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1 .TH PCREPATTERN 3 "05 November 2013" "PCRE 8.34"
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 .rs
6 .sp
7 The syntax and semantics of the regular expressions that are supported by PCRE
8 are described in detail below. There is a quick-reference syntax summary in the
9 .\" HREF
10 \fBpcresyntax\fP
11 .\"
12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13 also supports some alternative regular expression syntax (which does not
14 conflict with the Perl syntax) in order to provide some compatibility with
15 regular expressions in Python, .NET, and Oniguruma.
16 .P
17 Perl's regular expressions are described in its own documentation, and
18 regular expressions in general are covered in a number of books, some of which
19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20 published by O'Reilly, covers regular expressions in great detail. This
21 description of PCRE's regular expressions is intended as reference material.
22 .P
23 This document discusses the patterns that are supported by PCRE when one its
24 main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25 (16- or 32-bit), is used. PCRE also has alternative matching functions,
26 \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27 different algorithm that is not Perl-compatible. Some of the features discussed
28 below are not available when DFA matching is used. The advantages and
29 disadvantages of the alternative functions, and how they differ from the normal
30 functions, are discussed in the
31 .\" HREF
32 \fBpcrematching\fP
33 .\"
34 page.
35 .
36 .
37 .SH "SPECIAL START-OF-PATTERN ITEMS"
38 .rs
39 .sp
40 A number of options that can be passed to \fBpcre_compile()\fP can also be set
41 by special items at the start of a pattern. These are not Perl-compatible, but
42 are provided to make these options accessible to pattern writers who are not
43 able to change the program that processes the pattern. Any number of these
44 items may appear, but they must all be together right at the start of the
45 pattern string, and the letters must be in upper case.
46 .
47 .
48 .SS "UTF support"
49 .rs
50 .sp
51 The original operation of PCRE was on strings of one-byte characters. However,
52 there is now also support for UTF-8 strings in the original library, an
53 extra library that supports 16-bit and UTF-16 character strings, and a
54 third library that supports 32-bit and UTF-32 character strings. To use these
55 features, PCRE must be built to include appropriate support. When using UTF
56 strings you must either call the compiling function with the PCRE_UTF8,
57 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58 these special sequences:
59 .sp
60 (*UTF8)
61 (*UTF16)
62 (*UTF32)
63 (*UTF)
64 .sp
65 (*UTF) is a generic sequence that can be used with any of the libraries.
66 Starting a pattern with such a sequence is equivalent to setting the relevant
67 option. How setting a UTF mode affects pattern matching is mentioned in several
68 places below. There is also a summary of features in the
69 .\" HREF
70 \fBpcreunicode\fP
71 .\"
72 page.
73 .P
74 Some applications that allow their users to supply patterns may wish to
75 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76 option is set at compile time, (*UTF) etc. are not allowed, and their
77 appearance causes an error.
78 .
79 .
80 .SS "Unicode property support"
81 .rs
82 .sp
83 Another special sequence that may appear at the start of a pattern is
84 .sp
85 (*UCP)
86 .sp
87 This has the same effect as setting the PCRE_UCP option: it causes sequences
88 such as \ed and \ew to use Unicode properties to determine character types,
89 instead of recognizing only characters with codes less than 128 via a lookup
90 table.
91 .
92 .
93 .SS "Disabling start-up optimizations"
94 .rs
95 .sp
96 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
97 PCRE_NO_START_OPTIMIZE option either at compile or matching time.
98 .
99 .
100 .\" HTML <a name="newlines"></a>
101 .SS "Newline conventions"
102 .rs
103 .sp
104 PCRE supports five different conventions for indicating line breaks in
105 strings: a single CR (carriage return) character, a single LF (linefeed)
106 character, the two-character sequence CRLF, any of the three preceding, or any
107 Unicode newline sequence. The
108 .\" HREF
109 \fBpcreapi\fP
110 .\"
111 page has
112 .\" HTML <a href="pcreapi.html#newlines">
113 .\" </a>
114 further discussion
115 .\"
116 about newlines, and shows how to set the newline convention in the
117 \fIoptions\fP arguments for the compiling and matching functions.
118 .P
119 It is also possible to specify a newline convention by starting a pattern
120 string with one of the following five sequences:
121 .sp
122 (*CR) carriage return
123 (*LF) linefeed
124 (*CRLF) carriage return, followed by linefeed
125 (*ANYCRLF) any of the three above
126 (*ANY) all Unicode newline sequences
127 .sp
128 These override the default and the options given to the compiling function. For
129 example, on a Unix system where LF is the default newline sequence, the pattern
130 .sp
131 (*CR)a.b
132 .sp
133 changes the convention to CR. That pattern matches "a\enb" because LF is no
134 longer a newline. If more than one of these settings is present, the last one
135 is used.
136 .P
137 The newline convention affects where the circumflex and dollar assertions are
138 true. It also affects the interpretation of the dot metacharacter when
139 PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
140 what the \eR escape sequence matches. By default, this is any Unicode newline
141 sequence, for Perl compatibility. However, this can be changed; see the
142 description of \eR in the section entitled
143 .\" HTML <a href="#newlineseq">
144 .\" </a>
145 "Newline sequences"
146 .\"
147 below. A change of \eR setting can be combined with a change of newline
148 convention.
149 .
150 .
151 .SS "Setting match and recursion limits"
152 .rs
153 .sp
154 The caller of \fBpcre_exec()\fP can set a limit on the number of times the
155 internal \fBmatch()\fP function is called and on the maximum depth of
156 recursive calls. These facilities are provided to catch runaway matches that
157 are provoked by patterns with huge matching trees (a typical example is a
158 pattern with nested unlimited repeats) and to avoid running out of system stack
159 by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
160 gives an error return. The limits can also be set by items at the start of the
161 pattern of the form
162 .sp
163 (*LIMIT_MATCH=d)
164 (*LIMIT_RECURSION=d)
165 .sp
166 where d is any number of decimal digits. However, the value of the setting must
167 be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
168 for it to have any effect. In other words, the pattern writer can lower the
169 limits set by the programmer, but not raise them. If there is more than one
170 setting of one of these limits, the lower value is used.
171 .
172 .
173 .SH "EBCDIC CHARACTER CODES"
174 .rs
175 .sp
176 PCRE can be compiled to run in an environment that uses EBCDIC as its character
177 code rather than ASCII or Unicode (typically a mainframe system). In the
178 sections below, character code values are ASCII or Unicode; in an EBCDIC
179 environment these characters may have different code values, and there are no
180 code points greater than 255.
181 .
182 .
183 .SH "CHARACTERS AND METACHARACTERS"
184 .rs
185 .sp
186 A regular expression is a pattern that is matched against a subject string from
187 left to right. Most characters stand for themselves in a pattern, and match the
188 corresponding characters in the subject. As a trivial example, the pattern
189 .sp
190 The quick brown fox
191 .sp
192 matches a portion of a subject string that is identical to itself. When
193 caseless matching is specified (the PCRE_CASELESS option), letters are matched
194 independently of case. In a UTF mode, PCRE always understands the concept of
195 case for characters whose values are less than 128, so caseless matching is
196 always possible. For characters with higher values, the concept of case is
197 supported if PCRE is compiled with Unicode property support, but not otherwise.
198 If you want to use caseless matching for characters 128 and above, you must
199 ensure that PCRE is compiled with Unicode property support as well as with
200 UTF support.
201 .P
202 The power of regular expressions comes from the ability to include alternatives
203 and repetitions in the pattern. These are encoded in the pattern by the use of
204 \fImetacharacters\fP, which do not stand for themselves but instead are
205 interpreted in some special way.
206 .P
207 There are two different sets of metacharacters: those that are recognized
208 anywhere in the pattern except within square brackets, and those that are
209 recognized within square brackets. Outside square brackets, the metacharacters
210 are as follows:
211 .sp
212 \e general escape character with several uses
213 ^ assert start of string (or line, in multiline mode)
214 $ assert end of string (or line, in multiline mode)
215 . match any character except newline (by default)
216 [ start character class definition
217 | start of alternative branch
218 ( start subpattern
219 ) end subpattern
220 ? extends the meaning of (
221 also 0 or 1 quantifier
222 also quantifier minimizer
223 * 0 or more quantifier
224 + 1 or more quantifier
225 also "possessive quantifier"
226 { start min/max quantifier
227 .sp
228 Part of a pattern that is in square brackets is called a "character class". In
229 a character class the only metacharacters are:
230 .sp
231 \e general escape character
232 ^ negate the class, but only if the first character
233 - indicates character range
234 .\" JOIN
235 [ POSIX character class (only if followed by POSIX
236 syntax)
237 ] terminates the character class
238 .sp
239 The following sections describe the use of each of the metacharacters.
240 .
241 .
242 .SH BACKSLASH
243 .rs
244 .sp
245 The backslash character has several uses. Firstly, if it is followed by a
246 character that is not a number or a letter, it takes away any special meaning
247 that character may have. This use of backslash as an escape character applies
248 both inside and outside character classes.
