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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">EBCDIC CHARACTER CODES</a>
18 <li><a name="TOC3" href="#SEC3">NEWLINE CONVENTIONS</a>
19 <li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
20 <li><a name="TOC5" href="#SEC5">BACKSLASH</a>
21 <li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
22 <li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
23 <li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a>
25 <li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
26 <li><a name="TOC11" href="#SEC11">VERTICAL BAR</a>
27 <li><a name="TOC12" href="#SEC12">INTERNAL OPTION SETTING</a>
28 <li><a name="TOC13" href="#SEC13">SUBPATTERNS</a>
29 <li><a name="TOC14" href="#SEC14">DUPLICATE SUBPATTERN NUMBERS</a>
30 <li><a name="TOC15" href="#SEC15">NAMED SUBPATTERNS</a>
31 <li><a name="TOC16" href="#SEC16">REPETITION</a>
33 <li><a name="TOC18" href="#SEC18">BACK REFERENCES</a>
34 <li><a name="TOC19" href="#SEC19">ASSERTIONS</a>
35 <li><a name="TOC20" href="#SEC20">CONDITIONAL SUBPATTERNS</a>
36 <li><a name="TOC21" href="#SEC21">COMMENTS</a>
37 <li><a name="TOC22" href="#SEC22">RECURSIVE PATTERNS</a>
38 <li><a name="TOC23" href="#SEC23">SUBPATTERNS AS SUBROUTINES</a>
39 <li><a name="TOC24" href="#SEC24">ONIGURUMA SUBROUTINE SYNTAX</a>
40 <li><a name="TOC25" href="#SEC25">CALLOUTS</a>
41 <li><a name="TOC26" href="#SEC26">BACKTRACKING CONTROL</a>
42 <li><a name="TOC27" href="#SEC27">SEE ALSO</a>
43 <li><a name="TOC28" href="#SEC28">AUTHOR</a>
44 <li><a name="TOC29" href="#SEC29">REVISION</a>
45 </ul>
46 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
47 <P>
48 The syntax and semantics of the regular expressions that are supported by PCRE
49 are described in detail below. There is a quick-reference syntax summary in the
50 <a href="pcresyntax.html"><b>pcresyntax</b></a>
51 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
52 also supports some alternative regular expression syntax (which does not
53 conflict with the Perl syntax) in order to provide some compatibility with
54 regular expressions in Python, .NET, and Oniguruma.
55 </P>
56 <P>
57 Perl's regular expressions are described in its own documentation, and
58 regular expressions in general are covered in a number of books, some of which
59 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
60 published by O'Reilly, covers regular expressions in great detail. This
61 description of PCRE's regular expressions is intended as reference material.
62 </P>
63 <P>
64 The original operation of PCRE was on strings of one-byte characters. However,
65 there is now also support for UTF-8 strings in the original library, an
66 extra library that supports 16-bit and UTF-16 character strings, and a
67 third library that supports 32-bit and UTF-32 character strings. To use these
68 features, PCRE must be built to include appropriate support. When using UTF
69 strings you must either call the compiling function with the PCRE_UTF8,
70 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
71 these special sequences:
72 <pre>
73 (*UTF8)
74 (*UTF16)
75 (*UTF32)
76 (*UTF)
77 </pre>
78 (*UTF) is a generic sequence that can be used with any of the libraries.
79 Starting a pattern with such a sequence is equivalent to setting the relevant
80 option. This feature is not Perl-compatible. How setting a UTF mode affects
81 pattern matching is mentioned in several places below. There is also a summary
82 of features in the
83 <a href="pcreunicode.html"><b>pcreunicode</b></a>
84 page.
85 </P>
86 <P>
87 Another special sequence that may appear at the start of a pattern or in
88 combination with (*UTF8), (*UTF16), (*UTF32) or (*UTF) is:
89 <pre>
90 (*UCP)
91 </pre>
92 This has the same effect as setting the PCRE_UCP option: it causes sequences
93 such as \d and \w to use Unicode properties to determine character types,
94 instead of recognizing only characters with codes less than 128 via a lookup
95 table.
96 </P>
97 <P>
98 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
99 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
100 also some more of these special sequences that are concerned with the handling
101 of newlines; they are described below.
102 </P>
103 <P>
104 The remainder of this document discusses the patterns that are supported by
105 PCRE when one its main matching functions, <b>pcre_exec()</b> (8-bit) or
106 <b>pcre[16|32]_exec()</b> (16- or 32-bit), is used. PCRE also has alternative
107 matching functions, <b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>,
108 which match using a different algorithm that is not Perl-compatible. Some of
109 the features discussed below are not available when DFA matching is used. The
110 advantages and disadvantages of the alternative functions, and how they differ
111 from the normal functions, are discussed in the
112 <a href="pcrematching.html"><b>pcrematching</b></a>
113 page.
114 </P>
115 <br><a name="SEC2" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
116 <P>
117 PCRE can be compiled to run in an environment that uses EBCDIC as its character
118 code rather than ASCII or Unicode (typically a mainframe system). In the
119 sections below, character code values are ASCII or Unicode; in an EBCDIC
120 environment these characters may have different code values, and there are no
121 code points greater than 255.
122 <a name="newlines"></a></P>
123 <br><a name="SEC3" href="#TOC1">NEWLINE CONVENTIONS</a><br>
124 <P>
125 PCRE supports five different conventions for indicating line breaks in
126 strings: a single CR (carriage return) character, a single LF (linefeed)
127 character, the two-character sequence CRLF, any of the three preceding, or any
128 Unicode newline sequence. The
129 <a href="pcreapi.html"><b>pcreapi</b></a>
130 page has
131 <a href="pcreapi.html#newlines">further discussion</a>
132 about newlines, and shows how to set the newline convention in the
133 <i>options</i> arguments for the compiling and matching functions.
134 </P>
135 <P>
136 It is also possible to specify a newline convention by starting a pattern
137 string with one of the following five sequences:
138 <pre>
139 (*CR) carriage return
140 (*LF) linefeed
141 (*CRLF) carriage return, followed by linefeed
142 (*ANYCRLF) any of the three above
143 (*ANY) all Unicode newline sequences
144 </pre>
145 These override the default and the options given to the compiling function. For
146 example, on a Unix system where LF is the default newline sequence, the pattern
147 <pre>
148 (*CR)a.b
149 </pre>
150 changes the convention to CR. That pattern matches "a\nb" because LF is no
151 longer a newline. Note that these special settings, which are not
152 Perl-compatible, are recognized only at the very start of a pattern, and that
153 they must be in upper case. If more than one of them is present, the last one
154 is used.
155 </P>
156 <P>
157 The newline convention affects where the circumflex and dollar assertions are
158 true. It also affects the interpretation of the dot metacharacter when
159 PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect
160 what the \R escape sequence matches. By default, this is any Unicode newline
161 sequence, for Perl compatibility. However, this can be changed; see the
162 description of \R in the section entitled
163 <a href="#newlineseq">"Newline sequences"</a>
164 below. A change of \R setting can be combined with a change of newline
165 convention.
166 </P>
167 <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
168 <P>
169 A regular expression is a pattern that is matched against a subject string from
170 left to right. Most characters stand for themselves in a pattern, and match the
171 corresponding characters in the subject. As a trivial example, the pattern
172 <pre>
173 The quick brown fox
174 </pre>
175 matches a portion of a subject string that is identical to itself. When
176 caseless matching is specified (the PCRE_CASELESS option), letters are matched
177 independently of case. In a UTF mode, PCRE always understands the concept of
178 case for characters whose values are less than 128, so caseless matching is
179 always possible. For characters with higher values, the concept of case is
180 supported if PCRE is compiled with Unicode property support, but not otherwise.
181 If you want to use caseless matching for characters 128 and above, you must
182 ensure that PCRE is compiled with Unicode property support as well as with
183 UTF support.
184 </P>
185 <P>
186 The power of regular expressions comes from the ability to include alternatives
187 and repetitions in the pattern. These are encoded in the pattern by the use of
188 <i>metacharacters</i>, which do not stand for themselves but instead are
189 interpreted in some special way.
190 </P>
191 <P>
192 There are two different sets of metacharacters: those that are recognized
193 anywhere in the pattern except within square brackets, and those that are
194 recognized within square brackets. Outside square brackets, the metacharacters
195 are as follows:
196 <pre>
197 \ general escape character with several uses
198 ^ assert start of string (or line, in multiline mode)
199 $ assert end of string (or line, in multiline mode)
200 . match any character except newline (by default)
201 [ start character class definition
202 | start of alternative branch
203 ( start subpattern
204 ) end subpattern
205 ? extends the meaning of (
206 also 0 or 1 quantifier
207 also quantifier minimizer
208 * 0 or more quantifier
209 + 1 or more quantifier
210 also "possessive quantifier"
211 { start min/max quantifier
212 </pre>
213 Part of a pattern that is in square brackets is called a "character class". In
214 a character class the only metacharacters are:
215 <pre>
216 \ general escape character
217 ^ negate the class, but only if the first character
218 - indicates character range
219 [ POSIX character class (only if followed by POSIX syntax)
220 ] terminates the character class
221 </pre>
222 The following sections describe the use of each of the metacharacters.
223 </P>
224 <br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
225 <P>
226 The backslash character has several uses. Firstly, if it is followed by a
227 character that is not a number or a letter, it takes away any special meaning
228 that character may have. This use of backslash as an escape character applies
229 both inside and outside character classes.
230 </P>
231 <P>
232 For example, if you want to match a * character, you write \* in the pattern.
233 This escaping action applies whether or not the following character would
234 otherwise be interpreted as a metacharacter, so it is always safe to precede a
235 non-alphanumeric with backslash to specify that it stands for itself. In
236 particular, if you want to match a backslash, you write \\.
237 </P>
238 <P>
239 In a UTF mode, only ASCII numbers and letters have any special meaning after a
240 backslash. All other characters (in particular, those whose codepoints are
241 greater than 127) are treated as literals.
242 </P>
243 <P>
244 If a pattern is compiled with the PCRE_EXTENDED option, white space in the
245 pattern (other than in a character class) and characters between a # outside
246 a character class and the next newline are ignored. An escaping backslash can
247 be used to include a white space or # character as part of the pattern.
248 </P>
249 <P>
250 If you want to remove the special meaning from a sequence of characters, you
251 can do so by putting them between \Q and \E. This is different from Perl in
252 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
253 Perl, $ and @ cause variable interpolation. Note the following examples:
254 <pre>
255 Pattern PCRE matches Perl matches
257 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
258 \Qabc\$xyz\E abc\$xyz abc\$xyz
259 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
260 </pre>
261 The \Q...\E sequence is recognized both inside and outside character classes.
262 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
263 by \E later in the pattern, the literal interpretation continues to the end of
264 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
265 a character class, this causes an error, because the character class is not
266 terminated.
267 <a name="digitsafterbackslash"></a></P>
268 <br><b>
269 Non-printing characters
270 </b><br>
271 <P>
272 A second use of backslash provides a way of encoding non-printing characters
273 in patterns in a visible manner. There is no restriction on the appearance of
274 non-printing characters, apart from the binary zero that terminates a pattern,
275 but when a pattern is being prepared by text editing, it is often easier to use
276 one of the following escape sequences than the binary character it represents:
277 <pre>
278 \a alarm, that is, the BEL character (hex 07)
279 \cx "control-x", where x is any ASCII character
280 \e escape (hex 1B)
281 \f form feed (hex 0C)
282 \n linefeed (hex 0A)
283 \r carriage return (hex 0D)
284 \t tab (hex 09)
285 \ddd character with octal code ddd, or back reference
286 \xhh character with hex code hh
287 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
288 \uhhhh character with hex code hhhh (JavaScript mode only)
289 </pre>
290 The precise effect of \cx on ASCII characters is as follows: if x is a lower
291 case letter, it is converted to upper case. Then bit 6 of the character (hex
292 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
293 but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
294 data item (byte or 16-bit value) following \c has a value greater than 127, a
295 compile-time error occurs. This locks out non-ASCII characters in all modes.
296 </P>
297 <P>
298 The \c facility was designed for use with ASCII characters, but with the
299 extension to Unicode it is even less useful than it once was. It is, however,
300 recognized when PCRE is compiled in EBCDIC mode, where data items are always
301 bytes. In this mode, all values are valid after \c. If the next character is a
302 lower case letter, it is converted to upper case. Then the 0xc0 bits of the
303 byte are inverted. Thus \cA becomes hex 01, as in ASCII (A is C1), but because
304 the EBCDIC letters are disjoint, \cZ becomes hex 29 (Z is E9), and other
305 characters also generate different values.
306 </P>
307 <P>
308 By default, after \x, from zero to two hexadecimal digits are read (letters
309 can be in upper or lower case). Any number of hexadecimal digits may appear
310 between \x{ and }, but the character code is constrained as follows:
311 <pre>
312 8-bit non-UTF mode less than 0x100
313 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
314 16-bit non-UTF mode less than 0x10000
315 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
316 32-bit non-UTF mode less than 0x80000000
317 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
318 </pre>
319 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
320 "surrogate" codepoints), and 0xffef.
321 </P>
322 <P>
323 If characters other than hexadecimal digits appear between \x{ and }, or if
324 there is no terminating }, this form of escape is not recognized. Instead, the
325 initial \x will be interpreted as a basic hexadecimal escape, with no
326 following digits, giving a character whose value is zero.
327 </P>
328 <P>
329 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
330 as just described only when it is followed by two hexadecimal digits.
331 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
332 code points greater than 256 is provided by \u, which must be followed by
333 four hexadecimal digits; otherwise it matches a literal "u" character.
334 Character codes specified by \u in JavaScript mode are constrained in the same
335 was as those specified by \x in non-JavaScript mode.
336 </P>
337 <P>
338 Characters whose value is less than 256 can be defined by either of the two
339 syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
340 way they are handled. For example, \xdc is exactly the same as \x{dc} (or
341 \u00dc in JavaScript mode).
342 </P>
343 <P>
344 After \0 up to two further octal digits are read. If there are fewer than two
345 digits, just those that are present are used. Thus the sequence \0\x\07
346 specifies two binary zeros followed by a BEL character (code value 7). Make
347 sure you supply two digits after the initial zero if the pattern character that
348 follows is itself an octal digit.
