although "abcba" is correctly matched, if the subject is "ababa",
2359 PCRE finds the palindrome "aba" at the start, then fails at top level because
2360 the end of the string does not follow. Once again, it cannot jump back into the
2361 recursion to try other alternatives, so the entire match fails.
2362 </P>
2363 <P>
2364 The second way in which PCRE and Perl differ in their recursion processing is
2365 in the handling of captured values. In Perl, when a subpattern is called
2366 recursively or as a subpattern (see the next section), it has no access to any
2367 values that were captured outside the recursion, whereas in PCRE these values
2368 can be referenced. Consider this pattern:
2369 <pre>
2370 ^(.)(\1|a(?2))
2371 </pre>
2372 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2373 then in the second group, when the back reference \1 fails to match "b", the
2374 second alternative matches "a" and then recurses. In the recursion, \1 does
2375 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2376 match because inside the recursive call \1 cannot access the externally set
2377 value.
2378 <a name="subpatternsassubroutines"></a></P>
2379 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2380 <P>
2381 If the syntax for a recursive subpattern call (either by number or by
2382 name) is used outside the parentheses to which it refers, it operates like a
2383 subroutine in a programming language. The called subpattern may be defined
2384 before or after the reference. A numbered reference can be absolute or
2385 relative, as in these examples:
2386 <pre>
2387 (...(absolute)...)...(?2)...
2388 (...(relative)...)...(?-1)...
2389 (...(?+1)...(relative)...
2390 </pre>
2391 An earlier example pointed out that the pattern
2392 <pre>
2393 (sens|respons)e and \1ibility
2394 </pre>
2395 matches "sense and sensibility" and "response and responsibility", but not
2396 "sense and responsibility". If instead the pattern
2397 <pre>
2398 (sens|respons)e and (?1)ibility
2399 </pre>
2400 is used, it does match "sense and responsibility" as well as the other two
2401 strings. Another example is given in the discussion of DEFINE above.
2402 </P>
2403 <P>
2404 All subroutine calls, whether recursive or not, are always treated as atomic
2405 groups. That is, once a subroutine has matched some of the subject string, it
2406 is never re-entered, even if it contains untried alternatives and there is a
2407 subsequent matching failure. Any capturing parentheses that are set during the
2408 subroutine call revert to their previous values afterwards.
2409 </P>
2410 <P>
2411 Processing options such as case-independence are fixed when a subpattern is
2412 defined, so if it is used as a subroutine, such options cannot be changed for
2413 different calls. For example, consider this pattern:
2414 <pre>
2415 (abc)(?i:(?-1))
2416 </pre>
2417 It matches "abcabc". It does not match "abcABC" because the change of
2418 processing option does not affect the called subpattern.
2419 <a name="onigurumasubroutines"></a></P>
2420 <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2421 <P>
2422 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2423 a number enclosed either in angle brackets or single quotes, is an alternative
2424 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2425 are two of the examples used above, rewritten using this syntax:
2426 <pre>
2427 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2428 (sens|respons)e and \g'1'ibility
2429 </pre>
2430 PCRE supports an extension to Oniguruma: if a number is preceded by a
2431 plus or a minus sign it is taken as a relative reference. For example:
2432 <pre>
2433 (abc)(?i:\g&#60;-1&#62;)
2434 </pre>
2435 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2436 synonymous. The former is a back reference; the latter is a subroutine call.
2437 </P>
2438 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2439 <P>
2440 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2441 code to be obeyed in the middle of matching a regular expression. This makes it
2442 possible, amongst other things, to extract different substrings that match the
2443 same pair of parentheses when there is a repetition.
2444 </P>
2445 <P>
2446 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2447 code. The feature is called "callout". The caller of PCRE provides an external
2448 function by putting its entry point in the global variable <i>pcre_callout</i>.
2449 By default, this variable contains NULL, which disables all calling out.
2450 </P>
2451 <P>
2452 Within a regular expression, (?C) indicates the points at which the external
2453 function is to be called. If you want to identify different callout points, you
2454 can put a number less than 256 after the letter C. The default value is zero.
2455 For example, this pattern has two callout points:
2456 <pre>
2457 (?C1)abc(?C2)def
2458 </pre>
2459 If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2460 automatically installed before each item in the pattern. They are all numbered
2461 255.
2462 </P>
2463 <P>
2464 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2465 set), the external function is called. It is provided with the number of the
2466 callout, the position in the pattern, and, optionally, one item of data
2467 originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2468 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2469 description of the interface to the callout function is given in the
2470 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2471 documentation.