249 .P
250 For example, if you want to match a * character, you write \e* in the pattern.
251 This escaping action applies whether or not the following character would
252 otherwise be interpreted as a metacharacter, so it is always safe to precede a
253 non-alphanumeric with backslash to specify that it stands for itself. In
254 particular, if you want to match a backslash, you write \e\e.
255 .P
256 In a UTF mode, only ASCII numbers and letters have any special meaning after a
257 backslash. All other characters (in particular, those whose codepoints are
258 greater than 127) are treated as literals.
259 .P
260 If a pattern is compiled with the PCRE_EXTENDED option, white space in the
261 pattern (other than in a character class) and characters between a # outside
262 a character class and the next newline are ignored. An escaping backslash can
263 be used to include a white space or # character as part of the pattern.
264 .P
265 If you want to remove the special meaning from a sequence of characters, you
266 can do so by putting them between \eQ and \eE. This is different from Perl in
267 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
268 Perl, $ and @ cause variable interpolation. Note the following examples:
269 .sp
270 Pattern PCRE matches Perl matches
271 .sp
272 .\" JOIN
273 \eQabc$xyz\eE abc$xyz abc followed by the
274 contents of $xyz
275 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
276 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
277 .sp
278 The \eQ...\eE sequence is recognized both inside and outside character classes.
279 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
280 by \eE later in the pattern, the literal interpretation continues to the end of
281 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
282 a character class, this causes an error, because the character class is not
283 terminated.
284 .
285 .
286 .\" HTML <a name="digitsafterbackslash"></a>
287 .SS "Non-printing characters"
288 .rs
289 .sp
290 A second use of backslash provides a way of encoding non-printing characters
291 in patterns in a visible manner. There is no restriction on the appearance of
292 non-printing characters, apart from the binary zero that terminates a pattern,
293 but when a pattern is being prepared by text editing, it is often easier to use
294 one of the following escape sequences than the binary character it represents:
295 .sp
296 \ea alarm, that is, the BEL character (hex 07)
297 \ecx "control-x", where x is any ASCII character
298 \ee escape (hex 1B)
299 \ef form feed (hex 0C)
300 \en linefeed (hex 0A)
301 \er carriage return (hex 0D)
302 \et tab (hex 09)
303 \e0dd character with octal code 0dd
304 \eddd character with octal code ddd, or back reference
305 \eo{ddd..} character with octal code ddd..
306 \exhh character with hex code hh
307 \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
308 \euhhhh character with hex code hhhh (JavaScript mode only)
309 .sp
310 The precise effect of \ecx on ASCII characters is as follows: if x is a lower
311 case letter, it is converted to upper case. Then bit 6 of the character (hex
312 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
313 but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
314 data item (byte or 16-bit value) following \ec has a value greater than 127, a
315 compile-time error occurs. This locks out non-ASCII characters in all modes.
316 .P
317 The \ec facility was designed for use with ASCII characters, but with the
318 extension to Unicode it is even less useful than it once was. It is, however,
319 recognized when PCRE is compiled in EBCDIC mode, where data items are always
320 bytes. In this mode, all values are valid after \ec. If the next character is a
321 lower case letter, it is converted to upper case. Then the 0xc0 bits of the
322 byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
323 the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
324 characters also generate different values.
325 .P
326 After \e0 up to two further octal digits are read. If there are fewer than two
327 digits, just those that are present are used. Thus the sequence \e0\ex\e07
328 specifies two binary zeros followed by a BEL character (code value 7). Make
329 sure you supply two digits after the initial zero if the pattern character that
330 follows is itself an octal digit.
331 .P
332 The escape \eo must be followed by a sequence of octal digits, enclosed in
333 braces. An error occurs if this is not the case. This escape is a recent
334 addition to Perl; it provides way of specifying character code points as octal
335 numbers greater than 0777, and it also allows octal numbers and back references
336 to be unambiguously specified.
337 .P
338 For greater clarity and unambiguity, it is best to avoid following \e by a
339 digit greater than zero. Instead, use \eo{} or \ex{} to specify character
340 numbers, and \eg{} to specify back references. The following paragraphs
341 describe the old, ambiguous syntax.
342 .P
343 The handling of a backslash followed by a digit other than 0 is complicated,
344 and Perl has changed in recent releases, causing PCRE also to change. Outside a
345 character class, PCRE reads the digit and any following digits as a decimal
346 number. If the number is less than 8, or if there have been at least that many
347 previous capturing left parentheses in the expression, the entire sequence is
348 taken as a \fIback reference\fP. A description of how this works is given
349 .\" HTML <a href="#backreferences">
350 .\" </a>
351 later,
352 .\"
353 following the discussion of
354 .\" HTML <a href="#subpattern">
355 .\" </a>
356 parenthesized subpatterns.
357 .\"
358 .P
359 Inside a character class, or if the decimal number following \e is greater than
360 7 and there have not been that many capturing subpatterns, PCRE handles \e8 and
361 \e9 as the literal characters "8" and "9", and otherwise re-reads up to three
362 octal digits following the backslash, using them to generate a data character.
363 Any subsequent digits stand for themselves. For example:
364 .sp
365 \e040 is another way of writing an ASCII space
366 .\" JOIN
367 \e40 is the same, provided there are fewer than 40
368 previous capturing subpatterns
369 \e7 is always a back reference
370 .\" JOIN
371 \e11 might be a back reference, or another way of
372 writing a tab
373 \e011 is always a tab
374 \e0113 is a tab followed by the character "3"
375 .\" JOIN
376 \e113 might be a back reference, otherwise the
377 character with octal code 113
378 .\" JOIN
379 \e377 might be a back reference, otherwise
380 the value 255 (decimal)
381 .\" JOIN
382 \e81 is either a back reference, or the two
383 characters "8" and "1"
384 .sp
385 Note that octal values of 100 or greater that are specified using this syntax
386 must not be introduced by a leading zero, because no more than three octal
387 digits are ever read.
388 .P
389 By default, after \ex that is not followed by {, from zero to two hexadecimal
390 digits are read (letters can be in upper or lower case). Any number of
391 hexadecimal digits may appear between \ex{ and }. If a character other than
392 a hexadecimal digit appears between \ex{ and }, or if there is no terminating
393 }, an error occurs.
394 .P
395 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
396 as just described only when it is followed by two hexadecimal digits.
397 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
398 code points greater than 256 is provided by \eu, which must be followed by
399 four hexadecimal digits; otherwise it matches a literal "u" character.
400 .P
401 Characters whose value is less than 256 can be defined by either of the two
402 syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
403 way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
404 \eu00dc in JavaScript mode).
405 .
406 .
407 .SS "Constraints on character values"
408 .rs
409 .sp
410 Characters that are specified using octal or hexadecimal numbers are
411 limited to certain values, as follows:
412 .sp
413 8-bit non-UTF mode less than 0x100
414 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
415 16-bit non-UTF mode less than 0x10000
416 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
417 32-bit non-UTF mode less than 0x100000000
418 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
419 .sp
420 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
421 "surrogate" codepoints), and 0xffef.
422 .
423 .
424 .SS "Escape sequences in character classes"
425 .rs
426 .sp
427 All the sequences that define a single character value can be used both inside
428 and outside character classes. In addition, inside a character class, \eb is
429 interpreted as the backspace character (hex 08).
430 .P
431 \eN is not allowed in a character class. \eB, \eR, and \eX are not special
432 inside a character class. Like other unrecognized escape sequences, they are
433 treated as the literal characters "B", "R", and "X" by default, but cause an
434 error if the PCRE_EXTRA option is set. Outside a character class, these
435 sequences have different meanings.
436 .
437 .
438 .SS "Unsupported escape sequences"
439 .rs
440 .sp
441 In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
442 handler and used to modify the case of following characters. By default, PCRE
443 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
444 option is set, \eU matches a "U" character, and \eu can be used to define a
445 character by code point, as described in the previous section.
446 .
447 .
448 .SS "Absolute and relative back references"
449 .rs
450 .sp
451 The sequence \eg followed by an unsigned or a negative number, optionally
452 enclosed in braces, is an absolute or relative back reference. A named back
453 reference can be coded as \eg{name}. Back references are discussed
454 .\" HTML <a href="#backreferences">
455 .\" </a>
456 later,
457 .\"
458 following the discussion of
459 .\" HTML <a href="#subpattern">
460 .\" </a>
461 parenthesized subpatterns.
462 .\"
463 .
464 .
465 .SS "Absolute and relative subroutine calls"
466 .rs
467 .sp
468 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
469 a number enclosed either in angle brackets or single quotes, is an alternative
470 syntax for referencing a subpattern as a "subroutine". Details are discussed
471 .\" HTML <a href="#onigurumasubroutines">
472 .\" </a>
473 later.
474 .\"
475 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
476 synonymous. The former is a back reference; the latter is a
477 .\" HTML <a href="#subpatternsassubroutines">
478 .\" </a>
479 subroutine
480 .\"
481 call.
482 .
483 .