349 </P>
350 <P>
351 The handling of a backslash followed by a digit other than 0 is complicated.
352 Outside a character class, PCRE reads it and any following digits as a decimal
353 number. If the number is less than 10, or if there have been at least that many
354 previous capturing left parentheses in the expression, the entire sequence is
355 taken as a <i>back reference</i>. A description of how this works is given
356 <a href="#backreferences">later,</a>
357 following the discussion of
358 <a href="#subpattern">parenthesized subpatterns.</a>
359 </P>
360 <P>
361 Inside a character class, or if the decimal number is greater than 9 and there
362 have not been that many capturing subpatterns, PCRE re-reads up to three octal
363 digits following the backslash, and uses them to generate a data character. Any
364 subsequent digits stand for themselves. The value of the character is
365 constrained in the same way as characters specified in hexadecimal.
366 For example:
367 <pre>
368 \040 is another way of writing an ASCII space
369 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
370 \7 is always a back reference
371 \11 might be a back reference, or another way of writing a tab
372 \011 is always a tab
373 \0113 is a tab followed by the character "3"
374 \113 might be a back reference, otherwise the character with octal code 113
375 \377 might be a back reference, otherwise the value 255 (decimal)
376 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
377 </pre>
378 Note that octal values of 100 or greater must not be introduced by a leading
379 zero, because no more than three octal digits are ever read.
380 </P>
381 <P>
382 All the sequences that define a single character value can be used both inside
383 and outside character classes. In addition, inside a character class, \b is
384 interpreted as the backspace character (hex 08).
385 </P>
386 <P>
387 \N is not allowed in a character class. \B, \R, and \X are not special
388 inside a character class. Like other unrecognized escape sequences, they are
389 treated as the literal characters "B", "R", and "X" by default, but cause an
390 error if the PCRE_EXTRA option is set. Outside a character class, these
391 sequences have different meanings.
392 </P>
393 <br><b>
394 Unsupported escape sequences
395 </b><br>
396 <P>
397 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
398 handler and used to modify the case of following characters. By default, PCRE
399 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
400 option is set, \U matches a "U" character, and \u can be used to define a
401 character by code point, as described in the previous section.
402 </P>
403 <br><b>
404 Absolute and relative back references
405 </b><br>
406 <P>
407 The sequence \g followed by an unsigned or a negative number, optionally
408 enclosed in braces, is an absolute or relative back reference. A named back
409 reference can be coded as \g{name}. Back references are discussed
410 <a href="#backreferences">later,</a>
411 following the discussion of
412 <a href="#subpattern">parenthesized subpatterns.</a>
413 </P>
414 <br><b>
415 Absolute and relative subroutine calls
416 </b><br>
417 <P>
418 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
419 a number enclosed either in angle brackets or single quotes, is an alternative
420 syntax for referencing a subpattern as a "subroutine". Details are discussed
421 <a href="#onigurumasubroutines">later.</a>
422 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
423 synonymous. The former is a back reference; the latter is a
424 <a href="#subpatternsassubroutines">subroutine</a>
425 call.
426 <a name="genericchartypes"></a></P>
427 <br><b>
428 Generic character types
429 </b><br>
430 <P>
431 Another use of backslash is for specifying generic character types:
432 <pre>
433 \d any decimal digit
434 \D any character that is not a decimal digit
435 \h any horizontal white space character
436 \H any character that is not a horizontal white space character
437 \s any white space character
438 \S any character that is not a white space character
439 \v any vertical white space character
440 \V any character that is not a vertical white space character
441 \w any "word" character
442 \W any "non-word" character
443 </pre>
444 There is also the single sequence \N, which matches a non-newline character.
445 This is the same as
446 <a href="#fullstopdot">the "." metacharacter</a>
447 when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
448 PCRE does not support this.
449 </P>
450 <P>
451 Each pair of lower and upper case escape sequences partitions the complete set
452 of characters into two disjoint sets. Any given character matches one, and only
453 one, of each pair. The sequences can appear both inside and outside character
454 classes. They each match one character of the appropriate type. If the current
455 matching point is at the end of the subject string, all of them fail, because
456 there is no character to match.
457 </P>
458 <P>
459 For compatibility with Perl, \s does not match the VT character (code 11).
460 This makes it different from the the POSIX "space" class. The \s characters
461 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
462 included in a Perl script, \s may match the VT character. In PCRE, it never
463 does.
464 </P>
465 <P>
466 A "word" character is an underscore or any character that is a letter or digit.
467 By default, the definition of letters and digits is controlled by PCRE's
468 low-valued character tables, and may vary if locale-specific matching is taking
469 place (see
470 <a href="pcreapi.html#localesupport">"Locale support"</a>
471 in the
472 <a href="pcreapi.html"><b>pcreapi</b></a>
473 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
474 or "french" in Windows, some character codes greater than 128 are used for
475 accented letters, and these are then matched by \w. The use of locales with
476 Unicode is discouraged.
477 </P>
478 <P>
479 By default, in a UTF mode, characters with values greater than 128 never match
480 \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
481 their original meanings from before UTF support was available, mainly for
482 efficiency reasons. However, if PCRE is compiled with Unicode property support,
483 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
484 properties are used to determine character types, as follows:
485 <pre>
486 \d any character that \p{Nd} matches (decimal digit)
487 \s any character that \p{Z} matches, plus HT, LF, FF, CR
488 \w any character that \p{L} or \p{N} matches, plus underscore
489 </pre>
490 The upper case escapes match the inverse sets of characters. Note that \d
491 matches only decimal digits, whereas \w matches any Unicode digit, as well as
492 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
493 \B because they are defined in terms of \w and \W. Matching these sequences
494 is noticeably slower when PCRE_UCP is set.
495 </P>
496 <P>
497 The sequences \h, \H, \v, and \V are features that were added to Perl at
498 release 5.10. In contrast to the other sequences, which match only ASCII
499 characters by default, these always match certain high-valued codepoints,
500 whether or not PCRE_UCP is set. The horizontal space characters are:
501 <pre>
502 U+0009 Horizontal tab (HT)
503 U+0020 Space
504 U+00A0 Non-break space
505 U+1680 Ogham space mark
506 U+180E Mongolian vowel separator
507 U+2000 En quad
508 U+2001 Em quad
509 U+2002 En space
510 U+2003 Em space
511 U+2004 Three-per-em space
512 U+2005 Four-per-em space
513 U+2006 Six-per-em space
514 U+2007 Figure space
515 U+2008 Punctuation space
516 U+2009 Thin space
517 U+200A Hair space
518 U+202F Narrow no-break space
519 U+205F Medium mathematical space
520 U+3000 Ideographic space
521 </pre>
522 The vertical space characters are:
523 <pre>
524 U+000A Linefeed (LF)
525 U+000B Vertical tab (VT)
526 U+000C Form feed (FF)
527 U+000D Carriage return (CR)
528 U+0085 Next line (NEL)
529 U+2028 Line separator
530 U+2029 Paragraph separator
531 </pre>
532 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
533 relevant.
534 <a name="newlineseq"></a></P>
535 <br><b>
536 Newline sequences
537 </b><br>
538 <P>
539 Outside a character class, by default, the escape sequence \R matches any
540 Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
541 following:
542 <pre>
543 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
544 </pre>
545 This is an example of an "atomic group", details of which are given
546 <a href="#atomicgroup">below.</a>
547 This particular group matches either the two-character sequence CR followed by
548 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
549 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
550 line, U+0085). The two-character sequence is treated as a single unit that
551 cannot be split.
552 </P>
553 <P>
554 In other modes, two additional characters whose codepoints are greater than 255
555 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
556 Unicode character property support is not needed for these characters to be
557 recognized.
558 </P>
559 <P>
560 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
561 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
562 either at compile time or when the pattern is matched. (BSR is an abbrevation
563 for "backslash R".) This can be made the default when PCRE is built; if this is
564 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
565 It is also possible to specify these settings by starting a pattern string with
566 one of the following sequences:
567 <pre>
568 (*BSR_ANYCRLF) CR, LF, or CRLF only
569 (*BSR_UNICODE) any Unicode newline sequence
570 </pre>
571 These override the default and the options given to the compiling function, but
572 they can themselves be overridden by options given to a matching function. Note
573 that these special settings, which are not Perl-compatible, are recognized only
574 at the very start of a pattern, and that they must be in upper case. If more
575 than one of them is present, the last one is used. They can be combined with a
576 change of newline convention; for example, a pattern can start with:
577 <pre>
579 </pre>
580 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
581 (*UCP) special sequences. Inside a character class, \R is treated as an
582 unrecognized escape sequence, and so matches the letter "R" by default, but
583 causes an error if PCRE_EXTRA is set.
584 <a name="uniextseq"></a></P>
585 <br><b>
586 Unicode character properties
587 </b><br>
588 <P>
589 When PCRE is built with Unicode character property support, three additional
590 escape sequences that match characters with specific properties are available.
591 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
592 characters whose codepoints are less than 256, but they do work in this mode.
593 The extra escape sequences are:
594 <pre>
595 \p{<i>xx</i>} a character with the <i>xx</i> property
596 \P{<i>xx</i>} a character without the <i>xx</i> property
597 \X a Unicode extended grapheme cluster
598 </pre>
599 The property names represented by <i>xx</i> above are limited to the Unicode
600 script names, the general category properties, "Any", which matches any
601 character (including newline), and some special PCRE properties (described
602 in the
603 <a href="#extraprops">next section).</a>
604 Other Perl properties such as "InMusicalSymbols" are not currently supported by
605 PCRE. Note that \P{Any} does not match any characters, so always causes a
606 match failure.
607 </P>
608 <P>
609 Sets of Unicode characters are defined as belonging to certain scripts. A
610 character from one of these sets can be matched using a script name. For
611 example:
612 <pre>
613 \p{Greek}
614 \P{Han}
615 </pre>
616 Those that are not part of an identified script are lumped together as
617 "Common". The current list of scripts is:
618 </P>
619 <P>
620 Arabic,
621 Armenian,
622 Avestan,
623 Balinese,
624 Bamum,
625 Batak,
626 Bengali,
627 Bopomofo,
628 Brahmi,
629 Braille,
630 Buginese,
631 Buhid,
632 Canadian_Aboriginal,
633 Carian,
634 Chakma,
635 Cham,
636 Cherokee,
637 Common,
638 Coptic,
639 Cuneiform,
640 Cypriot,
641 Cyrillic,
642 Deseret,
643 Devanagari,
644 Egyptian_Hieroglyphs,
645 Ethiopic,
646 Georgian,
647 Glagolitic,
648 Gothic,
649 Greek,
650 Gujarati,
651 Gurmukhi,
652 Han,
653 Hangul,
654 Hanunoo,
655 Hebrew,
656 Hiragana,
657 Imperial_Aramaic,
658 Inherited,
659 Inscriptional_Pahlavi,
660 Inscriptional_Parthian,
661 Javanese,
662 Kaithi,
663 Kannada,
664 Katakana,
665 Kayah_Li,
666 Kharoshthi,
667 Khmer,
668 Lao,
669 Latin,
670 Lepcha,
671 Limbu,
672 Linear_B,
673 Lisu,
674 Lycian,
675 Lydian,
676 Malayalam,
677 Mandaic,
678 Meetei_Mayek,
679 Meroitic_Cursive,
680 Meroitic_Hieroglyphs,
681 Miao,
682 Mongolian,
683 Myanmar,
684 New_Tai_Lue,
685 Nko,
686 Ogham,
687 Old_Italic,
688 Old_Persian,
689 Old_South_Arabian,
690 Old_Turkic,
691 Ol_Chiki,
692 Oriya,
693 Osmanya,
694 Phags_Pa,
695 Phoenician,
696 Rejang,
697 Runic,
698 Samaritan,
699 Saurashtra,
700 Sharada,
701 Shavian,
702 Sinhala,
703 Sora_Sompeng,
704 Sundanese,
705 Syloti_Nagri,
706 Syriac,
707 Tagalog,
708 Tagbanwa,
709 Tai_Le,
710 Tai_Tham,
711 Tai_Viet,
712 Takri,
713 Tamil,
714 Telugu,
715 Thaana,
716 Thai,
717 Tibetan,
718 Tifinagh,
719 Ugaritic,
720 Vai,
721 Yi.
722 </P>
723 <P>
724 Each character has exactly one Unicode general category property, specified by
725 a two-letter abbreviation. For compatibility with Perl, negation can be
726 specified by including a circumflex between the opening brace and the property
727 name. For example, \p{^Lu} is the same as \P{Lu}.
728 </P>
729 <P>
730 If only one letter is specified with \p or \P, it includes all the general
731 category properties that start with that letter. In this case, in the absence
732 of negation, the curly brackets in the escape sequence are optional; these two
733 examples have the same effect:
734 <pre>
735 \p{L}
736 \pL
737 </pre>
738 The following general category property codes are supported:
739 <pre>
740 C Other
741 Cc Control
742 Cf Format
743 Cn Unassigned
744 Co Private use
745 Cs Surrogate
747 L Letter
748 Ll Lower case letter
749 Lm Modifier letter
750 Lo Other letter
751 Lt Title case letter
752 Lu Upper case letter
754 M Mark
755 Mc Spacing mark
756 Me Enclosing mark
757 Mn Non-spacing mark
759 N Number
760 Nd Decimal number
761 Nl Letter number
762 No Other number
764 P Punctuation
765 Pc Connector punctuation
766 Pd Dash punctuation
767 Pe Close punctuation
768 Pf Final punctuation
769 Pi Initial punctuation
770 Po Other punctuation
771 Ps Open punctuation
773 S Symbol
774 Sc Currency symbol
775 Sk Modifier symbol
776 Sm Mathematical symbol
777 So Other symbol
779 Z Separator
780 Zl Line separator
781 Zp Paragraph separator
782 Zs Space separator
783 </pre>
784 The special property L& is also supported: it matches a character that has
785 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
786 a modifier or "other".
787 </P>
788 <P>
789 The Cs (Surrogate) property applies only to characters in the range U+D800 to
790 U+DFFF. Such characters are not valid in Unicode strings and so
791 cannot be tested by PCRE, unless UTF validity checking has been turned off
792 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
793 PCRE_NO_UTF32_CHECK in the
794 <a href="pcreapi.html"><b>pcreapi</b></a>
795 page). Perl does not support the Cs property.