2472 <a name="backtrackcontrol"></a></P>
2473 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2474 <P>
2475 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2476 are described in the Perl documentation as "experimental and subject to change
2477 or removal in a future version of Perl". It goes on to say: "Their usage in
2478 production code should be noted to avoid problems during upgrades." The same
2479 remarks apply to the PCRE features described in this section.
2480 </P>
2481 <P>
2482 Since these verbs are specifically related to backtracking, most of them can be
2483 used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2484 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2485 failing negative assertion, they cause an error if encountered by
2486 <b>pcre_dfa_exec()</b>.
2487 </P>
2488 <P>
2489 If any of these verbs are used in an assertion or in a subpattern that is
2490 called as a subroutine (whether or not recursively), their effect is confined
2491 to that subpattern; it does not extend to the surrounding pattern, with one
2492 exception: a *MARK that is encountered in a positive assertion <i>is</i> passed
2493 back (compare capturing parentheses in assertions). Note that such subpatterns
2494 are processed as anchored at the point where they are tested. Note also that
2495 Perl's treatment of subroutines is different in some cases.
2496 </P>
2497 <P>
2498 The new verbs make use of what was previously invalid syntax: an opening
2499 parenthesis followed by an asterisk. They are generally of the form
2500 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2501 depending on whether or not an argument is present. A name is any sequence of
2502 characters that does not include a closing parenthesis. If the name is empty,
2503 that is, if the closing parenthesis immediately follows the colon, the effect
2504 is as if the colon were not there. Any number of these verbs may occur in a
2505 pattern.
2506 </P>
2507 <P>
2508 PCRE contains some optimizations that are used to speed up matching by running
2509 some checks at the start of each match attempt. For example, it may know the
2510 minimum length of matching subject, or that a particular character must be
2511 present. When one of these optimizations suppresses the running of a match, any
2512 included backtracking verbs will not, of course, be processed. You can suppress
2513 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2514 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2515 pattern with (*NO_START_OPT).
2516 </P>
2517 <br><b>
2518 Verbs that act immediately
2519 </b><br>
2520 <P>
2521 The following verbs act as soon as they are encountered. They may not be
2522 followed by a name.
2523 <pre>
2524 (*ACCEPT)
2525 </pre>
2526 This verb causes the match to end successfully, skipping the remainder of the
2527 pattern. However, when it is inside a subpattern that is called as a
2528 subroutine, only that subpattern is ended successfully. Matching then continues
2529 at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2530 far is captured. For example:
2531 <pre>
2532 A((?:A|B(*ACCEPT)|C)D)
2533 </pre>
2534 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2535 the outer parentheses.
2536 <pre>
2537 (*FAIL) or (*F)
2538 </pre>
2539 This verb causes a matching failure, forcing backtracking to occur. It is
2540 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2541 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2542 Perl features that are not present in PCRE. The nearest equivalent is the
2543 callout feature, as for example in this pattern:
2544 <pre>
2545 a+(?C)(*FAIL)
2546 </pre>
2547 A match with the string "aaaa" always fails, but the callout is taken before
2548 each backtrack happens (in this example, 10 times).
2549 </P>
2550 <br><b>
2551 Recording which path was taken
2552 </b><br>
2553 <P>
2554 There is one verb whose main purpose is to track how a match was arrived at,
2555 though it also has a secondary use in conjunction with advancing the match
2556 starting point (see (*SKIP) below).
2557 <pre>
2558 (*MARK:NAME) or (*:NAME)
2559 </pre>
2560 A name is always required with this verb. There may be as many instances of
2561 (*MARK) as you like in a pattern, and their names do not have to be unique.
2562 </P>
2563 <P>
2564 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2565 to the caller via the <i>pcre_extra</i> data structure, as described in the
2566 <a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a>
2567 in the
2568 <a href="pcreapi.html"><b>pcreapi</b></a>
2569 documentation. No data is returned for a partial match. Here is an example of
2570 <b>pcretest</b> output, where the /K modifier requests the retrieval and
2571 outputting of (*MARK) data:
2572 <pre>
2573 /X(*MARK:A)Y|X(*MARK:B)Z/K
2574 XY
2575 0: XY
2576 MK: A
2577 XZ
2578 0: XZ
2579 MK: B
2580 </pre>
2581 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2582 indicates which of the two alternatives matched. This is a more efficient way
2583 of obtaining this information than putting each alternative in its own
2584 capturing parentheses.