484 .\" HTML <a name="genericchartypes"></a>
485 .SS "Generic character types"
486 .rs
487 .sp
488 Another use of backslash is for specifying generic character types:
489 .sp
490 \ed any decimal digit
491 \eD any character that is not a decimal digit
492 \eh any horizontal white space character
493 \eH any character that is not a horizontal white space character
494 \es any white space character
495 \eS any character that is not a white space character
496 \ev any vertical white space character
497 \eV any character that is not a vertical white space character
498 \ew any "word" character
499 \eW any "non-word" character
500 .sp
501 There is also the single sequence \eN, which matches a non-newline character.
502 This is the same as
503 .\" HTML <a href="#fullstopdot">
504 .\" </a>
505 the "." metacharacter
506 .\"
507 when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
508 PCRE does not support this.
509 .P
510 Each pair of lower and upper case escape sequences partitions the complete set
511 of characters into two disjoint sets. Any given character matches one, and only
512 one, of each pair. The sequences can appear both inside and outside character
513 classes. They each match one character of the appropriate type. If the current
514 matching point is at the end of the subject string, all of them fail, because
515 there is no character to match.
516 .P
517 For compatibility with Perl, \es did not used to match the VT character (code
518 11), which made it different from the the POSIX "space" class. However, Perl
519 added VT at release 5.18, and PCRE followed suit at release 8.34. The \es
520 characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space (32).
521 .P
522 A "word" character is an underscore or any character that is a letter or digit.
523 By default, the definition of letters and digits is controlled by PCRE's
524 low-valued character tables, and may vary if locale-specific matching is taking
525 place (see
526 .\" HTML <a href="pcreapi.html#localesupport">
527 .\" </a>
528 "Locale support"
529 .\"
530 in the
531 .\" HREF
532 \fBpcreapi\fP
533 .\"
534 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
535 or "french" in Windows, some character codes greater than 128 are used for
536 accented letters, and these are then matched by \ew. The use of locales with
537 Unicode is discouraged.
538 .P
539 By default, in a UTF mode, characters with values greater than 128 never match
540 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
541 their original meanings from before UTF support was available, mainly for
542 efficiency reasons. However, if PCRE is compiled with Unicode property support,
543 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
544 properties are used to determine character types, as follows:
545 .sp
546 \ed any character that matches \ep{Nd} (decimal digit)
547 \es any character that matches \ep{Z} or \eh or \ev
548 \ew any character that matches \ep{L} or \ep{N}, plus underscore
549 .sp
550 The upper case escapes match the inverse sets of characters. Note that \ed
551 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
552 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
553 \eB because they are defined in terms of \ew and \eW. Matching these sequences
554 is noticeably slower when PCRE_UCP is set.
555 .P
556 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
557 release 5.10. In contrast to the other sequences, which match only ASCII
558 characters by default, these always match certain high-valued codepoints,
559 whether or not PCRE_UCP is set. The horizontal space characters are:
560 .sp
561 U+0009 Horizontal tab (HT)
562 U+0020 Space
563 U+00A0 Non-break space
564 U+1680 Ogham space mark
565 U+180E Mongolian vowel separator
566 U+2000 En quad
567 U+2001 Em quad
568 U+2002 En space
569 U+2003 Em space
570 U+2004 Three-per-em space
571 U+2005 Four-per-em space
572 U+2006 Six-per-em space
573 U+2007 Figure space
574 U+2008 Punctuation space
575 U+2009 Thin space
576 U+200A Hair space
577 U+202F Narrow no-break space
578 U+205F Medium mathematical space
579 U+3000 Ideographic space
580 .sp
581 The vertical space characters are:
582 .sp
583 U+000A Linefeed (LF)
584 U+000B Vertical tab (VT)
585 U+000C Form feed (FF)
586 U+000D Carriage return (CR)
587 U+0085 Next line (NEL)
588 U+2028 Line separator
589 U+2029 Paragraph separator
590 .sp
591 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
592 relevant.
593 .
594 .
595 .\" HTML <a name="newlineseq"></a>
596 .SS "Newline sequences"
597 .rs
598 .sp
599 Outside a character class, by default, the escape sequence \eR matches any
600 Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
601 following:
602 .sp
603 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
604 .sp
605 This is an example of an "atomic group", details of which are given
606 .\" HTML <a href="#atomicgroup">
607 .\" </a>
608 below.
609 .\"
610 This particular group matches either the two-character sequence CR followed by
611 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
612 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
613 line, U+0085). The two-character sequence is treated as a single unit that
614 cannot be split.
615 .P
616 In other modes, two additional characters whose codepoints are greater than 255
617 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
618 Unicode character property support is not needed for these characters to be
619 recognized.
620 .P
621 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
622 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
623 either at compile time or when the pattern is matched. (BSR is an abbrevation
624 for "backslash R".) This can be made the default when PCRE is built; if this is
625 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
626 It is also possible to specify these settings by starting a pattern string with
627 one of the following sequences:
628 .sp
629 (*BSR_ANYCRLF) CR, LF, or CRLF only
630 (*BSR_UNICODE) any Unicode newline sequence
631 .sp
632 These override the default and the options given to the compiling function, but
633 they can themselves be overridden by options given to a matching function. Note
634 that these special settings, which are not Perl-compatible, are recognized only
635 at the very start of a pattern, and that they must be in upper case. If more
636 than one of them is present, the last one is used. They can be combined with a
637 change of newline convention; for example, a pattern can start with:
638 .sp
639 (*ANY)(*BSR_ANYCRLF)
640 .sp
641 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
642 (*UCP) special sequences. Inside a character class, \eR is treated as an
643 unrecognized escape sequence, and so matches the letter "R" by default, but
644 causes an error if PCRE_EXTRA is set.
645 .
646 .
647 .\" HTML <a name="uniextseq"></a>
648 .SS Unicode character properties
649 .rs
650 .sp
651 When PCRE is built with Unicode character property support, three additional
652 escape sequences that match characters with specific properties are available.
653 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
654 characters whose codepoints are less than 256, but they do work in this mode.
655 The extra escape sequences are:
656 .sp
657 \ep{\fIxx\fP} a character with the \fIxx\fP property
658 \eP{\fIxx\fP} a character without the \fIxx\fP property
659 \eX a Unicode extended grapheme cluster
660 .sp
661 The property names represented by \fIxx\fP above are limited to the Unicode
662 script names, the general category properties, "Any", which matches any
663 character (including newline), and some special PCRE properties (described
664 in the
665 .\" HTML <a href="#extraprops">
666 .\" </a>
667 next section).
668 .\"
669 Other Perl properties such as "InMusicalSymbols" are not currently supported by
670 PCRE. Note that \eP{Any} does not match any characters, so always causes a
671 match failure.
672 .P
673 Sets of Unicode characters are defined as belonging to certain scripts. A
674 character from one of these sets can be matched using a script name. For
675 example:
676 .sp
677 \ep{Greek}
678 \eP{Han}
679 .sp
680 Those that are not part of an identified script are lumped together as
681 "Common". The current list of scripts is:
682 .P
683 Arabic,
684 Armenian,
685 Avestan,
686 Balinese,
687 Bamum,
688 Batak,
689 Bengali,
690 Bopomofo,
691 Brahmi,
692 Braille,
693 Buginese,
694 Buhid,
695 Canadian_Aboriginal,
696 Carian,
697 Chakma,
698 Cham,
699 Cherokee,
700 Common,
701 Coptic,
702 Cuneiform,
703 Cypriot,
704 Cyrillic,
705 Deseret,
706 Devanagari,
707 Egyptian_Hieroglyphs,
708 Ethiopic,
709 Georgian,
710 Glagolitic,
711 Gothic,
712 Greek,
713 Gujarati,
714 Gurmukhi,
715 Han,
716 Hangul,
717 Hanunoo,
718 Hebrew,
719 Hiragana,
720 Imperial_Aramaic,
721 Inherited,
722 Inscriptional_Pahlavi,
723 Inscriptional_Parthian,
724 Javanese,
725 Kaithi,
726 Kannada,
727 Katakana,
728 Kayah_Li,
729 Kharoshthi,
730 Khmer,
731 Lao,
732 Latin,
733 Lepcha,
734 Limbu,
735 Linear_B,
736 Lisu,
737 Lycian,
738 Lydian,
739 Malayalam,
740 Mandaic,
741 Meetei_Mayek,
742 Meroitic_Cursive,
743 Meroitic_Hieroglyphs,
744 Miao,
745 Mongolian,
746 Myanmar,
747 New_Tai_Lue,
748 Nko,
749 Ogham,
750 Old_Italic,
751 Old_Persian,
752 Old_South_Arabian,
753 Old_Turkic,
754 Ol_Chiki,
755 Oriya,
756 Osmanya,
757 Phags_Pa,
758 Phoenician,
759 Rejang,
760 Runic,
761 Samaritan,
762 Saurashtra,
763 Sharada,
764 Shavian,
765 Sinhala,
766 Sora_Sompeng,
767 Sundanese,
768 Syloti_Nagri,
769 Syriac,
770 Tagalog,
771 Tagbanwa,
772 Tai_Le,
773 Tai_Tham,
774 Tai_Viet,
775 Takri,
776 Tamil,
777 Telugu,
778 Thaana,
779 Thai,
780 Tibetan,
781 Tifinagh,
782 Ugaritic,
783 Vai,
784 Yi.
785 .P
786 Each character has exactly one Unicode general category property, specified by
787 a two-letter abbreviation. For compatibility with Perl, negation can be
788 specified by including a circumflex between the opening brace and the property
789 name. For example, \ep{^Lu} is the same as \eP{Lu}.