796 </P>
797 <P>
798 The long synonyms for property names that Perl supports (such as \p{Letter})
799 are not supported by PCRE, nor is it permitted to prefix any of these
800 properties with "Is".
801 </P>
802 <P>
803 No character that is in the Unicode table has the Cn (unassigned) property.
804 Instead, this property is assumed for any code point that is not in the
805 Unicode table.
806 </P>
807 <P>
808 Specifying caseless matching does not affect these escape sequences. For
809 example, \p{Lu} always matches only upper case letters.
810 </P>
811 <P>
812 Matching characters by Unicode property is not fast, because PCRE has to do a
813 multistage table lookup in order to find a character's property. That is why
814 the traditional escape sequences such as \d and \w do not use Unicode
815 properties in PCRE by default, though you can make them do so by setting the
816 PCRE_UCP option or by starting the pattern with (*UCP).
817 </P>
818 <br><b>
819 Extended grapheme clusters
820 </b><br>
821 <P>
822 The \X escape matches any number of Unicode characters that form an "extended
823 grapheme cluster", and treats the sequence as an atomic group
824 <a href="#atomicgroup">(see below).</a>
825 Up to and including release 8.31, PCRE matched an earlier, simpler definition
826 that was equivalent to
827 <pre>
828 (?&#62;\PM\pM*)
829 </pre>
830 That is, it matched a character without the "mark" property, followed by zero
831 or more characters with the "mark" property. Characters with the "mark"
832 property are typically non-spacing accents that affect the preceding character.
833 </P>
834 <P>
835 This simple definition was extended in Unicode to include more complicated
836 kinds of composite character by giving each character a grapheme breaking
837 property, and creating rules that use these properties to define the boundaries
838 of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches
839 one of these clusters.
840 </P>
841 <P>
842 \X always matches at least one character. Then it decides whether to add
843 additional characters according to the following rules for ending a cluster:
844 </P>
845 <P>
846 1. End at the end of the subject string.
847 </P>
848 <P>
849 2. Do not end between CR and LF; otherwise end after any control character.
850 </P>
851 <P>
852 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
853 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
854 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
855 character; an LVT or T character may be follwed only by a T character.
856 </P>
857 <P>
858 4. Do not end before extending characters or spacing marks. Characters with
859 the "mark" property always have the "extend" grapheme breaking property.
860 </P>
861 <P>
862 5. Do not end after prepend characters.
863 </P>
864 <P>
865 6. Otherwise, end the cluster.
866 <a name="extraprops"></a></P>
867 <br><b>
868 PCRE's additional properties
869 </b><br>
870 <P>
871 As well as the standard Unicode properties described above, PCRE supports four
872 more that make it possible to convert traditional escape sequences such as \w
873 and \s and POSIX character classes to use Unicode properties. PCRE uses these
874 non-standard, non-Perl properties internally when PCRE_UCP is set. They are:
875 <pre>
876 Xan Any alphanumeric character
877 Xps Any POSIX space character
878 Xsp Any Perl space character
879 Xwd Any Perl "word" character
880 </pre>
881 Xan matches characters that have either the L (letter) or the N (number)
882 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
883 carriage return, and any other character that has the Z (separator) property.
884 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
885 same characters as Xan, plus underscore.
886 <a name="resetmatchstart"></a></P>
887 <br><b>
888 Resetting the match start
889 </b><br>
890 <P>
891 The escape sequence \K causes any previously matched characters not to be
892 included in the final matched sequence. For example, the pattern:
893 <pre>
894 foo\Kbar
895 </pre>
896 matches "foobar", but reports that it has matched "bar". This feature is
897 similar to a lookbehind assertion
898 <a href="#lookbehind">(described below).</a>
899 However, in this case, the part of the subject before the real match does not
900 have to be of fixed length, as lookbehind assertions do. The use of \K does
901 not interfere with the setting of
902 <a href="#subpattern">captured substrings.</a>
903 For example, when the pattern
904 <pre>
905 (foo)\Kbar
906 </pre>
907 matches "foobar", the first substring is still set to "foo".
908 </P>
909 <P>
910 Perl documents that the use of \K within assertions is "not well defined". In
911 PCRE, \K is acted upon when it occurs inside positive assertions, but is
912 ignored in negative assertions.
913 <a name="smallassertions"></a></P>
914 <br><b>
915 Simple assertions
916 </b><br>
917 <P>
918 The final use of backslash is for certain simple assertions. An assertion
919 specifies a condition that has to be met at a particular point in a match,
920 without consuming any characters from the subject string. The use of
921 subpatterns for more complicated assertions is described
922 <a href="#bigassertions">below.</a>
923 The backslashed assertions are:
924 <pre>
925 \b matches at a word boundary
926 \B matches when not at a word boundary
927 \A matches at the start of the subject
928 \Z matches at the end of the subject
929 also matches before a newline at the end of the subject
930 \z matches only at the end of the subject
931 \G matches at the first matching position in the subject
932 </pre>
933 Inside a character class, \b has a different meaning; it matches the backspace
934 character. If any other of these assertions appears in a character class, by
935 default it matches the corresponding literal character (for example, \B
936 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
937 escape sequence" error is generated instead.
938 </P>
939 <P>
940 A word boundary is a position in the subject string where the current character
941 and the previous character do not both match \w or \W (i.e. one matches
942 \w and the other matches \W), or the start or end of the string if the
943 first or last character matches \w, respectively. In a UTF mode, the meanings
944 of \w and \W can be changed by setting the PCRE_UCP option. When this is
945 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
946 of word" or "end of word" metasequence. However, whatever follows \b normally
947 determines which it is. For example, the fragment \ba matches "a" at the start
948 of a word.
949 </P>
950 <P>
951 The \A, \Z, and \z assertions differ from the traditional circumflex and
952 dollar (described in the next section) in that they only ever match at the very
953 start and end of the subject string, whatever options are set. Thus, they are
954 independent of multiline mode. These three assertions are not affected by the
955 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
956 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
957 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
958 at a point other than the beginning of the subject, \A can never match. The
959 difference between \Z and \z is that \Z matches before a newline at the end
960 of the string as well as at the very end, whereas \z matches only at the end.
961 </P>
962 <P>
963 The \G assertion is true only when the current matching position is at the
964 start point of the match, as specified by the <i>startoffset</i> argument of
965 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
966 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
967 arguments, you can mimic Perl's /g option, and it is in this kind of
968 implementation where \G can be useful.
969 </P>
970 <P>
971 Note, however, that PCRE's interpretation of \G, as the start of the current
972 match, is subtly different from Perl's, which defines it as the end of the
973 previous match. In Perl, these can be different when the previously matched
974 string was empty. Because PCRE does just one match at a time, it cannot
975 reproduce this behaviour.
976 </P>
977 <P>
978 If all the alternatives of a pattern begin with \G, the expression is anchored
979 to the starting match position, and the "anchored" flag is set in the compiled
980 regular expression.
981 </P>
982 <br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
983 <P>
984 The circumflex and dollar metacharacters are zero-width assertions. That is,
985 they test for a particular condition being true without consuming any
986 characters from the subject string.
987 </P>
988 <P>
989 Outside a character class, in the default matching mode, the circumflex
990 character is an assertion that is true only if the current matching point is at
991 the start of the subject string. If the <i>startoffset</i> argument of
992 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
993 option is unset. Inside a character class, circumflex has an entirely different
994 meaning
995 <a href="#characterclass">(see below).</a>
996 </P>
997 <P>
998 Circumflex need not be the first character of the pattern if a number of
999 alternatives are involved, but it should be the first thing in each alternative
1000 in which it appears if the pattern is ever to match that branch. If all
1001 possible alternatives start with a circumflex, that is, if the pattern is
1002 constrained to match only at the start of the subject, it is said to be an
1003 "anchored" pattern. (There are also other constructs that can cause a pattern
1004 to be anchored.)
1005 </P>
1006 <P>
1007 The dollar character is an assertion that is true only if the current matching
1008 point is at the end of the subject string, or immediately before a newline at
1009 the end of the string (by default). Note, however, that it does not actually
1010 match the newline. Dollar need not be the last character of the pattern if a
1011 number of alternatives are involved, but it should be the last item in any
1012 branch in which it appears. Dollar has no special meaning in a character class.
1013 </P>
1014 <P>
1015 The meaning of dollar can be changed so that it matches only at the very end of
1016 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1017 does not affect the \Z assertion.
1018 </P>
1019 <P>
1020 The meanings of the circumflex and dollar characters are changed if the
1021 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1022 immediately after internal newlines as well as at the start of the subject
1023 string. It does not match after a newline that ends the string. A dollar
1024 matches before any newlines in the string, as well as at the very end, when
1025 PCRE_MULTILINE is set. When newline is specified as the two-character
1026 sequence CRLF, isolated CR and LF characters do not indicate newlines.
1027 </P>
1028 <P>
1029 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
1030 \n represents a newline) in multiline mode, but not otherwise. Consequently,
1031 patterns that are anchored in single line mode because all branches start with
1032 ^ are not anchored in multiline mode, and a match for circumflex is possible
1033 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
1034 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1035 </P>
1036 <P>
1037 Note that the sequences \A, \Z, and \z can be used to match the start and
1038 end of the subject in both modes, and if all branches of a pattern start with
1039 \A it is always anchored, whether or not PCRE_MULTILINE is set.
1040 <a name="fullstopdot"></a></P>
1041 <br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
1042 <P>
1043 Outside a character class, a dot in the pattern matches any one character in
1044 the subject string except (by default) a character that signifies the end of a
1045 line.
1046 </P>
1047 <P>
1048 When a line ending is defined as a single character, dot never matches that
1049 character; when the two-character sequence CRLF is used, dot does not match CR
1050 if it is immediately followed by LF, but otherwise it matches all characters
1051 (including isolated CRs and LFs). When any Unicode line endings are being
1052 recognized, dot does not match CR or LF or any of the other line ending
1053 characters.
1054 </P>
1055 <P>
1056 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1057 option is set, a dot matches any one character, without exception. If the
1058 two-character sequence CRLF is present in the subject string, it takes two dots
1059 to match it.
1060 </P>
1061 <P>
1062 The handling of dot is entirely independent of the handling of circumflex and
1063 dollar, the only relationship being that they both involve newlines. Dot has no
1064 special meaning in a character class.
1065 </P>
1066 <P>
1067 The escape sequence \N behaves like a dot, except that it is not affected by
1068 the PCRE_DOTALL option. In other words, it matches any character except one
1069 that signifies the end of a line. Perl also uses \N to match characters by
1070 name; PCRE does not support this.
1071 </P>
1072 <br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1073 <P>
1074 Outside a character class, the escape sequence \C matches any one data unit,
1075 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1076 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1077 a 32-bit unit. Unlike a dot, \C always
1078 matches line-ending characters. The feature is provided in Perl in order to
1079 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1080 used. Because \C breaks up characters into individual data units, matching one
1081 unit with \C in a UTF mode means that the rest of the string may start with a
1082 malformed UTF character. This has undefined results, because PCRE assumes that
1083 it is dealing with valid UTF strings (and by default it checks this at the
1084 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1085 PCRE_NO_UTF32_CHECK option is used).
1086 </P>
1087 <P>
1088 PCRE does not allow \C to appear in lookbehind assertions
1089 <a href="#lookbehind">(described below)</a>
1090 in a UTF mode, because this would make it impossible to calculate the length of
1091 the lookbehind.
1092 </P>
1093 <P>
1094 In general, the \C escape sequence is best avoided. However, one
1095 way of using it that avoids the problem of malformed UTF characters is to use a
1096 lookahead to check the length of the next character, as in this pattern, which
1097 could be used with a UTF-8 string (ignore white space and line breaks):
1098 <pre>
1099 (?| (?=[\x00-\x7f])(\C) |
1100 (?=[\x80-\x{7ff}])(\C)(\C) |
1101 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1102 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1103 </pre>
1104 A group that starts with (?| resets the capturing parentheses numbers in each
1105 alternative (see
1106 <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1107 below). The assertions at the start of each branch check the next UTF-8
1108 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1109 character's individual bytes are then captured by the appropriate number of
1110 groups.
1111 <a name="characterclass"></a></P>
1112 <br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1113 <P>
1114 An opening square bracket introduces a character class, terminated by a closing
1115 square bracket. A closing square bracket on its own is not special by default.
1116 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1117 bracket causes a compile-time error. If a closing square bracket is required as
1118 a member of the class, it should be the first data character in the class
1119 (after an initial circumflex, if present) or escaped with a backslash.
1120 </P>
1121 <P>
1122 A character class matches a single character in the subject. In a UTF mode, the
1123 character may be more than one data unit long. A matched character must be in
1124 the set of characters defined by the class, unless the first character in the
1125 class definition is a circumflex, in which case the subject character must not
1126 be in the set defined by the class. If a circumflex is actually required as a
1127 member of the class, ensure it is not the first character, or escape it with a
1128 backslash.
1129 </P>
1130 <P>
1131 For example, the character class [aeiou] matches any lower case vowel, while
1132 [^aeiou] matches any character that is not a lower case vowel. Note that a
1133 circumflex is just a convenient notation for specifying the characters that
1134 are in the class by enumerating those that are not. A class that starts with a
1135 circumflex is not an assertion; it still consumes a character from the subject
1136 string, and therefore it fails if the current pointer is at the end of the
1137 string.
1138 </P>
1139 <P>
1140 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1141 can be included in a class as a literal string of data units, or by using the
1142 \x{ escaping mechanism.
1143 </P>
1144 <P>
1145 When caseless matching is set, any letters in a class represent both their
1146 upper case and lower case versions, so for example, a caseless [aeiou] matches
1147 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1148 caseful version would. In a UTF mode, PCRE always understands the concept of
1149 case for characters whose values are less than 128, so caseless matching is
1150 always possible. For characters with higher values, the concept of case is
1151 supported if PCRE is compiled with Unicode property support, but not otherwise.
1152 If you want to use caseless matching in a UTF mode for characters 128 and
1153 above, you must ensure that PCRE is compiled with Unicode property support as
1154 well as with UTF support.
1155 </P>
1156 <P>
1157 Characters that might indicate line breaks are never treated in any special way
1158 when matching character classes, whatever line-ending sequence is in use, and
1159 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1160 such as [^a] always matches one of these characters.