2585 </P>
2586 <P>
2587 If (*MARK) is encountered in a positive assertion, its name is recorded and
2588 passed back if it is the last-encountered. This does not happen for negative
2589 assertions.
2590 </P>
2591 <P>
2592 A name may also be returned after a failed match if the final path through the
2593 pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2594 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2595 starting point for matching is advanced, the final check is often with an empty
2596 string, causing a failure before (*MARK) is reached. For example:
2597 <pre>
2598 /X(*MARK:A)Y|X(*MARK:B)Z/K
2599 XP
2600 No match
2601 </pre>
2602 There are three potential starting points for this match (starting with X,
2603 starting with P, and with an empty string). If the pattern is anchored, the
2604 result is different:
2605 <pre>
2606 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2607 XP
2608 No match, mark = B
2609 </pre>
2610 PCRE's start-of-match optimizations can also interfere with this. For example,
2611 if, as a result of a call to <b>pcre_study()</b>, it knows the minimum
2612 subject length for a match, a shorter subject will not be scanned at all.
2613 </P>
2614 <P>
2615 Note that similar anomalies (though different in detail) exist in Perl, no
2616 doubt for the same reasons. The use of (*MARK) data after a failed match of an
2617 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2618 </P>
2619 <br><b>
2620 Verbs that act after backtracking
2621 </b><br>
2622 <P>
2623 The following verbs do nothing when they are encountered. Matching continues
2624 with what follows, but if there is no subsequent match, causing a backtrack to
2625 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2626 the verb. However, when one of these verbs appears inside an atomic group, its
2627 effect is confined to that group, because once the group has been matched,
2628 there is never any backtracking into it. In this situation, backtracking can
2629 "jump back" to the left of the entire atomic group. (Remember also, as stated
2630 above, that this localization also applies in subroutine calls and assertions.)
2631 </P>
2632 <P>
2633 These verbs differ in exactly what kind of failure occurs when backtracking
2634 reaches them.
2635 <pre>
2636 (*COMMIT)
2637 </pre>
2638 This verb, which may not be followed by a name, causes the whole match to fail
2639 outright if the rest of the pattern does not match. Even if the pattern is
2640 unanchored, no further attempts to find a match by advancing the starting point
2641 take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2642 finding a match at the current starting point, or not at all. For example:
2643 <pre>
2644 a+(*COMMIT)b
2645 </pre>
2646 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2647 dynamic anchor, or "I've started, so I must finish." The name of the most
2648 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2649 match failure.
2650 </P>
2651 <P>
2652 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2653 unless PCRE's start-of-match optimizations are turned off, as shown in this
2654 <b>pcretest</b> example:
2655 <pre>
2656 /(*COMMIT)abc/
2657 xyzabc
2658 0: abc
2659 xyzabc\Y
2660 No match
2661 </pre>
2662 PCRE knows that any match must start with "a", so the optimization skips along
2663 the subject to "a" before running the first match attempt, which succeeds. When
2664 the optimization is disabled by the \Y escape in the second subject, the match
2665 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2666 starting points.
2667 <pre>
2668 (*PRUNE) or (*PRUNE:NAME)
2669 </pre>
2670 This verb causes the match to fail at the current starting position in the
2671 subject if the rest of the pattern does not match. If the pattern is
2672 unanchored, the normal "bumpalong" advance to the next starting character then
2673 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2674 reached, or when matching to the right of (*PRUNE), but if there is no match to
2675 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2676 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2677 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2678 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2679 match fails completely; the name is passed back if this is the final attempt.
2680 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2681 pattern (*PRUNE) has the same effect as (*COMMIT).
2682 <pre>
2683 (*SKIP)
2684 </pre>
2685 This verb, when given without a name, is like (*PRUNE), except that if the
2686 pattern is unanchored, the "bumpalong" advance is not to the next character,
2687 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2688 signifies that whatever text was matched leading up to it cannot be part of a
2689 successful match. Consider:
2690 <pre>
2691 a+(*SKIP)b
2692 </pre>
2693 If the subject is "aaaac...", after the first match attempt fails (starting at
2694 the first character in the string), the starting point skips on to start the
2695 next attempt at "c". Note that a possessive quantifer does not have the same
2696 effect as this example; although it would suppress backtracking during the
2697 first match attempt, the second attempt would start at the second character
2698 instead of skipping on to "c".