790 .P
791 If only one letter is specified with \ep or \eP, it includes all the general
792 category properties that start with that letter. In this case, in the absence
793 of negation, the curly brackets in the escape sequence are optional; these two
794 examples have the same effect:
795 .sp
796 \ep{L}
797 \epL
798 .sp
799 The following general category property codes are supported:
800 .sp
801 C Other
802 Cc Control
803 Cf Format
804 Cn Unassigned
805 Co Private use
806 Cs Surrogate
807 .sp
808 L Letter
809 Ll Lower case letter
810 Lm Modifier letter
811 Lo Other letter
812 Lt Title case letter
813 Lu Upper case letter
814 .sp
815 M Mark
816 Mc Spacing mark
817 Me Enclosing mark
818 Mn Non-spacing mark
819 .sp
820 N Number
821 Nd Decimal number
822 Nl Letter number
823 No Other number
824 .sp
825 P Punctuation
826 Pc Connector punctuation
827 Pd Dash punctuation
828 Pe Close punctuation
829 Pf Final punctuation
830 Pi Initial punctuation
831 Po Other punctuation
832 Ps Open punctuation
833 .sp
834 S Symbol
835 Sc Currency symbol
836 Sk Modifier symbol
837 Sm Mathematical symbol
838 So Other symbol
839 .sp
840 Z Separator
841 Zl Line separator
842 Zp Paragraph separator
843 Zs Space separator
844 .sp
845 The special property L& is also supported: it matches a character that has
846 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
847 a modifier or "other".
848 .P
849 The Cs (Surrogate) property applies only to characters in the range U+D800 to
850 U+DFFF. Such characters are not valid in Unicode strings and so
851 cannot be tested by PCRE, unless UTF validity checking has been turned off
852 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
853 PCRE_NO_UTF32_CHECK in the
854 .\" HREF
855 \fBpcreapi\fP
856 .\"
857 page). Perl does not support the Cs property.
858 .P
859 The long synonyms for property names that Perl supports (such as \ep{Letter})
860 are not supported by PCRE, nor is it permitted to prefix any of these
861 properties with "Is".
862 .P
863 No character that is in the Unicode table has the Cn (unassigned) property.
864 Instead, this property is assumed for any code point that is not in the
865 Unicode table.
866 .P
867 Specifying caseless matching does not affect these escape sequences. For
868 example, \ep{Lu} always matches only upper case letters. This is different from
869 the behaviour of current versions of Perl.
870 .P
871 Matching characters by Unicode property is not fast, because PCRE has to do a
872 multistage table lookup in order to find a character's property. That is why
873 the traditional escape sequences such as \ed and \ew do not use Unicode
874 properties in PCRE by default, though you can make them do so by setting the
875 PCRE_UCP option or by starting the pattern with (*UCP).
876 .
877 .
878 .SS Extended grapheme clusters
879 .rs
880 .sp
881 The \eX escape matches any number of Unicode characters that form an "extended
882 grapheme cluster", and treats the sequence as an atomic group
883 .\" HTML <a href="#atomicgroup">
884 .\" </a>
885 (see below).
886 .\"
887 Up to and including release 8.31, PCRE matched an earlier, simpler definition
888 that was equivalent to
889 .sp
890 (?>\ePM\epM*)
891 .sp
892 That is, it matched a character without the "mark" property, followed by zero
893 or more characters with the "mark" property. Characters with the "mark"
894 property are typically non-spacing accents that affect the preceding character.
895 .P
896 This simple definition was extended in Unicode to include more complicated
897 kinds of composite character by giving each character a grapheme breaking
898 property, and creating rules that use these properties to define the boundaries
899 of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
900 one of these clusters.
901 .P
902 \eX always matches at least one character. Then it decides whether to add
903 additional characters according to the following rules for ending a cluster:
904 .P
905 1. End at the end of the subject string.
906 .P
907 2. Do not end between CR and LF; otherwise end after any control character.
908 .P
909 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
910 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
911 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
912 character; an LVT or T character may be follwed only by a T character.
913 .P
914 4. Do not end before extending characters or spacing marks. Characters with
915 the "mark" property always have the "extend" grapheme breaking property.
916 .P
917 5. Do not end after prepend characters.
918 .P
919 6. Otherwise, end the cluster.
920 .
921 .
922 .\" HTML <a name="extraprops"></a>
923 .SS PCRE's additional properties
924 .rs
925 .sp
926 As well as the standard Unicode properties described above, PCRE supports four
927 more that make it possible to convert traditional escape sequences such as \ew
928 and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
929 properties internally when PCRE_UCP is set. However, they may also be used
930 explicitly. These properties are:
931 .sp
932 Xan Any alphanumeric character
933 Xps Any POSIX space character
934 Xsp Any Perl space character
935 Xwd Any Perl "word" character
936 .sp
937 Xan matches characters that have either the L (letter) or the N (number)
938 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
939 carriage return, and any other character that has the Z (separator) property.
940 Xsp is the same as Xps; it used to exclude vertical tab, for Perl
941 compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
942 matches the same characters as Xan, plus underscore.
943 .P
944 There is another non-standard property, Xuc, which matches any character that
945 can be represented by a Universal Character Name in C++ and other programming
946 languages. These are the characters $, @, ` (grave accent), and all characters
947 with Unicode code points greater than or equal to U+00A0, except for the
948 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
949 excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
950 where H is a hexadecimal digit. Note that the Xuc property does not match these
951 sequences but the characters that they represent.)
952 .
953 .
954 .\" HTML <a name="resetmatchstart"></a>
955 .SS "Resetting the match start"
956 .rs
957 .sp
958 The escape sequence \eK causes any previously matched characters not to be
959 included in the final matched sequence. For example, the pattern:
960 .sp
961 foo\eKbar
962 .sp
963 matches "foobar", but reports that it has matched "bar". This feature is
964 similar to a lookbehind assertion
965 .\" HTML <a href="#lookbehind">
966 .\" </a>
967 (described below).
968 .\"
969 However, in this case, the part of the subject before the real match does not
970 have to be of fixed length, as lookbehind assertions do. The use of \eK does
971 not interfere with the setting of
972 .\" HTML <a href="#subpattern">
973 .\" </a>
974 captured substrings.
975 .\"
976 For example, when the pattern
977 .sp
978 (foo)\eKbar
979 .sp
980 matches "foobar", the first substring is still set to "foo".
981 .P
982 Perl documents that the use of \eK within assertions is "not well defined". In
983 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
984 ignored in negative assertions.
985 .
986 .
987 .\" HTML <a name="smallassertions"></a>
988 .SS "Simple assertions"
989 .rs
990 .sp
991 The final use of backslash is for certain simple assertions. An assertion
992 specifies a condition that has to be met at a particular point in a match,
993 without consuming any characters from the subject string. The use of
994 subpatterns for more complicated assertions is described
995 .\" HTML <a href="#bigassertions">
996 .\" </a>
997 below.
998 .\"
999 The backslashed assertions are:
1000 .sp
1001 \eb matches at a word boundary
1002 \eB matches when not at a word boundary
1003 \eA matches at the start of the subject
1004 \eZ matches at the end of the subject
1005 also matches before a newline at the end of the subject
1006 \ez matches only at the end of the subject
1007 \eG matches at the first matching position in the subject
1008 .sp
1009 Inside a character class, \eb has a different meaning; it matches the backspace
1010 character. If any other of these assertions appears in a character class, by
1011 default it matches the corresponding literal character (for example, \eB
1012 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1013 escape sequence" error is generated instead.
1014 .P
1015 A word boundary is a position in the subject string where the current character
1016 and the previous character do not both match \ew or \eW (i.e. one matches
1017 \ew and the other matches \eW), or the start or end of the string if the
1018 first or last character matches \ew, respectively. In a UTF mode, the meanings
1019 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1020 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1021 of word" or "end of word" metasequence. However, whatever follows \eb normally
1022 determines which it is. For example, the fragment \eba matches "a" at the start
1023 of a word.
1024 .P
1025 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1026 dollar (described in the next section) in that they only ever match at the very
1027 start and end of the subject string, whatever options are set. Thus, they are
1028 independent of multiline mode. These three assertions are not affected by the
1029 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1030 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
1031 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
1032 at a point other than the beginning of the subject, \eA can never match. The
1033 difference between \eZ and \ez is that \eZ matches before a newline at the end
1034 of the string as well as at the very end, whereas \ez matches only at the end.
1035 .P
1036 The \eG assertion is true only when the current matching position is at the
1037 start point of the match, as specified by the \fIstartoffset\fP argument of
1038 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
1039 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
1040 arguments, you can mimic Perl's /g option, and it is in this kind of
1041 implementation where \eG can be useful.
1042 .P
1043 Note, however, that PCRE's interpretation of \eG, as the start of the current
1044 match, is subtly different from Perl's, which defines it as the end of the
1045 previous match. In Perl, these can be different when the previously matched
1046 string was empty. Because PCRE does just one match at a time, it cannot
1047 reproduce this behaviour.