1161 </P>
1162 <P>
1163 The minus (hyphen) character can be used to specify a range of characters in a
1164 character class. For example, [d-m] matches any letter between d and m,
1165 inclusive. If a minus character is required in a class, it must be escaped with
1166 a backslash or appear in a position where it cannot be interpreted as
1167 indicating a range, typically as the first or last character in the class.
1168 </P>
1169 <P>
1170 It is not possible to have the literal character "]" as the end character of a
1171 range. A pattern such as [W-]46] is interpreted as a class of two characters
1172 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1173 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1174 the end of range, so [W-\]46] is interpreted as a class containing a range
1175 followed by two other characters. The octal or hexadecimal representation of
1176 "]" can also be used to end a range.
1177 </P>
1178 <P>
1179 Ranges operate in the collating sequence of character values. They can also be
1180 used for characters specified numerically, for example [\000-\037]. Ranges
1181 can include any characters that are valid for the current mode.
1182 </P>
1183 <P>
1184 If a range that includes letters is used when caseless matching is set, it
1185 matches the letters in either case. For example, [W-c] is equivalent to
1186 [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1187 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1188 characters in both cases. In UTF modes, PCRE supports the concept of case for
1189 characters with values greater than 128 only when it is compiled with Unicode
1190 property support.
1191 </P>
1192 <P>
1193 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1194 \V, \w, and \W may appear in a character class, and add the characters that
1195 they match to the class. For example, [\dABCDEF] matches any hexadecimal
1196 digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1197 and their upper case partners, just as it does when they appear outside a
1198 character class, as described in the section entitled
1199 <a href="#genericchartypes">"Generic character types"</a>
1200 above. The escape sequence \b has a different meaning inside a character
1201 class; it matches the backspace character. The sequences \B, \N, \R, and \X
1202 are not special inside a character class. Like any other unrecognized escape
1203 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1204 default, but cause an error if the PCRE_EXTRA option is set.
1205 </P>
1206 <P>
1207 A circumflex can conveniently be used with the upper case character types to
1208 specify a more restricted set of characters than the matching lower case type.
1209 For example, the class [^\W_] matches any letter or digit, but not underscore,
1210 whereas [\w] includes underscore. A positive character class should be read as
1211 "something OR something OR ..." and a negative class as "NOT something AND NOT
1212 something AND NOT ...".
1213 </P>
1214 <P>
1215 The only metacharacters that are recognized in character classes are backslash,
1216 hyphen (only where it can be interpreted as specifying a range), circumflex
1217 (only at the start), opening square bracket (only when it can be interpreted as
1218 introducing a POSIX class name - see the next section), and the terminating
1219 closing square bracket. However, escaping other non-alphanumeric characters
1220 does no harm.
1221 </P>
1222 <br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1223 <P>
1224 Perl supports the POSIX notation for character classes. This uses names
1225 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1226 this notation. For example,
1227 <pre>
1228 [01[:alpha:]%]
1229 </pre>
1230 matches "0", "1", any alphabetic character, or "%". The supported class names
1231 are:
1232 <pre>
1233 alnum letters and digits
1234 alpha letters
1235 ascii character codes 0 - 127
1236 blank space or tab only
1237 cntrl control characters
1238 digit decimal digits (same as \d)
1239 graph printing characters, excluding space
1240 lower lower case letters
1241 print printing characters, including space
1242 punct printing characters, excluding letters and digits and space
1243 space white space (not quite the same as \s)
1244 upper upper case letters
1245 word "word" characters (same as \w)
1246 xdigit hexadecimal digits
1247 </pre>
1248 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1249 space (32). Notice that this list includes the VT character (code 11). This
1250 makes "space" different to \s, which does not include VT (for Perl
1251 compatibility).
1252 </P>
1253 <P>
1254 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1255 5.8. Another Perl extension is negation, which is indicated by a ^ character
1256 after the colon. For example,
1257 <pre>
1258 [12[:^digit:]]
1259 </pre>
1260 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1261 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1262 supported, and an error is given if they are encountered.
1263 </P>
1264 <P>
1265 By default, in UTF modes, characters with values greater than 128 do not match
1266 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1267 to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1268 character properties are used. This is achieved by replacing the POSIX classes
1269 by other sequences, as follows:
1270 <pre>
1271 [:alnum:] becomes \p{Xan}
1272 [:alpha:] becomes \p{L}
1273 [:blank:] becomes \h
1274 [:digit:] becomes \p{Nd}
1275 [:lower:] becomes \p{Ll}
1276 [:space:] becomes \p{Xps}
1277 [:upper:] becomes \p{Lu}
1278 [:word:] becomes \p{Xwd}
1279 </pre>
1280 Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1281 classes are unchanged, and match only characters with code points less than
1282 128.
1283 </P>
1284 <br><a name="SEC11" href="#TOC1">VERTICAL BAR</a><br>
1285 <P>
1286 Vertical bar characters are used to separate alternative patterns. For example,
1287 the pattern
1288 <pre>
1289 gilbert|sullivan
1290 </pre>
1291 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1292 and an empty alternative is permitted (matching the empty string). The matching
1293 process tries each alternative in turn, from left to right, and the first one
1294 that succeeds is used. If the alternatives are within a subpattern
1295 <a href="#subpattern">(defined below),</a>
1296 "succeeds" means matching the rest of the main pattern as well as the
1297 alternative in the subpattern.
1298 </P>
1299 <br><a name="SEC12" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1300 <P>
1301 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1302 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1303 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1304 The option letters are
1305 <pre>
1306 i for PCRE_CASELESS
1308 s for PCRE_DOTALL
1309 x for PCRE_EXTENDED
1310 </pre>
1311 For example, (?im) sets caseless, multiline matching. It is also possible to
1312 unset these options by preceding the letter with a hyphen, and a combined
1313 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1314 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1315 permitted. If a letter appears both before and after the hyphen, the option is
1316 unset.
1317 </P>
1318 <P>
1319 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1320 changed in the same way as the Perl-compatible options by using the characters
1321 J, U and X respectively.
1322 </P>
1323 <P>
1324 When one of these option changes occurs at top level (that is, not inside
1325 subpattern parentheses), the change applies to the remainder of the pattern
1326 that follows. If the change is placed right at the start of a pattern, PCRE
1327 extracts it into the global options (and it will therefore show up in data
1328 extracted by the <b>pcre_fullinfo()</b> function).
1329 </P>
1330 <P>
1331 An option change within a subpattern (see below for a description of
1332 subpatterns) affects only that part of the subpattern that follows it, so
1333 <pre>
1334 (a(?i)b)c
1335 </pre>
1336 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1337 By this means, options can be made to have different settings in different
1338 parts of the pattern. Any changes made in one alternative do carry on
1339 into subsequent branches within the same subpattern. For example,
1340 <pre>
1341 (a(?i)b|c)
1342 </pre>
1343 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1344 branch is abandoned before the option setting. This is because the effects of
1345 option settings happen at compile time. There would be some very weird
1346 behaviour otherwise.
1347 </P>
1348 <P>
1349 <b>Note:</b> There are other PCRE-specific options that can be set by the
1350 application when the compiling or matching functions are called. In some cases
1351 the pattern can contain special leading sequences such as (*CRLF) to override
1352 what the application has set or what has been defaulted. Details are given in
1353 the section entitled
1354 <a href="#newlineseq">"Newline sequences"</a>
1355 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1356 sequences that can be used to set UTF and Unicode property modes; they are
1357 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1358 options, respectively. The (*UTF) sequence is a generic version that can be
1359 used with any of the libraries.
1360 <a name="subpattern"></a></P>
1361 <br><a name="SEC13" href="#TOC1">SUBPATTERNS</a><br>
1362 <P>
1363 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1364 Turning part of a pattern into a subpattern does two things:
1365 <br>
1366 <br>
1367 1. It localizes a set of alternatives. For example, the pattern
1368 <pre>
1369 cat(aract|erpillar|)
1370 </pre>
1371 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1372 match "cataract", "erpillar" or an empty string.
1373 <br>
1374 <br>
1375 2. It sets up the subpattern as a capturing subpattern. This means that, when
1376 the whole pattern matches, that portion of the subject string that matched the
1377 subpattern is passed back to the caller via the <i>ovector</i> argument of the
1378 matching function. (This applies only to the traditional matching functions;
1379 the DFA matching functions do not support capturing.)
1380 </P>
1381 <P>
1382 Opening parentheses are counted from left to right (starting from 1) to obtain
1383 numbers for the capturing subpatterns. For example, if the string "the red
1384 king" is matched against the pattern
1385 <pre>
1386 the ((red|white) (king|queen))
1387 </pre>
1388 the captured substrings are "red king", "red", and "king", and are numbered 1,
1389 2, and 3, respectively.
1390 </P>
1391 <P>
1392 The fact that plain parentheses fulfil two functions is not always helpful.
1393 There are often times when a grouping subpattern is required without a
1394 capturing requirement. If an opening parenthesis is followed by a question mark
1395 and a colon, the subpattern does not do any capturing, and is not counted when
1396 computing the number of any subsequent capturing subpatterns. For example, if
1397 the string "the white queen" is matched against the pattern
1398 <pre>
1399 the ((?:red|white) (king|queen))
1400 </pre>
1401 the captured substrings are "white queen" and "queen", and are numbered 1 and
1402 2. The maximum number of capturing subpatterns is 65535.
1403 </P>
1404 <P>
1405 As a convenient shorthand, if any option settings are required at the start of
1406 a non-capturing subpattern, the option letters may appear between the "?" and
1407 the ":". Thus the two patterns
1408 <pre>
1409 (?i:saturday|sunday)
1410 (?:(?i)saturday|sunday)
1411 </pre>
1412 match exactly the same set of strings. Because alternative branches are tried
1413 from left to right, and options are not reset until the end of the subpattern
1414 is reached, an option setting in one branch does affect subsequent branches, so
1415 the above patterns match "SUNDAY" as well as "Saturday".
1416 <a name="dupsubpatternnumber"></a></P>
1417 <br><a name="SEC14" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1418 <P>
1419 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1420 the same numbers for its capturing parentheses. Such a subpattern starts with
1421 (?| and is itself a non-capturing subpattern. For example, consider this
1422 pattern:
1423 <pre>
1424 (?|(Sat)ur|(Sun))day
1425 </pre>
1426 Because the two alternatives are inside a (?| group, both sets of capturing
1427 parentheses are numbered one. Thus, when the pattern matches, you can look
1428 at captured substring number one, whichever alternative matched. This construct
1429 is useful when you want to capture part, but not all, of one of a number of
1430 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1431 number is reset at the start of each branch. The numbers of any capturing
1432 parentheses that follow the subpattern start after the highest number used in
1433 any branch. The following example is taken from the Perl documentation. The
1434 numbers underneath show in which buffer the captured content will be stored.
1435 <pre>
1436 # before ---------------branch-reset----------- after
1437 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1438 # 1 2 2 3 2 3 4
1439 </pre>
1440 A back reference to a numbered subpattern uses the most recent value that is
1441 set for that number by any subpattern. The following pattern matches "abcabc"
1442 or "defdef":
1443 <pre>
1444 /(?|(abc)|(def))\1/
1445 </pre>
1446 In contrast, a subroutine call to a numbered subpattern always refers to the
1447 first one in the pattern with the given number. The following pattern matches
1448 "abcabc" or "defabc":
1449 <pre>
1450 /(?|(abc)|(def))(?1)/
1451 </pre>
1452 If a
1453 <a href="#conditions">condition test</a>
1454 for a subpattern's having matched refers to a non-unique number, the test is
1455 true if any of the subpatterns of that number have matched.
1456 </P>
1457 <P>
1458 An alternative approach to using this "branch reset" feature is to use
1459 duplicate named subpatterns, as described in the next section.
1460 </P>
1461 <br><a name="SEC15" href="#TOC1">NAMED SUBPATTERNS</a><br>
1462 <P>
1463 Identifying capturing parentheses by number is simple, but it can be very hard
1464 to keep track of the numbers in complicated regular expressions. Furthermore,
1465 if an expression is modified, the numbers may change. To help with this
1466 difficulty, PCRE supports the naming of subpatterns. This feature was not
1467 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1468 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1469 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1470 have different names, but PCRE does not.
1471 </P>
1472 <P>
1473 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1474 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1475 parentheses from other parts of the pattern, such as
1476 <a href="#backreferences">back references,</a>
1477 <a href="#recursion">recursion,</a>
1478 and
1479 <a href="#conditions">conditions,</a>
1480 can be made by name as well as by number.
1481 </P>
1482 <P>
1483 Names consist of up to 32 alphanumeric characters and underscores. Named
1484 capturing parentheses are still allocated numbers as well as names, exactly as
1485 if the names were not present. The PCRE API provides function calls for
1486 extracting the name-to-number translation table from a compiled pattern. There
1487 is also a convenience function for extracting a captured substring by name.
1488 </P>
1489 <P>
1490 By default, a name must be unique within a pattern, but it is possible to relax
1491 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1492 names are also always permitted for subpatterns with the same number, set up as
1493 described in the previous section.) Duplicate names can be useful for patterns
1494 where only one instance of the named parentheses can match. Suppose you want to
1495 match the name of a weekday, either as a 3-letter abbreviation or as the full
1496 name, and in both cases you want to extract the abbreviation. This pattern
1497 (ignoring the line breaks) does the job:
1498 <pre>
1499 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1500 (?&#60;DN&#62;Tue)(?:sday)?|
1501 (?&#60;DN&#62;Wed)(?:nesday)?|
1502 (?&#60;DN&#62;Thu)(?:rsday)?|
1503 (?&#60;DN&#62;Sat)(?:urday)?
1504 </pre>
1505 There are five capturing substrings, but only one is ever set after a match.
1506 (An alternative way of solving this problem is to use a "branch reset"
1507 subpattern, as described in the previous section.)
1508 </P>
1509 <P>
1510 The convenience function for extracting the data by name returns the substring
1511 for the first (and in this example, the only) subpattern of that name that
1512 matched. This saves searching to find which numbered subpattern it was.