2699 <pre>
2700 (*SKIP:NAME)
2701 </pre>
2702 When (*SKIP) has an associated name, its behaviour is modified. If the
2703 following pattern fails to match, the previous path through the pattern is
2704 searched for the most recent (*MARK) that has the same name. If one is found,
2705 the "bumpalong" advance is to the subject position that corresponds to that
2706 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2707 matching name is found, normal "bumpalong" of one character happens (that is,
2708 the (*SKIP) is ignored).
2709 <pre>
2710 (*THEN) or (*THEN:NAME)
2711 </pre>
2712 This verb causes a skip to the next innermost alternative if the rest of the
2713 pattern does not match. That is, it cancels pending backtracking, but only
2714 within the current alternative. Its name comes from the observation that it can
2715 be used for a pattern-based if-then-else block:
2716 <pre>
2717 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2718 </pre>
2719 If the COND1 pattern matches, FOO is tried (and possibly further items after
2720 the end of the group if FOO succeeds); on failure, the matcher skips to the
2721 second alternative and tries COND2, without backtracking into COND1. The
2722 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2723 overall match fails. If (*THEN) is not inside an alternation, it acts like
2724 (*PRUNE).
2725 </P>
2726 <P>
2727 Note that a subpattern that does not contain a | character is just a part of
2728 the enclosing alternative; it is not a nested alternation with only one
2729 alternative. The effect of (*THEN) extends beyond such a subpattern to the
2730 enclosing alternative. Consider this pattern, where A, B, etc. are complex
2731 pattern fragments that do not contain any | characters at this level:
2732 <pre>
2733 A (B(*THEN)C) | D
2734 </pre>
2735 If A and B are matched, but there is a failure in C, matching does not
2736 backtrack into A; instead it moves to the next alternative, that is, D.
2737 However, if the subpattern containing (*THEN) is given an alternative, it
2738 behaves differently:
2739 <pre>
2740 A (B(*THEN)C | (*FAIL)) | D
2741 </pre>
2742 The effect of (*THEN) is now confined to the inner subpattern. After a failure
2743 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2744 because there are no more alternatives to try. In this case, matching does now
2745 backtrack into A.
2746 </P>
2747 <P>
2748 Note also that a conditional subpattern is not considered as having two
2749 alternatives, because only one is ever used. In other words, the | character in
2750 a conditional subpattern has a different meaning. Ignoring white space,
2751 consider:
2752 <pre>
2753 ^.*? (?(?=a) a | b(*THEN)c )
2754 </pre>
2755 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2756 it initially matches zero characters. The condition (?=a) then fails, the
2757 character "b" is matched, but "c" is not. At this point, matching does not
2758 backtrack to .*? as might perhaps be expected from the presence of the |
2759 character. The conditional subpattern is part of the single alternative that
2760 comprises the whole pattern, and so the match fails. (If there was a backtrack
2761 into .*?, allowing it to match "b", the match would succeed.)
2762 </P>
2763 <P>
2764 The verbs just described provide four different "strengths" of control when
2765 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2766 next alternative. (*PRUNE) comes next, failing the match at the current
2767 starting position, but allowing an advance to the next character (for an
2768 unanchored pattern). (*SKIP) is similar, except that the advance may be more
2769 than one character. (*COMMIT) is the strongest, causing the entire match to
2770 fail.
2771 </P>
2772 <P>
2773 If more than one such verb is present in a pattern, the "strongest" one wins.
2774 For example, consider this pattern, where A, B, etc. are complex pattern
2775 fragments:
2776 <pre>
2777 (A(*COMMIT)B(*THEN)C|D)
2778 </pre>
2779 Once A has matched, PCRE is committed to this match, at the current starting
2780 position. If subsequently B matches, but C does not, the normal (*THEN) action
2781 of trying the next alternative (that is, D) does not happen because (*COMMIT)
2782 overrides.
2783 </P>
2784 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2785 <P>
2786 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2787 <b>pcresyntax</b>(3), <b>pcre</b>(3).
2788 </P>
2789 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2790 <P>
2791 Philip Hazel
2792 <br>
2793 University Computing Service
2794 <br>
2795 Cambridge CB2 3QH, England.
2796 <br>
2797 </P>
2798 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2799 <P>
2800 Last updated: 09 October 2011
2801 <br>
2802 Copyright &copy; 1997-2011 University of Cambridge.
2803 <br>
2804 <p>
2805 Return to the <a href="index.html">PCRE index page</a>.
2806 </p>


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