1048 .P
1049 If all the alternatives of a pattern begin with \eG, the expression is anchored
1050 to the starting match position, and the "anchored" flag is set in the compiled
1051 regular expression.
1052 .
1053 .
1054 .SH "CIRCUMFLEX AND DOLLAR"
1055 .rs
1056 .sp
1057 The circumflex and dollar metacharacters are zero-width assertions. That is,
1058 they test for a particular condition being true without consuming any
1059 characters from the subject string.
1060 .P
1061 Outside a character class, in the default matching mode, the circumflex
1062 character is an assertion that is true only if the current matching point is at
1063 the start of the subject string. If the \fIstartoffset\fP argument of
1064 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1065 option is unset. Inside a character class, circumflex has an entirely different
1066 meaning
1067 .\" HTML <a href="#characterclass">
1068 .\" </a>
1069 (see below).
1070 .\"
1071 .P
1072 Circumflex need not be the first character of the pattern if a number of
1073 alternatives are involved, but it should be the first thing in each alternative
1074 in which it appears if the pattern is ever to match that branch. If all
1075 possible alternatives start with a circumflex, that is, if the pattern is
1076 constrained to match only at the start of the subject, it is said to be an
1077 "anchored" pattern. (There are also other constructs that can cause a pattern
1078 to be anchored.)
1079 .P
1080 The dollar character is an assertion that is true only if the current matching
1081 point is at the end of the subject string, or immediately before a newline at
1082 the end of the string (by default). Note, however, that it does not actually
1083 match the newline. Dollar need not be the last character of the pattern if a
1084 number of alternatives are involved, but it should be the last item in any
1085 branch in which it appears. Dollar has no special meaning in a character class.
1086 .P
1087 The meaning of dollar can be changed so that it matches only at the very end of
1088 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1089 does not affect the \eZ assertion.
1090 .P
1091 The meanings of the circumflex and dollar characters are changed if the
1092 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1093 immediately after internal newlines as well as at the start of the subject
1094 string. It does not match after a newline that ends the string. A dollar
1095 matches before any newlines in the string, as well as at the very end, when
1096 PCRE_MULTILINE is set. When newline is specified as the two-character
1097 sequence CRLF, isolated CR and LF characters do not indicate newlines.
1098 .P
1099 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
1100 \en represents a newline) in multiline mode, but not otherwise. Consequently,
1101 patterns that are anchored in single line mode because all branches start with
1102 ^ are not anchored in multiline mode, and a match for circumflex is possible
1103 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
1104 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1105 .P
1106 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
1107 end of the subject in both modes, and if all branches of a pattern start with
1108 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1109 .
1110 .
1111 .\" HTML <a name="fullstopdot"></a>
1112 .SH "FULL STOP (PERIOD, DOT) AND \eN"
1113 .rs
1114 .sp
1115 Outside a character class, a dot in the pattern matches any one character in
1116 the subject string except (by default) a character that signifies the end of a
1117 line.
1118 .P
1119 When a line ending is defined as a single character, dot never matches that
1120 character; when the two-character sequence CRLF is used, dot does not match CR
1121 if it is immediately followed by LF, but otherwise it matches all characters
1122 (including isolated CRs and LFs). When any Unicode line endings are being
1123 recognized, dot does not match CR or LF or any of the other line ending
1124 characters.
1125 .P
1126 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1127 option is set, a dot matches any one character, without exception. If the
1128 two-character sequence CRLF is present in the subject string, it takes two dots
1129 to match it.
1130 .P
1131 The handling of dot is entirely independent of the handling of circumflex and
1132 dollar, the only relationship being that they both involve newlines. Dot has no
1133 special meaning in a character class.
1134 .P
1135 The escape sequence \eN behaves like a dot, except that it is not affected by
1136 the PCRE_DOTALL option. In other words, it matches any character except one
1137 that signifies the end of a line. Perl also uses \eN to match characters by
1138 name; PCRE does not support this.
1139 .
1140 .
1141 .SH "MATCHING A SINGLE DATA UNIT"
1142 .rs
1143 .sp
1144 Outside a character class, the escape sequence \eC matches any one data unit,
1145 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1146 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1147 a 32-bit unit. Unlike a dot, \eC always
1148 matches line-ending characters. The feature is provided in Perl in order to
1149 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1150 used. Because \eC breaks up characters into individual data units, matching one
1151 unit with \eC in a UTF mode means that the rest of the string may start with a
1152 malformed UTF character. This has undefined results, because PCRE assumes that
1153 it is dealing with valid UTF strings (and by default it checks this at the
1154 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1155 PCRE_NO_UTF32_CHECK option is used).
1156 .P
1157 PCRE does not allow \eC to appear in lookbehind assertions
1158 .\" HTML <a href="#lookbehind">
1159 .\" </a>
1160 (described below)
1161 .\"
1162 in a UTF mode, because this would make it impossible to calculate the length of
1163 the lookbehind.
1164 .P
1165 In general, the \eC escape sequence is best avoided. However, one
1166 way of using it that avoids the problem of malformed UTF characters is to use a
1167 lookahead to check the length of the next character, as in this pattern, which
1168 could be used with a UTF-8 string (ignore white space and line breaks):
1169 .sp
1170 (?| (?=[\ex00-\ex7f])(\eC) |
1171 (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1172 (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1173 (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1174 .sp
1175 A group that starts with (?| resets the capturing parentheses numbers in each
1176 alternative (see
1177 .\" HTML <a href="#dupsubpatternnumber">
1178 .\" </a>
1179 "Duplicate Subpattern Numbers"
1180 .\"
1181 below). The assertions at the start of each branch check the next UTF-8
1182 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1183 character's individual bytes are then captured by the appropriate number of
1184 groups.
1185 .
1186 .
1187 .\" HTML <a name="characterclass"></a>
1188 .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1189 .rs
1190 .sp
1191 An opening square bracket introduces a character class, terminated by a closing
1192 square bracket. A closing square bracket on its own is not special by default.
1193 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1194 bracket causes a compile-time error. If a closing square bracket is required as
1195 a member of the class, it should be the first data character in the class
1196 (after an initial circumflex, if present) or escaped with a backslash.
1197 .P
1198 A character class matches a single character in the subject. In a UTF mode, the
1199 character may be more than one data unit long. A matched character must be in
1200 the set of characters defined by the class, unless the first character in the
1201 class definition is a circumflex, in which case the subject character must not
1202 be in the set defined by the class. If a circumflex is actually required as a
1203 member of the class, ensure it is not the first character, or escape it with a
1204 backslash.
1205 .P
1206 For example, the character class [aeiou] matches any lower case vowel, while
1207 [^aeiou] matches any character that is not a lower case vowel. Note that a
1208 circumflex is just a convenient notation for specifying the characters that
1209 are in the class by enumerating those that are not. A class that starts with a
1210 circumflex is not an assertion; it still consumes a character from the subject
1211 string, and therefore it fails if the current pointer is at the end of the
1212 string.
1213 .P
1214 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1215 can be included in a class as a literal string of data units, or by using the
1216 \ex{ escaping mechanism.
1217 .P
1218 When caseless matching is set, any letters in a class represent both their
1219 upper case and lower case versions, so for example, a caseless [aeiou] matches
1220 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1221 caseful version would. In a UTF mode, PCRE always understands the concept of
1222 case for characters whose values are less than 128, so caseless matching is
1223 always possible. For characters with higher values, the concept of case is
1224 supported if PCRE is compiled with Unicode property support, but not otherwise.
1225 If you want to use caseless matching in a UTF mode for characters 128 and
1226 above, you must ensure that PCRE is compiled with Unicode property support as
1227 well as with UTF support.
1228 .P
1229 Characters that might indicate line breaks are never treated in any special way
1230 when matching character classes, whatever line-ending sequence is in use, and
1231 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1232 such as [^a] always matches one of these characters.
1233 .P
1234 The minus (hyphen) character can be used to specify a range of characters in a
1235 character class. For example, [d-m] matches any letter between d and m,
1236 inclusive. If a minus character is required in a class, it must be escaped with
1237 a backslash or appear in a position where it cannot be interpreted as
1238 indicating a range, typically as the first or last character in the class.
1239 .P
1240 It is not possible to have the literal character "]" as the end character of a
1241 range. A pattern such as [W-]46] is interpreted as a class of two characters
1242 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1243 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1244 the end of range, so [W-\e]46] is interpreted as a class containing a range
1245 followed by two other characters. The octal or hexadecimal representation of
1246 "]" can also be used to end a range.
1247 .P
1248 Ranges operate in the collating sequence of character values. They can also be
1249 used for characters specified numerically, for example [\e000-\e037]. Ranges
1250 can include any characters that are valid for the current mode.
1251 .P
1252 If a range that includes letters is used when caseless matching is set, it
1253 matches the letters in either case. For example, [W-c] is equivalent to
1254 [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1255 tables for a French locale are in use, [\exc8-\excb] matches accented E
1256 characters in both cases. In UTF modes, PCRE supports the concept of case for
1257 characters with values greater than 128 only when it is compiled with Unicode
1258 property support.