1513 </P>
1514 <P>
1515 If you make a back reference to a non-unique named subpattern from elsewhere in
1516 the pattern, the one that corresponds to the first occurrence of the name is
1517 used. In the absence of duplicate numbers (see the previous section) this is
1518 the one with the lowest number. If you use a named reference in a condition
1519 test (see the
1520 <a href="#conditions">section about conditions</a>
1521 below), either to check whether a subpattern has matched, or to check for
1522 recursion, all subpatterns with the same name are tested. If the condition is
1523 true for any one of them, the overall condition is true. This is the same
1524 behaviour as testing by number. For further details of the interfaces for
1525 handling named subpatterns, see the
1526 <a href="pcreapi.html"><b>pcreapi</b></a>
1527 documentation.
1528 </P>
1529 <P>
1530 <b>Warning:</b> You cannot use different names to distinguish between two
1531 subpatterns with the same number because PCRE uses only the numbers when
1532 matching. For this reason, an error is given at compile time if different names
1533 are given to subpatterns with the same number. However, you can give the same
1534 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1535 </P>
1536 <br><a name="SEC16" href="#TOC1">REPETITION</a><br>
1537 <P>
1538 Repetition is specified by quantifiers, which can follow any of the following
1539 items:
1540 <pre>
1541 a literal data character
1542 the dot metacharacter
1543 the \C escape sequence
1544 the \X escape sequence
1545 the \R escape sequence
1546 an escape such as \d or \pL that matches a single character
1547 a character class
1548 a back reference (see next section)
1549 a parenthesized subpattern (including assertions)
1550 a subroutine call to a subpattern (recursive or otherwise)
1551 </pre>
1552 The general repetition quantifier specifies a minimum and maximum number of
1553 permitted matches, by giving the two numbers in curly brackets (braces),
1554 separated by a comma. The numbers must be less than 65536, and the first must
1555 be less than or equal to the second. For example:
1556 <pre>
1557 z{2,4}
1558 </pre>
1559 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1560 character. If the second number is omitted, but the comma is present, there is
1561 no upper limit; if the second number and the comma are both omitted, the
1562 quantifier specifies an exact number of required matches. Thus
1563 <pre>
1564 [aeiou]{3,}
1565 </pre>
1566 matches at least 3 successive vowels, but may match many more, while
1567 <pre>
1568 \d{8}
1569 </pre>
1570 matches exactly 8 digits. An opening curly bracket that appears in a position
1571 where a quantifier is not allowed, or one that does not match the syntax of a
1572 quantifier, is taken as a literal character. For example, {,6} is not a
1573 quantifier, but a literal string of four characters.
1574 </P>
1575 <P>
1576 In UTF modes, quantifiers apply to characters rather than to individual data
1577 units. Thus, for example, \x{100}{2} matches two characters, each of
1578 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1579 \X{3} matches three Unicode extended grapheme clusters, each of which may be
1580 several data units long (and they may be of different lengths).
1581 </P>
1582 <P>
1583 The quantifier {0} is permitted, causing the expression to behave as if the
1584 previous item and the quantifier were not present. This may be useful for
1585 subpatterns that are referenced as
1586 <a href="#subpatternsassubroutines">subroutines</a>
1587 from elsewhere in the pattern (but see also the section entitled
1588 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1589 below). Items other than subpatterns that have a {0} quantifier are omitted
1590 from the compiled pattern.
1591 </P>
1592 <P>
1593 For convenience, the three most common quantifiers have single-character
1594 abbreviations:
1595 <pre>
1596 * is equivalent to {0,}
1597 + is equivalent to {1,}
1598 ? is equivalent to {0,1}
1599 </pre>
1600 It is possible to construct infinite loops by following a subpattern that can
1601 match no characters with a quantifier that has no upper limit, for example:
1602 <pre>
1603 (a?)*
1604 </pre>
1605 Earlier versions of Perl and PCRE used to give an error at compile time for
1606 such patterns. However, because there are cases where this can be useful, such
1607 patterns are now accepted, but if any repetition of the subpattern does in fact
1608 match no characters, the loop is forcibly broken.
1609 </P>
1610 <P>
1611 By default, the quantifiers are "greedy", that is, they match as much as
1612 possible (up to the maximum number of permitted times), without causing the
1613 rest of the pattern to fail. The classic example of where this gives problems
1614 is in trying to match comments in C programs. These appear between /* and */
1615 and within the comment, individual * and / characters may appear. An attempt to
1616 match C comments by applying the pattern
1617 <pre>
1618 /\*.*\*/
1619 </pre>
1620 to the string
1621 <pre>
1622 /* first comment */ not comment /* second comment */
1623 </pre>
1624 fails, because it matches the entire string owing to the greediness of the .*
1625 item.
1626 </P>
1627 <P>
1628 However, if a quantifier is followed by a question mark, it ceases to be
1629 greedy, and instead matches the minimum number of times possible, so the
1630 pattern
1631 <pre>
1632 /\*.*?\*/
1633 </pre>
1634 does the right thing with the C comments. The meaning of the various
1635 quantifiers is not otherwise changed, just the preferred number of matches.
1636 Do not confuse this use of question mark with its use as a quantifier in its
1637 own right. Because it has two uses, it can sometimes appear doubled, as in
1638 <pre>
1639 \d??\d
1640 </pre>
1641 which matches one digit by preference, but can match two if that is the only
1642 way the rest of the pattern matches.
1643 </P>
1644 <P>
1645 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1646 the quantifiers are not greedy by default, but individual ones can be made
1647 greedy by following them with a question mark. In other words, it inverts the
1648 default behaviour.
1649 </P>
1650 <P>
1651 When a parenthesized subpattern is quantified with a minimum repeat count that
1652 is greater than 1 or with a limited maximum, more memory is required for the
1653 compiled pattern, in proportion to the size of the minimum or maximum.
1654 </P>
1655 <P>
1656 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1657 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1658 implicitly anchored, because whatever follows will be tried against every
1659 character position in the subject string, so there is no point in retrying the
1660 overall match at any position after the first. PCRE normally treats such a
1661 pattern as though it were preceded by \A.
1662 </P>
1663 <P>
1664 In cases where it is known that the subject string contains no newlines, it is
1665 worth setting PCRE_DOTALL in order to obtain this optimization, or
1666 alternatively using ^ to indicate anchoring explicitly.
1667 </P>
1668 <P>
1669 However, there are some cases where the optimization cannot be used. When .*
1670 is inside capturing parentheses that are the subject of a back reference
1671 elsewhere in the pattern, a match at the start may fail where a later one
1672 succeeds. Consider, for example:
1673 <pre>
1674 (.*)abc\1
1675 </pre>
1676 If the subject is "xyz123abc123" the match point is the fourth character. For
1677 this reason, such a pattern is not implicitly anchored.
1678 </P>
1679 <P>
1680 Another case where implicit anchoring is not applied is when the leading .* is
1681 inside an atomic group. Once again, a match at the start may fail where a later
1682 one succeeds. Consider this pattern:
1683 <pre>
1684 (?&#62;.*?a)b
1685 </pre>
1686 It matches "ab" in the subject "aab". The use of the backtracking control verbs
1687 (*PRUNE) and (*SKIP) also disable this optimization.
1688 </P>
1689 <P>
1690 When a capturing subpattern is repeated, the value captured is the substring
1691 that matched the final iteration. For example, after
1692 <pre>
1693 (tweedle[dume]{3}\s*)+
1694 </pre>
1695 has matched "tweedledum tweedledee" the value of the captured substring is
1696 "tweedledee". However, if there are nested capturing subpatterns, the
1697 corresponding captured values may have been set in previous iterations. For
1698 example, after
1699 <pre>
1700 /(a|(b))+/
1701 </pre>
1702 matches "aba" the value of the second captured substring is "b".
1703 <a name="atomicgroup"></a></P>
1704 <br><a name="SEC17" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1705 <P>
1706 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1707 repetition, failure of what follows normally causes the repeated item to be
1708 re-evaluated to see if a different number of repeats allows the rest of the
1709 pattern to match. Sometimes it is useful to prevent this, either to change the
1710 nature of the match, or to cause it fail earlier than it otherwise might, when
1711 the author of the pattern knows there is no point in carrying on.
1712 </P>
1713 <P>
1714 Consider, for example, the pattern \d+foo when applied to the subject line
1715 <pre>
1716 123456bar
1717 </pre>
1718 After matching all 6 digits and then failing to match "foo", the normal
1719 action of the matcher is to try again with only 5 digits matching the \d+
1720 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1721 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1722 that once a subpattern has matched, it is not to be re-evaluated in this way.
1723 </P>
1724 <P>
1725 If we use atomic grouping for the previous example, the matcher gives up
1726 immediately on failing to match "foo" the first time. The notation is a kind of
1727 special parenthesis, starting with (?&#62; as in this example:
1728 <pre>
1729 (?&#62;\d+)foo
1730 </pre>
1731 This kind of parenthesis "locks up" the part of the pattern it contains once
1732 it has matched, and a failure further into the pattern is prevented from
1733 backtracking into it. Backtracking past it to previous items, however, works as
1734 normal.
1735 </P>
1736 <P>
1737 An alternative description is that a subpattern of this type matches the string
1738 of characters that an identical standalone pattern would match, if anchored at
1739 the current point in the subject string.
1740 </P>
1741 <P>
1742 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1743 the above example can be thought of as a maximizing repeat that must swallow
1744 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1745 number of digits they match in order to make the rest of the pattern match,
1746 (?&#62;\d+) can only match an entire sequence of digits.
1747 </P>
1748 <P>
1749 Atomic groups in general can of course contain arbitrarily complicated
1750 subpatterns, and can be nested. However, when the subpattern for an atomic
1751 group is just a single repeated item, as in the example above, a simpler
1752 notation, called a "possessive quantifier" can be used. This consists of an
1753 additional + character following a quantifier. Using this notation, the
1754 previous example can be rewritten as
1755 <pre>
1756 \d++foo
1757 </pre>
1758 Note that a possessive quantifier can be used with an entire group, for
1759 example:
1760 <pre>
1761 (abc|xyz){2,3}+
1762 </pre>
1763 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1764 option is ignored. They are a convenient notation for the simpler forms of
1765 atomic group. However, there is no difference in the meaning of a possessive
1766 quantifier and the equivalent atomic group, though there may be a performance
1767 difference; possessive quantifiers should be slightly faster.
1768 </P>
1769 <P>
1770 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1771 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1772 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1773 package, and PCRE copied it from there. It ultimately found its way into Perl
1774 at release 5.10.
1775 </P>
1776 <P>
1777 PCRE has an optimization that automatically "possessifies" certain simple
1778 pattern constructs. For example, the sequence A+B is treated as A++B because
1779 there is no point in backtracking into a sequence of A's when B must follow.
1780 </P>
1781 <P>
1782 When a pattern contains an unlimited repeat inside a subpattern that can itself
1783 be repeated an unlimited number of times, the use of an atomic group is the
1784 only way to avoid some failing matches taking a very long time indeed. The
1785 pattern
1786 <pre>
1787 (\D+|&#60;\d+&#62;)*[!?]
1788 </pre>
1789 matches an unlimited number of substrings that either consist of non-digits, or
1790 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1791 quickly. However, if it is applied to
1792 <pre>
1793 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1794 </pre>
1795 it takes a long time before reporting failure. This is because the string can
1796 be divided between the internal \D+ repeat and the external * repeat in a
1797 large number of ways, and all have to be tried. (The example uses [!?] rather
1798 than a single character at the end, because both PCRE and Perl have an
1799 optimization that allows for fast failure when a single character is used. They
1800 remember the last single character that is required for a match, and fail early
1801 if it is not present in the string.) If the pattern is changed so that it uses
1802 an atomic group, like this:
1803 <pre>
1804 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1805 </pre>
1806 sequences of non-digits cannot be broken, and failure happens quickly.
1807 <a name="backreferences"></a></P>
1808 <br><a name="SEC18" href="#TOC1">BACK REFERENCES</a><br>
1809 <P>
1810 Outside a character class, a backslash followed by a digit greater than 0 (and
1811 possibly further digits) is a back reference to a capturing subpattern earlier
1812 (that is, to its left) in the pattern, provided there have been that many
1813 previous capturing left parentheses.
1814 </P>
1815 <P>
1816 However, if the decimal number following the backslash is less than 10, it is
1817 always taken as a back reference, and causes an error only if there are not
1818 that many capturing left parentheses in the entire pattern. In other words, the
1819 parentheses that are referenced need not be to the left of the reference for
1820 numbers less than 10. A "forward back reference" of this type can make sense
1821 when a repetition is involved and the subpattern to the right has participated
1822 in an earlier iteration.
1823 </P>
1824 <P>
1825 It is not possible to have a numerical "forward back reference" to a subpattern
1826 whose number is 10 or more using this syntax because a sequence such as \50 is
1827 interpreted as a character defined in octal. See the subsection entitled
1828 "Non-printing characters"
1829 <a href="#digitsafterbackslash">above</a>
1830 for further details of the handling of digits following a backslash. There is
1831 no such problem when named parentheses are used. A back reference to any
1832 subpattern is possible using named parentheses (see below).
1833 </P>
1834 <P>
1835 Another way of avoiding the ambiguity inherent in the use of digits following a
1836 backslash is to use the \g escape sequence. This escape must be followed by an
1837 unsigned number or a negative number, optionally enclosed in braces. These
1838 examples are all identical:
1839 <pre>
1840 (ring), \1
1841 (ring), \g1
1842 (ring), \g{1}
1843 </pre>
1844 An unsigned number specifies an absolute reference without the ambiguity that
1845 is present in the older syntax. It is also useful when literal digits follow
1846 the reference. A negative number is a relative reference. Consider this
1847 example:
1848 <pre>
1849 (abc(def)ghi)\g{-1}
1850 </pre>
1851 The sequence \g{-1} is a reference to the most recently started capturing
1852 subpattern before \g, that is, is it equivalent to \2 in this example.
1853 Similarly, \g{-2} would be equivalent to \1. The use of relative references
1854 can be helpful in long patterns, and also in patterns that are created by
1855 joining together fragments that contain references within themselves.
1856 </P>
1857 <P>
1858 A back reference matches whatever actually matched the capturing subpattern in
1859 the current subject string, rather than anything matching the subpattern
1860 itself (see
1861 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1862 below for a way of doing that). So the pattern
1863 <pre>
1864 (sens|respons)e and \1ibility
1865 </pre>
1866 matches "sense and sensibility" and "response and responsibility", but not
1867 "sense and responsibility". If caseful matching is in force at the time of the
1868 back reference, the case of letters is relevant. For example,
1869 <pre>
1870 ((?i)rah)\s+\1
1871 </pre>
1872 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1873 capturing subpattern is matched caselessly.
