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

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