1259 .P
1260 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1261 \eV, \ew, and \eW may appear in a character class, and add the characters that
1262 they match to the class. For example, [\edABCDEF] matches any hexadecimal
1263 digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1264 and their upper case partners, just as it does when they appear outside a
1265 character class, as described in the section entitled
1266 .\" HTML <a href="#genericchartypes">
1267 .\" </a>
1268 "Generic character types"
1269 .\"
1270 above. The escape sequence \eb has a different meaning inside a character
1271 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1272 are not special inside a character class. Like any other unrecognized escape
1273 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1274 default, but cause an error if the PCRE_EXTRA option is set.
1275 .P
1276 A circumflex can conveniently be used with the upper case character types to
1277 specify a more restricted set of characters than the matching lower case type.
1278 For example, the class [^\eW_] matches any letter or digit, but not underscore,
1279 whereas [\ew] includes underscore. A positive character class should be read as
1280 "something OR something OR ..." and a negative class as "NOT something AND NOT
1281 something AND NOT ...".
1282 .P
1283 The only metacharacters that are recognized in character classes are backslash,
1284 hyphen (only where it can be interpreted as specifying a range), circumflex
1285 (only at the start), opening square bracket (only when it can be interpreted as
1286 introducing a POSIX class name - see the next section), and the terminating
1287 closing square bracket. However, escaping other non-alphanumeric characters
1288 does no harm.
1289 .
1290 .
1291 .SH "POSIX CHARACTER CLASSES"
1292 .rs
1293 .sp
1294 Perl supports the POSIX notation for character classes. This uses names
1295 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1296 this notation. For example,
1297 .sp
1298 [01[:alpha:]%]
1299 .sp
1300 matches "0", "1", any alphabetic character, or "%". The supported class names
1301 are:
1302 .sp
1303 alnum letters and digits
1304 alpha letters
1305 ascii character codes 0 - 127
1306 blank space or tab only
1307 cntrl control characters
1308 digit decimal digits (same as \ed)
1309 graph printing characters, excluding space
1310 lower lower case letters
1311 print printing characters, including space
1312 punct printing characters, excluding letters and digits and space
1313 space white space (the same as \es from PCRE 8.34)
1314 upper upper case letters
1315 word "word" characters (same as \ew)
1316 xdigit hexadecimal digits
1317 .sp
1318 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1319 space (32). "Space" used to be different to \es, which did not include VT, for
1320 Perl compatibility. However, Perl changed at release 5.18, and PCRE followed at
1321 release 8.34. "Space" and \es now match the same set of characters.
1322 .P
1323 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1324 5.8. Another Perl extension is negation, which is indicated by a ^ character
1325 after the colon. For example,
1326 .sp
1327 [12[:^digit:]]
1328 .sp
1329 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1330 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1331 supported, and an error is given if they are encountered.
1332 .P
1333 By default, in UTF modes, characters with values greater than 128 do not match
1334 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1335 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1336 character properties are used. This is achieved by replacing certain POSIX
1337 classes by other sequences, as follows:
1338 .sp
1339 [:alnum:] becomes \ep{Xan}
1340 [:alpha:] becomes \ep{L}
1341 [:blank:] becomes \eh
1342 [:digit:] becomes \ep{Nd}
1343 [:lower:] becomes \ep{Ll}
1344 [:space:] becomes \ep{Xps}
1345 [:upper:] becomes \ep{Lu}
1346 [:word:] becomes \ep{Xwd}
1347 .sp
1348 Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1349 classes are handled specially in UCP mode:
1350 .TP 10
1351 [:graph:]
1352 This matches characters that have glyphs that mark the page when printed. In
1353 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1354 properties, except for:
1355 .sp
1356 U+061C Arabic Letter Mark
1357 U+180E Mongolian Vowel Separator
1358 U+2066 - U+2069 Various "isolate"s
1359 .sp
1360 .TP 10
1361 [:print:]
1362 This matches the same characters as [:graph:] plus space characters that are
1363 not controls, that is, characters with the Zs property.
1364 .TP 10
1365 [:punct:]
1366 This matches all characters that have the Unicode P (punctuation) property,
1367 plus those characters whose code points are less than 128 that have the S
1368 (Symbol) property.
1369 .P
1370 The other POSIX classes are unchanged, and match only characters with code
1371 points less than 128.
1372 .
1373 .
1374 .SH "VERTICAL BAR"
1375 .rs
1376 .sp
1377 Vertical bar characters are used to separate alternative patterns. For example,
1378 the pattern
1379 .sp
1380 gilbert|sullivan
1381 .sp
1382 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1383 and an empty alternative is permitted (matching the empty string). The matching
1384 process tries each alternative in turn, from left to right, and the first one
1385 that succeeds is used. If the alternatives are within a subpattern
1386 .\" HTML <a href="#subpattern">
1387 .\" </a>
1388 (defined below),
1389 .\"
1390 "succeeds" means matching the rest of the main pattern as well as the
1391 alternative in the subpattern.
1392 .
1393 .
1394 .SH "INTERNAL OPTION SETTING"
1395 .rs
1396 .sp
1397 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1398 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1399 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1400 The option letters are
1401 .sp
1402 i for PCRE_CASELESS
1403 m for PCRE_MULTILINE
1404 s for PCRE_DOTALL
1405 x for PCRE_EXTENDED
1406 .sp
1407 For example, (?im) sets caseless, multiline matching. It is also possible to
1408 unset these options by preceding the letter with a hyphen, and a combined
1409 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1410 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1411 permitted. If a letter appears both before and after the hyphen, the option is
1412 unset.
1413 .P
1414 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1415 changed in the same way as the Perl-compatible options by using the characters
1416 J, U and X respectively.
1417 .P
1418 When one of these option changes occurs at top level (that is, not inside
1419 subpattern parentheses), the change applies to the remainder of the pattern
1420 that follows. If the change is placed right at the start of a pattern, PCRE
1421 extracts it into the global options (and it will therefore show up in data
1422 extracted by the \fBpcre_fullinfo()\fP function).
1423 .P
1424 An option change within a subpattern (see below for a description of
1425 subpatterns) affects only that part of the subpattern that follows it, so
1426 .sp
1427 (a(?i)b)c
1428 .sp
1429 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1430 By this means, options can be made to have different settings in different
1431 parts of the pattern. Any changes made in one alternative do carry on
1432 into subsequent branches within the same subpattern. For example,
1433 .sp
1434 (a(?i)b|c)
1435 .sp
1436 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1437 branch is abandoned before the option setting. This is because the effects of
1438 option settings happen at compile time. There would be some very weird
1439 behaviour otherwise.
1440 .P
1441 \fBNote:\fP There are other PCRE-specific options that can be set by the
1442 application when the compiling or matching functions are called. In some cases
1443 the pattern can contain special leading sequences such as (*CRLF) to override
1444 what the application has set or what has been defaulted. Details are given in
1445 the section entitled
1446 .\" HTML <a href="#newlineseq">
1447 .\" </a>
1448 "Newline sequences"
1449 .\"
1450 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1451 sequences that can be used to set UTF and Unicode property modes; they are
1452 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1453 options, respectively. The (*UTF) sequence is a generic version that can be
1454 used with any of the libraries. However, the application can set the
1455 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1456 .
1457 .
1458 .\" HTML <a name="subpattern"></a>
1459 .SH SUBPATTERNS
1460 .rs
1461 .sp
1462 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1463 Turning part of a pattern into a subpattern does two things:
1464 .sp
1465 1. It localizes a set of alternatives. For example, the pattern
1466 .sp
1467 cat(aract|erpillar|)
1468 .sp
1469 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1470 match "cataract", "erpillar" or an empty string.
1471 .sp
1472 2. It sets up the subpattern as a capturing subpattern. This means that, when
1473 the whole pattern matches, that portion of the subject string that matched the
1474 subpattern is passed back to the caller via the \fIovector\fP argument of the
1475 matching function. (This applies only to the traditional matching functions;
1476 the DFA matching functions do not support capturing.)
1477 .P
1478 Opening parentheses are counted from left to right (starting from 1) to obtain
1479 numbers for the capturing subpatterns. For example, if the string "the red
1480 king" is matched against the pattern
1481 .sp
1482 the ((red|white) (king|queen))
1483 .sp
1484 the captured substrings are "red king", "red", and "king", and are numbered 1,
1485 2, and 3, respectively.
1486 .P
1487 The fact that plain parentheses fulfil two functions is not always helpful.
1488 There are often times when a grouping subpattern is required without a
1489 capturing requirement. If an opening parenthesis is followed by a question mark
1490 and a colon, the subpattern does not do any capturing, and is not counted when
1491 computing the number of any subsequent capturing subpatterns. For example, if
1492 the string "the white queen" is matched against the pattern
1493 .sp
1494 the ((?:red|white) (king|queen))
1495 .sp
1496 the captured substrings are "white queen" and "queen", and are numbered 1 and
1497 2. The maximum number of capturing subpatterns is 65535.