1874 </P>
1875 <P>
1876 There are several different ways of writing back references to named
1877 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1878 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1879 back reference syntax, in which \g can be used for both numeric and named
1880 references, is also supported. We could rewrite the above example in any of
1881 the following ways:
1882 <pre>
1883 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1884 (?'p1'(?i)rah)\s+\k{p1}
1885 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1886 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1887 </pre>
1888 A subpattern that is referenced by name may appear in the pattern before or
1889 after the reference.
1890 </P>
1891 <P>
1892 There may be more than one back reference to the same subpattern. If a
1893 subpattern has not actually been used in a particular match, any back
1894 references to it always fail by default. For example, the pattern
1895 <pre>
1896 (a|(bc))\2
1897 </pre>
1898 always fails if it starts to match "a" rather than "bc". However, if the
1899 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1900 unset value matches an empty string.
1901 </P>
1902 <P>
1903 Because there may be many capturing parentheses in a pattern, all digits
1904 following a backslash are taken as part of a potential back reference number.
1905 If the pattern continues with a digit character, some delimiter must be used to
1906 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1907 white space. Otherwise, the \g{ syntax or an empty comment (see
1908 <a href="#comments">"Comments"</a>
1909 below) can be used.
1910 </P>
1911 <br><b>
1912 Recursive back references
1913 </b><br>
1914 <P>
1915 A back reference that occurs inside the parentheses to which it refers fails
1916 when the subpattern is first used, so, for example, (a\1) never matches.
1917 However, such references can be useful inside repeated subpatterns. For
1918 example, the pattern
1919 <pre>
1920 (a|b\1)+
1921 </pre>
1922 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1923 the subpattern, the back reference matches the character string corresponding
1924 to the previous iteration. In order for this to work, the pattern must be such
1925 that the first iteration does not need to match the back reference. This can be
1926 done using alternation, as in the example above, or by a quantifier with a
1927 minimum of zero.
1928 </P>
1929 <P>
1930 Back references of this type cause the group that they reference to be treated
1931 as an
1932 <a href="#atomicgroup">atomic group.</a>
1933 Once the whole group has been matched, a subsequent matching failure cannot
1934 cause backtracking into the middle of the group.
1935 <a name="bigassertions"></a></P>
1936 <br><a name="SEC19" href="#TOC1">ASSERTIONS</a><br>
1937 <P>
1938 An assertion is a test on the characters following or preceding the current
1939 matching point that does not actually consume any characters. The simple
1940 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1941 <a href="#smallassertions">above.</a>
1942 </P>
1943 <P>
1944 More complicated assertions are coded as subpatterns. There are two kinds:
1945 those that look ahead of the current position in the subject string, and those
1946 that look behind it. An assertion subpattern is matched in the normal way,
1947 except that it does not cause the current matching position to be changed.
1948 </P>
1949 <P>
1950 Assertion subpatterns are not capturing subpatterns. If such an assertion
1951 contains capturing subpatterns within it, these are counted for the purposes of
1952 numbering the capturing subpatterns in the whole pattern. However, substring
1953 capturing is carried out only for positive assertions, because it does not make
1954 sense for negative assertions.
1955 </P>
1956 <P>
1957 For compatibility with Perl, assertion subpatterns may be repeated; though
1958 it makes no sense to assert the same thing several times, the side effect of
1959 capturing parentheses may occasionally be useful. In practice, there only three
1960 cases:
1961 <br>
1962 <br>
1963 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1964 However, it may contain internal capturing parenthesized groups that are called
1965 from elsewhere via the
1966 <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1967 <br>
1968 <br>
1969 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1970 were {0,1}. At run time, the rest of the pattern match is tried with and
1971 without the assertion, the order depending on the greediness of the quantifier.
1972 <br>
1973 <br>
1974 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1975 The assertion is obeyed just once when encountered during matching.
1976 </P>
1977 <br><b>
1978 Lookahead assertions
1979 </b><br>
1980 <P>
1981 Lookahead assertions start with (?= for positive assertions and (?! for
1982 negative assertions. For example,
1983 <pre>
1984 \w+(?=;)
1985 </pre>
1986 matches a word followed by a semicolon, but does not include the semicolon in
1987 the match, and
1988 <pre>
1989 foo(?!bar)
1990 </pre>
1991 matches any occurrence of "foo" that is not followed by "bar". Note that the
1992 apparently similar pattern
1993 <pre>
1994 (?!foo)bar
1995 </pre>
1996 does not find an occurrence of "bar" that is preceded by something other than
1997 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1998 (?!foo) is always true when the next three characters are "bar". A
1999 lookbehind assertion is needed to achieve the other effect.
2000 </P>
2001 <P>
2002 If you want to force a matching failure at some point in a pattern, the most
2003 convenient way to do it is with (?!) because an empty string always matches, so
2004 an assertion that requires there not to be an empty string must always fail.
2005 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2006 <a name="lookbehind"></a></P>
2007 <br><b>
2008 Lookbehind assertions
2009 </b><br>
2010 <P>
2011 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
2012 negative assertions. For example,
2013 <pre>
2014 (?&#60;!foo)bar
2015 </pre>
2016 does find an occurrence of "bar" that is not preceded by "foo". The contents of
2017 a lookbehind assertion are restricted such that all the strings it matches must
2018 have a fixed length. However, if there are several top-level alternatives, they
2019 do not all have to have the same fixed length. Thus
2020 <pre>
2021 (?&#60;=bullock|donkey)
2022 </pre>
2023 is permitted, but
2024 <pre>
2025 (?&#60;!dogs?|cats?)
2026 </pre>
2027 causes an error at compile time. Branches that match different length strings
2028 are permitted only at the top level of a lookbehind assertion. This is an
2029 extension compared with Perl, which requires all branches to match the same
2030 length of string. An assertion such as
2031 <pre>
2032 (?&#60;=ab(c|de))
2033 </pre>
2034 is not permitted, because its single top-level branch can match two different
2035 lengths, but it is acceptable to PCRE if rewritten to use two top-level
2036 branches:
2037 <pre>
2038 (?&#60;=abc|abde)
2039 </pre>
2040 In some cases, the escape sequence \K
2041 <a href="#resetmatchstart">(see above)</a>
2042 can be used instead of a lookbehind assertion to get round the fixed-length
2043 restriction.
2044 </P>
2045 <P>
2046 The implementation of lookbehind assertions is, for each alternative, to
2047 temporarily move the current position back by the fixed length and then try to
2048 match. If there are insufficient characters before the current position, the
2049 assertion fails.
2050 </P>
2051 <P>
2052 In a UTF mode, PCRE does not allow the \C escape (which matches a single data
2053 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2054 it impossible to calculate the length of the lookbehind. The \X and \R
2055 escapes, which can match different numbers of data units, are also not
2056 permitted.
2057 </P>
2058 <P>
2059 <a href="#subpatternsassubroutines">"Subroutine"</a>
2060 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2061 as the subpattern matches a fixed-length string.
2062 <a href="#recursion">Recursion,</a>
2063 however, is not supported.
2064 </P>
2065 <P>
2066 Possessive quantifiers can be used in conjunction with lookbehind assertions to
2067 specify efficient matching of fixed-length strings at the end of subject
2068 strings. Consider a simple pattern such as
2069 <pre>
2070 abcd$
2071 </pre>
2072 when applied to a long string that does not match. Because matching proceeds
2073 from left to right, PCRE will look for each "a" in the subject and then see if
2074 what follows matches the rest of the pattern. If the pattern is specified as
2075 <pre>
2076 ^.*abcd$
2077 </pre>
2078 the initial .* matches the entire string at first, but when this fails (because
2079 there is no following "a"), it backtracks to match all but the last character,
2080 then all but the last two characters, and so on. Once again the search for "a"
2081 covers the entire string, from right to left, so we are no better off. However,
2082 if the pattern is written as
2083 <pre>
2084 ^.*+(?&#60;=abcd)
2085 </pre>
2086 there can be no backtracking for the .*+ item; it can match only the entire
2087 string. The subsequent lookbehind assertion does a single test on the last four
2088 characters. If it fails, the match fails immediately. For long strings, this
2089 approach makes a significant difference to the processing time.
2090 </P>
2091 <br><b>
2092 Using multiple assertions
2093 </b><br>
2094 <P>
2095 Several assertions (of any sort) may occur in succession. For example,
2096 <pre>
2097 (?&#60;=\d{3})(?&#60;!999)foo
2098 </pre>
2099 matches "foo" preceded by three digits that are not "999". Notice that each of
2100 the assertions is applied independently at the same point in the subject
2101 string. First there is a check that the previous three characters are all
2102 digits, and then there is a check that the same three characters are not "999".
2103 This pattern does <i>not</i> match "foo" preceded by six characters, the first
2104 of which are digits and the last three of which are not "999". For example, it
2105 doesn't match "123abcfoo". A pattern to do that is
2106 <pre>
2107 (?&#60;=\d{3}...)(?&#60;!999)foo
2108 </pre>
2109 This time the first assertion looks at the preceding six characters, checking
2110 that the first three are digits, and then the second assertion checks that the
2111 preceding three characters are not "999".
2112 </P>
2113 <P>
2114 Assertions can be nested in any combination. For example,
2115 <pre>
2116 (?&#60;=(?&#60;!foo)bar)baz
2117 </pre>
2118 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2119 preceded by "foo", while
2120 <pre>
2121 (?&#60;=\d{3}(?!999)...)foo
2122 </pre>
2123 is another pattern that matches "foo" preceded by three digits and any three
2124 characters that are not "999".
2125 <a name="conditions"></a></P>
2126 <br><a name="SEC20" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2127 <P>
2128 It is possible to cause the matching process to obey a subpattern
2129 conditionally or to choose between two alternative subpatterns, depending on
2130 the result of an assertion, or whether a specific capturing subpattern has
2131 already been matched. The two possible forms of conditional subpattern are:
2132 <pre>
2133 (?(condition)yes-pattern)
2134 (?(condition)yes-pattern|no-pattern)
2135 </pre>
2136 If the condition is satisfied, the yes-pattern is used; otherwise the
2137 no-pattern (if present) is used. If there are more than two alternatives in the
2138 subpattern, a compile-time error occurs. Each of the two alternatives may
2139 itself contain nested subpatterns of any form, including conditional
2140 subpatterns; the restriction to two alternatives applies only at the level of
2141 the condition. This pattern fragment is an example where the alternatives are
2142 complex:
2143 <pre>
2144 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2146 </PRE>
2147 </P>
2148 <P>
2149 There are four kinds of condition: references to subpatterns, references to
2150 recursion, a pseudo-condition called DEFINE, and assertions.
2151 </P>
2152 <br><b>
2153 Checking for a used subpattern by number
2154 </b><br>
2155 <P>
2156 If the text between the parentheses consists of a sequence of digits, the
2157 condition is true if a capturing subpattern of that number has previously
2158 matched. If there is more than one capturing subpattern with the same number
2159 (see the earlier
2160 <a href="#recursion">section about duplicate subpattern numbers),</a>
2161 the condition is true if any of them have matched. An alternative notation is
2162 to precede the digits with a plus or minus sign. In this case, the subpattern
2163 number is relative rather than absolute. The most recently opened parentheses
2164 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2165 loops it can also make sense to refer to subsequent groups. The next
2166 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2167 zero in any of these forms is not used; it provokes a compile-time error.)
2168 </P>
2169 <P>
2170 Consider the following pattern, which contains non-significant white space to
2171 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2172 three parts for ease of discussion:
2173 <pre>
2174 ( \( )? [^()]+ (?(1) \) )
2175 </pre>
2176 The first part matches an optional opening parenthesis, and if that
2177 character is present, sets it as the first captured substring. The second part
2178 matches one or more characters that are not parentheses. The third part is a
2179 conditional subpattern that tests whether or not the first set of parentheses
2180 matched. If they did, that is, if subject started with an opening parenthesis,
2181 the condition is true, and so the yes-pattern is executed and a closing
2182 parenthesis is required. Otherwise, since no-pattern is not present, the
2183 subpattern matches nothing. In other words, this pattern matches a sequence of
2184 non-parentheses, optionally enclosed in parentheses.
2185 </P>
2186 <P>
2187 If you were embedding this pattern in a larger one, you could use a relative
2188 reference:
2189 <pre>
2190 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2191 </pre>
2192 This makes the fragment independent of the parentheses in the larger pattern.
2193 </P>
2194 <br><b>
2195 Checking for a used subpattern by name
2196 </b><br>
2197 <P>
2198 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2199 subpattern by name. For compatibility with earlier versions of PCRE, which had
2200 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2201 there is a possible ambiguity with this syntax, because subpattern names may
2202 consist entirely of digits. PCRE looks first for a named subpattern; if it
2203 cannot find one and the name consists entirely of digits, PCRE looks for a
2204 subpattern of that number, which must be greater than zero. Using subpattern
2205 names that consist entirely of digits is not recommended.
2206 </P>
2207 <P>
2208 Rewriting the above example to use a named subpattern gives this:
2209 <pre>
2210 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
2211 </pre>
2212 If the name used in a condition of this kind is a duplicate, the test is
2213 applied to all subpatterns of the same name, and is true if any one of them has
2214 matched.
2215 </P>
2216 <br><b>
2217 Checking for pattern recursion
2218 </b><br>
2219 <P>
2220 If the condition is the string (R), and there is no subpattern with the name R,
2221 the condition is true if a recursive call to the whole pattern or any
2222 subpattern has been made. If digits or a name preceded by ampersand follow the
2223 letter R, for example:
2224 <pre>
2225 (?(R3)...) or (?(R&name)...)
2226 </pre>
2227 the condition is true if the most recent recursion is into a subpattern whose
2228 number or name is given. This condition does not check the entire recursion
2229 stack. If the name used in a condition of this kind is a duplicate, the test is
2230 applied to all subpatterns of the same name, and is true if any one of them is
2231 the most recent recursion.
2232 </P>
2233 <P>
2234 At "top level", all these recursion test conditions are false.
2235 <a href="#recursion">The syntax for recursive patterns</a>
2236 is described below.
2237 <a name="subdefine"></a></P>
2238 <br><b>
2239 Defining subpatterns for use by reference only
2240 </b><br>
2241 <P>
2242 If the condition is the string (DEFINE), and there is no subpattern with the
2243 name DEFINE, the condition is always false. In this case, there may be only one
2244 alternative in the subpattern. It is always skipped if control reaches this
2245 point in the pattern; the idea of DEFINE is that it can be used to define
2246 subroutines that can be referenced from elsewhere. (The use of
2247 <a href="#subpatternsassubroutines">subroutines</a>
2248 is described below.) For example, a pattern to match an IPv4 address such as
2249 "" could be written like this (ignore white space and line
2250 breaks):
2251 <pre>
2252 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2253 \b (?&byte) (\.(?&byte)){3} \b
2254 </pre>
2255 The first part of the pattern is a DEFINE group inside which a another group
2256 named "byte" is defined. This matches an individual component of an IPv4
2257 address (a number less than 256). When matching takes place, this part of the
2258 pattern is skipped because DEFINE acts like a false condition. The rest of the
2259 pattern uses references to the named group to match the four dot-separated
2260 components of an IPv4 address, insisting on a word boundary at each end.
2261 </P>
2262 <br><b>
2263 Assertion conditions
2264 </b><br>
2265 <P>
2266 If the condition is not in any of the above formats, it must be an assertion.
2267 This may be a positive or negative lookahead or lookbehind assertion. Consider
2268 this pattern, again containing non-significant white space, and with the two
2269 alternatives on the second line:
2270 <pre>
2271 (?(?=[^a-z]*[a-z])
2272 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2273 </pre>
2274 The condition is a positive lookahead assertion that matches an optional
2275 sequence of non-letters followed by a letter. In other words, it tests for the
2276 presence of at least one letter in the subject. If a letter is found, the
2277 subject is matched against the first alternative; otherwise it is matched
2278 against the second. This pattern matches strings in one of the two forms
2279 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2280 <a name="comments"></a></P>
2281 <br><a name="SEC21" href="#TOC1">COMMENTS</a><br>
2282 <P>
2283 There are two ways of including comments in patterns that are processed by
2284 PCRE. In both cases, the start of the comment must not be in a character class,
2285 nor in the middle of any other sequence of related characters such as (?: or a
2286 subpattern name or number. The characters that make up a comment play no part
2287 in the pattern matching.
2288 </P>
2289 <P>
2290 The sequence (?# marks the start of a comment that continues up to the next
2291 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2292 option is set, an unescaped # character also introduces a comment, which in
2293 this case continues to immediately after the next newline character or
2294 character sequence in the pattern. Which characters are interpreted as newlines
2295 is controlled by the options passed to a compiling function or by a special
2296 sequence at the start of the pattern, as described in the section entitled
2297 <a href="#newlines">"Newline conventions"</a>
2298 above. Note that the end of this type of comment is a literal newline sequence
2299 in the pattern; escape sequences that happen to represent a newline do not
2300 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2301 default newline convention is in force:
2302 <pre>
2303 abc #comment \n still comment
2304 </pre>
2305 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2306 a newline in the pattern. The sequence \n is still literal at this stage, so
2307 it does not terminate the comment. Only an actual character with the code value
2308 0x0a (the default newline) does so.
2309 <a name="recursion"></a></P>
2310 <br><a name="SEC22" href="#TOC1">RECURSIVE PATTERNS</a><br>
2311 <P>
2312 Consider the problem of matching a string in parentheses, allowing for
2313 unlimited nested parentheses. Without the use of recursion, the best that can
2314 be done is to use a pattern that matches up to some fixed depth of nesting. It
2315 is not possible to handle an arbitrary nesting depth.
2316 </P>
2317 <P>
2318 For some time, Perl has provided a facility that allows regular expressions to
2319 recurse (amongst other things). It does this by interpolating Perl code in the
2320 expression at run time, and the code can refer to the expression itself. A Perl
2321 pattern using code interpolation to solve the parentheses problem can be
2322 created like this:
2323 <pre>
2324 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2325 </pre>
2326 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2327 recursively to the pattern in which it appears.
2328 </P>
2329 <P>
2330 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2331 supports special syntax for recursion of the entire pattern, and also for
2332 individual subpattern recursion. After its introduction in PCRE and Python,
2333 this kind of recursion was subsequently introduced into Perl at release 5.10.
2334 </P>
2335 <P>
2336 A special item that consists of (? followed by a number greater than zero and a
2337 closing parenthesis is a recursive subroutine call of the subpattern of the
2338 given number, provided that it occurs inside that subpattern. (If not, it is a
2339 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2340 call, which is described in the next section.) The special item (?R) or (?0) is
2341 a recursive call of the entire regular expression.
2342 </P>
2343 <P>
2344 This PCRE pattern solves the nested parentheses problem (assume the
2345 PCRE_EXTENDED option is set so that white space is ignored):
2346 <pre>
2347 \( ( [^()]++ | (?R) )* \)
2348 </pre>
2349 First it matches an opening parenthesis. Then it matches any number of
2350 substrings which can either be a sequence of non-parentheses, or a recursive
2351 match of the pattern itself (that is, a correctly parenthesized substring).
2352 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2353 to avoid backtracking into sequences of non-parentheses.
2354 </P>
2355 <P>
2356 If this were part of a larger pattern, you would not want to recurse the entire
2357 pattern, so instead you could use this:
2358 <pre>
2359 ( \( ( [^()]++ | (?1) )* \) )
2360 </pre>
2361 We have put the pattern into parentheses, and caused the recursion to refer to
2362 them instead of the whole pattern.
2363 </P>
2364 <P>
2365 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2366 is made easier by the use of relative references. Instead of (?1) in the
2367 pattern above you can write (?-2) to refer to the second most recently opened
2368 parentheses preceding the recursion. In other words, a negative number counts
2369 capturing parentheses leftwards from the point at which it is encountered.
2370 </P>
2371 <P>
2372 It is also possible to refer to subsequently opened parentheses, by writing
2373 references such as (?+2). However, these cannot be recursive because the
2374 reference is not inside the parentheses that are referenced. They are always
2375 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2376 calls, as described in the next section.
2377 </P>
2378 <P>
2379 An alternative approach is to use named parentheses instead. The Perl syntax
2380 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2381 could rewrite the above example as follows:
2382 <pre>
2383 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2384 </pre>
2385 If there is more than one subpattern with the same name, the earliest one is
2386 used.
2387 </P>
2388 <P>
2389 This particular example pattern that we have been looking at contains nested
2390 unlimited repeats, and so the use of a possessive quantifier for matching
2391 strings of non-parentheses is important when applying the pattern to strings
2392 that do not match. For example, when this pattern is applied to
2393 <pre>
2394 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2395 </pre>
2396 it yields "no match" quickly. However, if a possessive quantifier is not used,
2397 the match runs for a very long time indeed because there are so many different
2398 ways the + and * repeats can carve up the subject, and all have to be tested
2399 before failure can be reported.
2400 </P>
2401 <P>
2402 At the end of a match, the values of capturing parentheses are those from
2403 the outermost level. If you want to obtain intermediate values, a callout
2404 function can be used (see below and the
2405 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2406 documentation). If the pattern above is matched against
2407 <pre>
2408 (ab(cd)ef)
2409 </pre>
2410 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2411 the last value taken on at the top level. If a capturing subpattern is not
2412 matched at the top level, its final captured value is unset, even if it was
2413 (temporarily) set at a deeper level during the matching process.
2414 </P>
2415 <P>
2416 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2417 obtain extra memory to store data during a recursion, which it does by using
2418 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2419 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2420 </P>
2421 <P>
2422 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2423 Consider this pattern, which matches text in angle brackets, allowing for
2424 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2425 recursing), whereas any characters are permitted at the outer level.
2426 <pre>
2427 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2428 </pre>
2429 In this pattern, (?(R) is the start of a conditional subpattern, with two
2430 different alternatives for the recursive and non-recursive cases. The (?R) item
2431 is the actual recursive call.
2432 <a name="recursiondifference"></a></P>
2433 <br><b>
2434 Differences in recursion processing between PCRE and Perl
2435 </b><br>
2436 <P>
2437 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2438 (like Python, but unlike Perl), a recursive subpattern call is always treated
2439 as an atomic group. That is, once it has matched some of the subject string, it
2440 is never re-entered, even if it contains untried alternatives and there is a
2441 subsequent matching failure. This can be illustrated by the following pattern,
2442 which purports to match a palindromic string that contains an odd number of
2443 characters (for example, "a", "aba", "abcba", "abcdcba"):
2444 <pre>
2445 ^(.|(.)(?1)\2)$
2446 </pre>
2447 The idea is that it either matches a single character, or two identical
2448 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2449 it does not if the pattern is longer than three characters. Consider the
2450 subject string "abcba":
2451 </P>
2452 <P>
2453 At the top level, the first character is matched, but as it is not at the end
2454 of the string, the first alternative fails; the second alternative is taken
2455 and the recursion kicks in. The recursive call to subpattern 1 successfully
2456 matches the next character ("b"). (Note that the beginning and end of line
2457 tests are not part of the recursion).
2458 </P>
2459 <P>
2460 Back at the top level, the next character ("c") is compared with what
2461 subpattern 2 matched, which was "a". This fails. Because the recursion is
2462 treated as an atomic group, there are now no backtracking points, and so the
2463 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2464 try the second alternative.) However, if the pattern is written with the
2465 alternatives in the other order, things are different:
2466 <pre>
2467 ^((.)(?1)\2|.)$
2468 </pre>
2469 This time, the recursing alternative is tried first, and continues to recurse
2470 until it runs out of characters, at which point the recursion fails. But this
2471 time we do have another alternative to try at the higher level. That is the big
2472 difference: in the previous case the remaining alternative is at a deeper
2473 recursion level, which PCRE cannot use.
2474 </P>
2475 <P>
2476 To change the pattern so that it matches all palindromic strings, not just
2477 those with an odd number of characters, it is tempting to change the pattern to
2478 this:
2479 <pre>
2480 ^((.)(?1)\2|.?)$
2481 </pre>
2482 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2483 deeper recursion has matched a single character, it cannot be entered again in
2484 order to match an empty string. The solution is to separate the two cases, and
2485 write out the odd and even cases as alternatives at the higher level:
2486 <pre>
2487 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2488 </pre>
2489 If you want to match typical palindromic phrases, the pattern has to ignore all
2490 non-word characters, which can be done like this:
2491 <pre>
2492 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2493 </pre>
2494 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2495 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2496 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2497 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2498 more) to match typical phrases, and Perl takes so long that you think it has
2499 gone into a loop.
2500 </P>
2501 <P>
2502 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2503 string does not start with a palindrome that is shorter than the entire string.
2504 For example, although "abcba" is correctly matched, if the subject is "ababa",
2505 PCRE finds the palindrome "aba" at the start, then fails at top level because
2506 the end of the string does not follow. Once again, it cannot jump back into the
2507 recursion to try other alternatives, so the entire match fails.
2508 </P>
2509 <P>
2510 The second way in which PCRE and Perl differ in their recursion processing is
2511 in the handling of captured values. In Perl, when a subpattern is called
2512 recursively or as a subpattern (see the next section), it has no access to any
2513 values that were captured outside the recursion, whereas in PCRE these values
2514 can be referenced. Consider this pattern:
2515 <pre>
2516 ^(.)(\1|a(?2))
2517 </pre>
2518 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2519 then in the second group, when the back reference \1 fails to match "b", the
2520 second alternative matches "a" and then recurses. In the recursion, \1 does
2521 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2522 match because inside the recursive call \1 cannot access the externally set
2523 value.
2524 <a name="subpatternsassubroutines"></a></P>
2525 <br><a name="SEC23" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2526 <P>
2527 If the syntax for a recursive subpattern call (either by number or by
2528 name) is used outside the parentheses to which it refers, it operates like a
2529 subroutine in a programming language. The called subpattern may be defined
2530 before or after the reference. A numbered reference can be absolute or
2531 relative, as in these examples:
2532 <pre>
2533 (...(absolute)...)...(?2)...
2534 (...(relative)...)...(?-1)...
2535 (...(?+1)...(relative)...
2536 </pre>
2537 An earlier example pointed out that the pattern
2538 <pre>
2539 (sens|respons)e and \1ibility
2540 </pre>
2541 matches "sense and sensibility" and "response and responsibility", but not
2542 "sense and responsibility". If instead the pattern
2543 <pre>
2544 (sens|respons)e and (?1)ibility
2545 </pre>
2546 is used, it does match "sense and responsibility" as well as the other two
2547 strings. Another example is given in the discussion of DEFINE above.
2548 </P>
2549 <P>
2550 All subroutine calls, whether recursive or not, are always treated as atomic
2551 groups. That is, once a subroutine has matched some of the subject string, it
2552 is never re-entered, even if it contains untried alternatives and there is a
2553 subsequent matching failure. Any capturing parentheses that are set during the
2554 subroutine call revert to their previous values afterwards.
2555 </P>
2556 <P>
2557 Processing options such as case-independence are fixed when a subpattern is
2558 defined, so if it is used as a subroutine, such options cannot be changed for
2559 different calls. For example, consider this pattern:
2560 <pre>
2561 (abc)(?i:(?-1))
2562 </pre>
2563 It matches "abcabc". It does not match "abcABC" because the change of
2564 processing option does not affect the called subpattern.
2565 <a name="onigurumasubroutines"></a></P>
2566 <br><a name="SEC24" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2567 <P>
2568 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2569 a number enclosed either in angle brackets or single quotes, is an alternative
2570 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2571 are two of the examples used above, rewritten using this syntax:
2572 <pre>
2573 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2574 (sens|respons)e and \g'1'ibility
2575 </pre>
2576 PCRE supports an extension to Oniguruma: if a number is preceded by a
2577 plus or a minus sign it is taken as a relative reference. For example:
2578 <pre>
2579 (abc)(?i:\g&#60;-1&#62;)
2580 </pre>
2581 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2582 synonymous. The former is a back reference; the latter is a subroutine call.
2583 </P>
2584 <br><a name="SEC25" href="#TOC1">CALLOUTS</a><br>
2585 <P>
2586 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2587 code to be obeyed in the middle of matching a regular expression. This makes it
2588 possible, amongst other things, to extract different substrings that match the
2589 same pair of parentheses when there is a repetition.