1498 .P
1499 As a convenient shorthand, if any option settings are required at the start of
1500 a non-capturing subpattern, the option letters may appear between the "?" and
1501 the ":". Thus the two patterns
1502 .sp
1503 (?i:saturday|sunday)
1504 (?:(?i)saturday|sunday)
1505 .sp
1506 match exactly the same set of strings. Because alternative branches are tried
1507 from left to right, and options are not reset until the end of the subpattern
1508 is reached, an option setting in one branch does affect subsequent branches, so
1509 the above patterns match "SUNDAY" as well as "Saturday".
1510 .
1511 .
1512 .\" HTML <a name="dupsubpatternnumber"></a>
1513 .SH "DUPLICATE SUBPATTERN NUMBERS"
1514 .rs
1515 .sp
1516 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1517 the same numbers for its capturing parentheses. Such a subpattern starts with
1518 (?| and is itself a non-capturing subpattern. For example, consider this
1519 pattern:
1520 .sp
1521 (?|(Sat)ur|(Sun))day
1522 .sp
1523 Because the two alternatives are inside a (?| group, both sets of capturing
1524 parentheses are numbered one. Thus, when the pattern matches, you can look
1525 at captured substring number one, whichever alternative matched. This construct
1526 is useful when you want to capture part, but not all, of one of a number of
1527 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1528 number is reset at the start of each branch. The numbers of any capturing
1529 parentheses that follow the subpattern start after the highest number used in
1530 any branch. The following example is taken from the Perl documentation. The
1531 numbers underneath show in which buffer the captured content will be stored.
1532 .sp
1533 # before ---------------branch-reset----------- after
1534 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1535 # 1 2 2 3 2 3 4
1536 .sp
1537 A back reference to a numbered subpattern uses the most recent value that is
1538 set for that number by any subpattern. The following pattern matches "abcabc"
1539 or "defdef":
1540 .sp
1541 /(?|(abc)|(def))\e1/
1542 .sp
1543 In contrast, a subroutine call to a numbered subpattern always refers to the
1544 first one in the pattern with the given number. The following pattern matches
1545 "abcabc" or "defabc":
1546 .sp
1547 /(?|(abc)|(def))(?1)/
1548 .sp
1549 If a
1550 .\" HTML <a href="#conditions">
1551 .\" </a>
1552 condition test
1553 .\"
1554 for a subpattern's having matched refers to a non-unique number, the test is
1555 true if any of the subpatterns of that number have matched.
1556 .P
1557 An alternative approach to using this "branch reset" feature is to use
1558 duplicate named subpatterns, as described in the next section.
1559 .
1560 .
1561 .SH "NAMED SUBPATTERNS"
1562 .rs
1563 .sp
1564 Identifying capturing parentheses by number is simple, but it can be very hard
1565 to keep track of the numbers in complicated regular expressions. Furthermore,
1566 if an expression is modified, the numbers may change. To help with this
1567 difficulty, PCRE supports the naming of subpatterns. This feature was not
1568 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1569 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1570 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1571 have different names, but PCRE does not.
1572 .P
1573 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1574 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1575 parentheses from other parts of the pattern, such as
1576 .\" HTML <a href="#backreferences">
1577 .\" </a>
1578 back references,
1579 .\"
1580 .\" HTML <a href="#recursion">
1581 .\" </a>
1582 recursion,
1583 .\"
1584 and
1585 .\" HTML <a href="#conditions">
1586 .\" </a>
1587 conditions,
1588 .\"
1589 can be made by name as well as by number.
1590 .P
1591 Names consist of up to 32 alphanumeric characters and underscores. Named
1592 capturing parentheses are still allocated numbers as well as names, exactly as
1593 if the names were not present. The PCRE API provides function calls for
1594 extracting the name-to-number translation table from a compiled pattern. There
1595 is also a convenience function for extracting a captured substring by name.
1596 .P
1597 By default, a name must be unique within a pattern, but it is possible to relax
1598 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1599 names are also always permitted for subpatterns with the same number, set up as
1600 described in the previous section.) Duplicate names can be useful for patterns
1601 where only one instance of the named parentheses can match. Suppose you want to
1602 match the name of a weekday, either as a 3-letter abbreviation or as the full
1603 name, and in both cases you want to extract the abbreviation. This pattern
1604 (ignoring the line breaks) does the job:
1605 .sp
1606 (?<DN>Mon|Fri|Sun)(?:day)?|
1607 (?<DN>Tue)(?:sday)?|
1608 (?<DN>Wed)(?:nesday)?|
1609 (?<DN>Thu)(?:rsday)?|
1610 (?<DN>Sat)(?:urday)?
1611 .sp
1612 There are five capturing substrings, but only one is ever set after a match.
1613 (An alternative way of solving this problem is to use a "branch reset"
1614 subpattern, as described in the previous section.)
1615 .P
1616 The convenience function for extracting the data by name returns the substring
1617 for the first (and in this example, the only) subpattern of that name that
1618 matched. This saves searching to find which numbered subpattern it was.
1619 .P
1620 If you make a back reference to a non-unique named subpattern from elsewhere in
1621 the pattern, the subpatterns to which the name refers are checked in the order
1622 in which they appear in the overall pattern. The first one that is set is used
1623 for the reference. For example, this pattern matches both "foofoo" and
1624 "barbar" but not "foobar" or "barfoo":
1625 .sp
1626 (?:(?<n>foo)|(?<n>bar))\k<n>
1627 .sp
1628 .P
1629 If you make a subroutine call to a non-unique named subpattern, the one that
1630 corresponds to the first occurrence of the name is used. In the absence of
1631 duplicate numbers (see the previous section) this is the one with the lowest
1632 number.
1633 .P
1634 If you use a named reference in a condition
1635 test (see the
1636 .\"
1637 .\" HTML <a href="#conditions">
1638 .\" </a>
1639 section about conditions
1640 .\"
1641 below), either to check whether a subpattern has matched, or to check for
1642 recursion, all subpatterns with the same name are tested. If the condition is
1643 true for any one of them, the overall condition is true. This is the same
1644 behaviour as testing by number. For further details of the interfaces for
1645 handling named subpatterns, see the
1646 .\" HREF
1647 \fBpcreapi\fP
1648 .\"
1649 documentation.
1650 .P
1651 \fBWarning:\fP You cannot use different names to distinguish between two
1652 subpatterns with the same number because PCRE uses only the numbers when
1653 matching. For this reason, an error is given at compile time if different names
1654 are given to subpatterns with the same number. However, you can always give the
1655 same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1656 set.
1657 .
1658 .
1659 .SH REPETITION
1660 .rs
1661 .sp
1662 Repetition is specified by quantifiers, which can follow any of the following
1663 items:
1664 .sp
1665 a literal data character
1666 the dot metacharacter
1667 the \eC escape sequence
1668 the \eX escape sequence
1669 the \eR escape sequence
1670 an escape such as \ed or \epL that matches a single character
1671 a character class
1672 a back reference (see next section)
1673 a parenthesized subpattern (including assertions)
1674 a subroutine call to a subpattern (recursive or otherwise)
1675 .sp
1676 The general repetition quantifier specifies a minimum and maximum number of
1677 permitted matches, by giving the two numbers in curly brackets (braces),
1678 separated by a comma. The numbers must be less than 65536, and the first must
1679 be less than or equal to the second. For example:
1680 .sp
1681 z{2,4}
1682 .sp
1683 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1684 character. If the second number is omitted, but the comma is present, there is
1685 no upper limit; if the second number and the comma are both omitted, the
1686 quantifier specifies an exact number of required matches. Thus
1687 .sp
1688 [aeiou]{3,}
1689 .sp
1690 matches at least 3 successive vowels, but may match many more, while
1691 .sp
1692 \ed{8}
1693 .sp
1694 matches exactly 8 digits. An opening curly bracket that appears in a position
1695 where a quantifier is not allowed, or one that does not match the syntax of a
1696 quantifier, is taken as a literal character. For example, {,6} is not a
1697 quantifier, but a literal string of four characters.
1698 .P
1699 In UTF modes, quantifiers apply to characters rather than to individual data
1700 units. Thus, for example, \ex{100}{2} matches two characters, each of
1701 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1702 \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1703 several data units long (and they may be of different lengths).
1704 .P
1705 The quantifier {0} is permitted, causing the expression to behave as if the
1706 previous item and the quantifier were not present. This may be useful for
1707 subpatterns that are referenced as
1708 .\" HTML <a href="#subpatternsassubroutines">
1709 .\" </a>
1710 subroutines
1711 .\"
1712 from elsewhere in the pattern (but see also the section entitled
1713 .\" HTML <a href="#subdefine">
1714 .\" </a>
1715 "Defining subpatterns for use by reference only"
1716 .\"
1717 below). Items other than subpatterns that have a {0} quantifier are omitted
1718 from the compiled pattern.
1719 .P
1720 For convenience, the three most common quantifiers have single-character
1721 abbreviations:
1722 .sp
1723 * is equivalent to {0,}
1724 + is equivalent to {1,}
1725 ? is equivalent to {0,1}
1726 .sp
1727 It is possible to construct infinite loops by following a subpattern that can
1728 match no characters with a quantifier that has no upper limit, for example:
1729 .sp
1730 (a?)*
1731 .sp
1732 Earlier versions of Perl and PCRE used to give an error at compile time for
1733 such patterns. However, because there are cases where this can be useful, such
1734 patterns are now accepted, but if any repetition of the subpattern does in fact
1735 match no characters, the loop is forcibly broken.