2590 </P>
2591 <P>
2592 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2593 code. The feature is called "callout". The caller of PCRE provides an external
2594 function by putting its entry point in the global variable <i>pcre_callout</i>
2595 (8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library).
2596 By default, this variable contains NULL, which disables all calling out.
2597 </P>
2598 <P>
2599 Within a regular expression, (?C) indicates the points at which the external
2600 function is to be called. If you want to identify different callout points, you
2601 can put a number less than 256 after the letter C. The default value is zero.
2602 For example, this pattern has two callout points:
2603 <pre>
2604 (?C1)abc(?C2)def
2605 </pre>
2606 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2607 automatically installed before each item in the pattern. They are all numbered
2608 255.
2609 </P>
2610 <P>
2611 During matching, when PCRE reaches a callout point, the external function is
2612 called. It is provided with the number of the callout, the position in the
2613 pattern, and, optionally, one item of data originally supplied by the caller of
2614 the matching function. The callout function may cause matching to proceed, to
2615 backtrack, or to fail altogether. A complete description of the interface to
2616 the callout function is given in the
2617 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2618 documentation.
2619 <a name="backtrackcontrol"></a></P>
2620 <br><a name="SEC26" href="#TOC1">BACKTRACKING CONTROL</a><br>
2621 <P>
2622 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2623 are described in the Perl documentation as "experimental and subject to change
2624 or removal in a future version of Perl". It goes on to say: "Their usage in
2625 production code should be noted to avoid problems during upgrades." The same
2626 remarks apply to the PCRE features described in this section.
2627 </P>
2628 <P>
2629 Since these verbs are specifically related to backtracking, most of them can be
2630 used only when the pattern is to be matched using one of the traditional
2631 matching functions, which use a backtracking algorithm. With the exception of
2632 (*FAIL), which behaves like a failing negative assertion, they cause an error
2633 if encountered by a DFA matching function.
2634 </P>
2635 <P>
2636 If any of these verbs are used in an assertion or in a subpattern that is
2637 called as a subroutine (whether or not recursively), their effect is confined
2638 to that subpattern; it does not extend to the surrounding pattern, with one
2639 exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2640 a successful positive assertion <i>is</i> passed back when a match succeeds
2641 (compare capturing parentheses in assertions). Note that such subpatterns are
2642 processed as anchored at the point where they are tested. Note also that Perl's
2643 treatment of subroutines and assertions is different in some cases.
2644 </P>
2645 <P>
2646 The new verbs make use of what was previously invalid syntax: an opening
2647 parenthesis followed by an asterisk. They are generally of the form
2648 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2649 depending on whether or not an argument is present. A name is any sequence of
2650 characters that does not include a closing parenthesis. The maximum length of
2651 name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit library.
2652 If the name is empty, that is, if the closing parenthesis immediately follows
2653 the colon, the effect is as if the colon were not there. Any number of these
2654 verbs may occur in a pattern.
2655 <a name="nooptimize"></a></P>
2656 <br><b>
2657 Optimizations that affect backtracking verbs
2658 </b><br>
2659 <P>
2660 PCRE contains some optimizations that are used to speed up matching by running
2661 some checks at the start of each match attempt. For example, it may know the
2662 minimum length of matching subject, or that a particular character must be
2663 present. When one of these optimizations suppresses the running of a match, any
2664 included backtracking verbs will not, of course, be processed. You can suppress
2665 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2666 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2667 pattern with (*NO_START_OPT). There is more discussion of this option in the
2668 section entitled
2669 <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
2670 in the
2671 <a href="pcreapi.html"><b>pcreapi</b></a>
2672 documentation.
2673 </P>
2674 <P>
2675 Experiments with Perl suggest that it too has similar optimizations, sometimes
2676 leading to anomalous results.
2677 </P>
2678 <br><b>
2679 Verbs that act immediately
2680 </b><br>
2681 <P>
2682 The following verbs act as soon as they are encountered. They may not be
2683 followed by a name.
2684 <pre>
2685 (*ACCEPT)
2686 </pre>
2687 This verb causes the match to end successfully, skipping the remainder of the
2688 pattern. However, when it is inside a subpattern that is called as a
2689 subroutine, only that subpattern is ended successfully. Matching then continues
2690 at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2691 far is captured. For example:
2692 <pre>
2693 A((?:A|B(*ACCEPT)|C)D)
2694 </pre>
2695 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2696 the outer parentheses.
2697 <pre>
2698 (*FAIL) or (*F)
2699 </pre>
2700 This verb causes a matching failure, forcing backtracking to occur. It is
2701 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2702 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2703 Perl features that are not present in PCRE. The nearest equivalent is the
2704 callout feature, as for example in this pattern:
2705 <pre>
2706 a+(?C)(*FAIL)
2707 </pre>
2708 A match with the string "aaaa" always fails, but the callout is taken before
2709 each backtrack happens (in this example, 10 times).
2710 </P>
2711 <br><b>
2712 Recording which path was taken
2713 </b><br>
2714 <P>
2715 There is one verb whose main purpose is to track how a match was arrived at,
2716 though it also has a secondary use in conjunction with advancing the match
2717 starting point (see (*SKIP) below).
2718 <pre>
2719 (*MARK:NAME) or (*:NAME)
2720 </pre>
2721 A name is always required with this verb. There may be as many instances of
2722 (*MARK) as you like in a pattern, and their names do not have to be unique.
2723 </P>
2724 <P>
2725 When a match succeeds, the name of the last-encountered (*MARK) on the matching
2726 path is passed back to the caller as described in the section entitled
2727 <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
2728 in the
2729 <a href="pcreapi.html"><b>pcreapi</b></a>
2730 documentation. Here is an example of <b>pcretest</b> output, where the /K
2731 modifier requests the retrieval and outputting of (*MARK) data:
2732 <pre>
2733 re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2734 data&#62; XY
2735 0: XY
2736 MK: A
2737 XZ
2738 0: XZ
2739 MK: B
2740 </pre>
2741 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2742 indicates which of the two alternatives matched. This is a more efficient way
2743 of obtaining this information than putting each alternative in its own
2744 capturing parentheses.
2745 </P>
2746 <P>
2747 If (*MARK) is encountered in a positive assertion, its name is recorded and
2748 passed back if it is the last-encountered. This does not happen for negative
2749 assertions.
2750 </P>
2751 <P>
2752 After a partial match or a failed match, the name of the last encountered
2753 (*MARK) in the entire match process is returned. For example:
2754 <pre>
2755 re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2756 data&#62; XP
2757 No match, mark = B
2758 </pre>
2759 Note that in this unanchored example the mark is retained from the match
2760 attempt that started at the letter "X" in the subject. Subsequent match
2761 attempts starting at "P" and then with an empty string do not get as far as the
2762 (*MARK) item, but nevertheless do not reset it.
2763 </P>
2764 <P>
2765 If you are interested in (*MARK) values after failed matches, you should
2766 probably set the PCRE_NO_START_OPTIMIZE option
2767 <a href="#nooptimize">(see above)</a>
2768 to ensure that the match is always attempted.
2769 </P>
2770 <br><b>
2771 Verbs that act after backtracking
2772 </b><br>
2773 <P>
2774 The following verbs do nothing when they are encountered. Matching continues
2775 with what follows, but if there is no subsequent match, causing a backtrack to
2776 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2777 the verb. However, when one of these verbs appears inside an atomic group, its
2778 effect is confined to that group, because once the group has been matched,
2779 there is never any backtracking into it. In this situation, backtracking can
2780 "jump back" to the left of the entire atomic group. (Remember also, as stated
2781 above, that this localization also applies in subroutine calls and assertions.)
2782 </P>
2783 <P>
2784 These verbs differ in exactly what kind of failure occurs when backtracking
2785 reaches them.
2786 <pre>
2787 (*COMMIT)
2788 </pre>
2789 This verb, which may not be followed by a name, causes the whole match to fail
2790 outright if the rest of the pattern does not match. Even if the pattern is
2791 unanchored, no further attempts to find a match by advancing the starting point
2792 take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2793 finding a match at the current starting point, or not at all. For example:
2794 <pre>
2795 a+(*COMMIT)b
2796 </pre>
2797 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2798 dynamic anchor, or "I've started, so I must finish." The name of the most
2799 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2800 match failure.
2801 </P>
2802 <P>
2803 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2804 unless PCRE's start-of-match optimizations are turned off, as shown in this
2805 <b>pcretest</b> example:
2806 <pre>
2807 re&#62; /(*COMMIT)abc/
2808 data&#62; xyzabc
2809 0: abc
2810 xyzabc\Y
2811 No match
2812 </pre>
2813 PCRE knows that any match must start with "a", so the optimization skips along
2814 the subject to "a" before running the first match attempt, which succeeds. When
2815 the optimization is disabled by the \Y escape in the second subject, the match
2816 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2817 starting points.
2818 <pre>
2819 (*PRUNE) or (*PRUNE:NAME)
2820 </pre>
2821 This verb causes the match to fail at the current starting position in the
2822 subject if the rest of the pattern does not match. If the pattern is
2823 unanchored, the normal "bumpalong" advance to the next starting character then
2824 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2825 reached, or when matching to the right of (*PRUNE), but if there is no match to
2826 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2827 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2828 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2829 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2830 anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2831 <pre>
2832 (*SKIP)
2833 </pre>
2834 This verb, when given without a name, is like (*PRUNE), except that if the
2835 pattern is unanchored, the "bumpalong" advance is not to the next character,
2836 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2837 signifies that whatever text was matched leading up to it cannot be part of a
2838 successful match. Consider:
2839 <pre>
2840 a+(*SKIP)b
2841 </pre>
2842 If the subject is "aaaac...", after the first match attempt fails (starting at
2843 the first character in the string), the starting point skips on to start the
2844 next attempt at "c". Note that a possessive quantifer does not have the same
2845 effect as this example; although it would suppress backtracking during the
2846 first match attempt, the second attempt would start at the second character
2847 instead of skipping on to "c".
2848 <pre>
2849 (*SKIP:NAME)
2850 </pre>
2851 When (*SKIP) has an associated name, its behaviour is modified. If the
2852 following pattern fails to match, the previous path through the pattern is
2853 searched for the most recent (*MARK) that has the same name. If one is found,
2854 the "bumpalong" advance is to the subject position that corresponds to that
2855 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2856 matching name is found, the (*SKIP) is ignored.
2857 <pre>
2858 (*THEN) or (*THEN:NAME)
2859 </pre>
2860 This verb causes a skip to the next innermost alternative if the rest of the
2861 pattern does not match. That is, it cancels pending backtracking, but only
2862 within the current alternative. Its name comes from the observation that it can
2863 be used for a pattern-based if-then-else block:
2864 <pre>
2865 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2866 </pre>
2867 If the COND1 pattern matches, FOO is tried (and possibly further items after
2868 the end of the group if FOO succeeds); on failure, the matcher skips to the
2869 second alternative and tries COND2, without backtracking into COND1. The
2870 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2871 If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2872 </P>
2873 <P>
2874 Note that a subpattern that does not contain a | character is just a part of
2875 the enclosing alternative; it is not a nested alternation with only one
2876 alternative. The effect of (*THEN) extends beyond such a subpattern to the
2877 enclosing alternative. Consider this pattern, where A, B, etc. are complex
2878 pattern fragments that do not contain any | characters at this level:
2879 <pre>
2880 A (B(*THEN)C) | D
2881 </pre>
2882 If A and B are matched, but there is a failure in C, matching does not
2883 backtrack into A; instead it moves to the next alternative, that is, D.
2884 However, if the subpattern containing (*THEN) is given an alternative, it
2885 behaves differently:
2886 <pre>
2887 A (B(*THEN)C | (*FAIL)) | D
2888 </pre>
2889 The effect of (*THEN) is now confined to the inner subpattern. After a failure
2890 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2891 because there are no more alternatives to try. In this case, matching does now
2892 backtrack into A.
2893 </P>
2894 <P>
2895 Note also that a conditional subpattern is not considered as having two
2896 alternatives, because only one is ever used. In other words, the | character in
2897 a conditional subpattern has a different meaning. Ignoring white space,
2898 consider:
2899 <pre>
2900 ^.*? (?(?=a) a | b(*THEN)c )
2901 </pre>
2902 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2903 it initially matches zero characters. The condition (?=a) then fails, the
2904 character "b" is matched, but "c" is not. At this point, matching does not
2905 backtrack to .*? as might perhaps be expected from the presence of the |
2906 character. The conditional subpattern is part of the single alternative that
2907 comprises the whole pattern, and so the match fails. (If there was a backtrack
2908 into .*?, allowing it to match "b", the match would succeed.)
2909 </P>
2910 <P>
2911 The verbs just described provide four different "strengths" of control when
2912 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2913 next alternative. (*PRUNE) comes next, failing the match at the current
2914 starting position, but allowing an advance to the next character (for an
2915 unanchored pattern). (*SKIP) is similar, except that the advance may be more
2916 than one character. (*COMMIT) is the strongest, causing the entire match to
2917 fail.
2918 </P>
2919 <P>
2920 If more than one such verb is present in a pattern, the "strongest" one wins.
2921 For example, consider this pattern, where A, B, etc. are complex pattern
2922 fragments:
2923 <pre>
2924 (A(*COMMIT)B(*THEN)C|D)
2925 </pre>
2926 Once A has matched, PCRE is committed to this match, at the current starting
2927 position. If subsequently B matches, but C does not, the normal (*THEN) action
2928 of trying the next alternative (that is, D) does not happen because (*COMMIT)
2929 overrides.
2930 </P>
2931 <br><a name="SEC27" href="#TOC1">SEE ALSO</a><br>
2932 <P>
2933 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2934 <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>.
2935 </P>
2936 <br><a name="SEC28" href="#TOC1">AUTHOR</a><br>
2937 <P>
2938 Philip Hazel
2939 <br>
2940 University Computing Service
2941 <br>
2942 Cambridge CB2 3QH, England.
2943 <br>
2944 </P>
2945 <br><a name="SEC29" href="#TOC1">REVISION</a><br>
2946 <P>
2947 Last updated: 11 November 2012
2948 <br>
2949 Copyright &copy; 1997-2012 University of Cambridge.
2950 <br>
2951 <p>
2952 Return to the <a href="index.html">PCRE index page</a>.
2953 </p>


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