1736 .P
1737 By default, the quantifiers are "greedy", that is, they match as much as
1738 possible (up to the maximum number of permitted times), without causing the
1739 rest of the pattern to fail. The classic example of where this gives problems
1740 is in trying to match comments in C programs. These appear between /* and */
1741 and within the comment, individual * and / characters may appear. An attempt to
1742 match C comments by applying the pattern
1743 .sp
1744 /\e*.*\e*/
1745 .sp
1746 to the string
1747 .sp
1748 /* first comment */ not comment /* second comment */
1749 .sp
1750 fails, because it matches the entire string owing to the greediness of the .*
1751 item.
1752 .P
1753 However, if a quantifier is followed by a question mark, it ceases to be
1754 greedy, and instead matches the minimum number of times possible, so the
1755 pattern
1756 .sp
1757 /\e*.*?\e*/
1758 .sp
1759 does the right thing with the C comments. The meaning of the various
1760 quantifiers is not otherwise changed, just the preferred number of matches.
1761 Do not confuse this use of question mark with its use as a quantifier in its
1762 own right. Because it has two uses, it can sometimes appear doubled, as in
1763 .sp
1764 \ed??\ed
1765 .sp
1766 which matches one digit by preference, but can match two if that is the only
1767 way the rest of the pattern matches.
1768 .P
1769 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1770 the quantifiers are not greedy by default, but individual ones can be made
1771 greedy by following them with a question mark. In other words, it inverts the
1772 default behaviour.
1773 .P
1774 When a parenthesized subpattern is quantified with a minimum repeat count that
1775 is greater than 1 or with a limited maximum, more memory is required for the
1776 compiled pattern, in proportion to the size of the minimum or maximum.
1777 .P
1778 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1779 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1780 implicitly anchored, because whatever follows will be tried against every
1781 character position in the subject string, so there is no point in retrying the
1782 overall match at any position after the first. PCRE normally treats such a
1783 pattern as though it were preceded by \eA.
1784 .P
1785 In cases where it is known that the subject string contains no newlines, it is
1786 worth setting PCRE_DOTALL in order to obtain this optimization, or
1787 alternatively using ^ to indicate anchoring explicitly.
1788 .P
1789 However, there are some cases where the optimization cannot be used. When .*
1790 is inside capturing parentheses that are the subject of a back reference
1791 elsewhere in the pattern, a match at the start may fail where a later one
1792 succeeds. Consider, for example:
1793 .sp
1794 (.*)abc\e1
1795 .sp
1796 If the subject is "xyz123abc123" the match point is the fourth character. For
1797 this reason, such a pattern is not implicitly anchored.
1798 .P
1799 Another case where implicit anchoring is not applied is when the leading .* is
1800 inside an atomic group. Once again, a match at the start may fail where a later
1801 one succeeds. Consider this pattern:
1802 .sp
1803 (?>.*?a)b
1804 .sp
1805 It matches "ab" in the subject "aab". The use of the backtracking control verbs
1806 (*PRUNE) and (*SKIP) also disable this optimization.
1807 .P
1808 When a capturing subpattern is repeated, the value captured is the substring
1809 that matched the final iteration. For example, after
1810 .sp
1811 (tweedle[dume]{3}\es*)+
1812 .sp
1813 has matched "tweedledum tweedledee" the value of the captured substring is
1814 "tweedledee". However, if there are nested capturing subpatterns, the
1815 corresponding captured values may have been set in previous iterations. For
1816 example, after
1817 .sp
1818 /(a|(b))+/
1819 .sp
1820 matches "aba" the value of the second captured substring is "b".
1821 .
1822 .
1823 .\" HTML <a name="atomicgroup"></a>
1824 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1825 .rs
1826 .sp
1827 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1828 repetition, failure of what follows normally causes the repeated item to be
1829 re-evaluated to see if a different number of repeats allows the rest of the
1830 pattern to match. Sometimes it is useful to prevent this, either to change the
1831 nature of the match, or to cause it fail earlier than it otherwise might, when
1832 the author of the pattern knows there is no point in carrying on.
1833 .P
1834 Consider, for example, the pattern \ed+foo when applied to the subject line
1835 .sp
1836 123456bar
1837 .sp
1838 After matching all 6 digits and then failing to match "foo", the normal
1839 action of the matcher is to try again with only 5 digits matching the \ed+
1840 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1841 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1842 that once a subpattern has matched, it is not to be re-evaluated in this way.
1843 .P
1844 If we use atomic grouping for the previous example, the matcher gives up
1845 immediately on failing to match "foo" the first time. The notation is a kind of
1846 special parenthesis, starting with (?> as in this example:
1847 .sp
1848 (?>\ed+)foo
1849 .sp
1850 This kind of parenthesis "locks up" the part of the pattern it contains once
1851 it has matched, and a failure further into the pattern is prevented from
1852 backtracking into it. Backtracking past it to previous items, however, works as
1853 normal.
1854 .P
1855 An alternative description is that a subpattern of this type matches the string
1856 of characters that an identical standalone pattern would match, if anchored at
1857 the current point in the subject string.
1858 .P
1859 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1860 the above example can be thought of as a maximizing repeat that must swallow
1861 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1862 number of digits they match in order to make the rest of the pattern match,
1863 (?>\ed+) can only match an entire sequence of digits.
1864 .P
1865 Atomic groups in general can of course contain arbitrarily complicated
1866 subpatterns, and can be nested. However, when the subpattern for an atomic
1867 group is just a single repeated item, as in the example above, a simpler
1868 notation, called a "possessive quantifier" can be used. This consists of an
1869 additional + character following a quantifier. Using this notation, the
1870 previous example can be rewritten as
1871 .sp
1872 \ed++foo
1873 .sp
1874 Note that a possessive quantifier can be used with an entire group, for
1875 example:
1876 .sp
1877 (abc|xyz){2,3}+
1878 .sp
1879 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1880 option is ignored. They are a convenient notation for the simpler forms of
1881 atomic group. However, there is no difference in the meaning of a possessive
1882 quantifier and the equivalent atomic group, though there may be a performance
1883 difference; possessive quantifiers should be slightly faster.
1884 .P
1885 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1886 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1887 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1888 package, and PCRE copied it from there. It ultimately found its way into Perl
1889 at release 5.10.
1890 .P
1891 PCRE has an optimization that automatically "possessifies" certain simple
1892 pattern constructs. For example, the sequence A+B is treated as A++B because
1893 there is no point in backtracking into a sequence of A's when B must follow.
1894 .P
1895 When a pattern contains an unlimited repeat inside a subpattern that can itself
1896 be repeated an unlimited number of times, the use of an atomic group is the
1897 only way to avoid some failing matches taking a very long time indeed. The
1898 pattern
1899 .sp
1900 (\eD+|<\ed+>)*[!?]
1901 .sp
1902 matches an unlimited number of substrings that either consist of non-digits, or
1903 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1904 quickly. However, if it is applied to
1905 .sp
1906 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1907 .sp
1908 it takes a long time before reporting failure. This is because the string can
1909 be divided between the internal \eD+ repeat and the external * repeat in a
1910 large number of ways, and all have to be tried. (The example uses [!?] rather
1911 than a single character at the end, because both PCRE and Perl have an
1912 optimization that allows for fast failure when a single character is used. They
1913 remember the last single character that is required for a match, and fail early
1914 if it is not present in the string.) If the pattern is changed so that it uses
1915 an atomic group, like this:
1916 .sp
1917 ((?>\eD+)|<\ed+>)*[!?]
1918 .sp
1919 sequences of non-digits cannot be broken, and failure happens quickly.
1920 .
1921 .
1922 .\" HTML <a name="backreferences"></a>
1923 .SH "BACK REFERENCES"
1924 .rs
1925 .sp
1926 Outside a character class, a backslash followed by a digit greater than 0 (and
1927 possibly further digits) is a back reference to a capturing subpattern earlier
1928 (that is, to its left) in the pattern, provided there have been that many
1929 previous capturing left parentheses.
1930 .P
1931 However, if the decimal number following the backslash is less than 10, it is
1932 always taken as a back reference, and causes an error only if there are not
1933 that many capturing left parentheses in the entire pattern. In other words, the
1934 parentheses that are referenced need not be to the left of the reference for
1935 numbers less than 10. A "forward back reference" of this type can make sense
1936 when a repetition is involved and the subpattern to the right has participated
1937 in an earlier iteration.
1938 .P
1939 It is not possible to have a numerical "forward back reference" to a subpattern
1940 whose number is 10 or more using this syntax because a sequence such as \e50 is
1941 interpreted as a character defined in octal. See the subsection entitled
1942 "Non-printing characters"
1943 .\" HTML <a href="#digitsafterbackslash">
1944 .\" </a>
1945 above
1946 .\"
1947 for further details of the handling of digits following a backslash. There is
1948 no such problem when named parentheses are used. A back reference to any
1949 subpattern is possible using named parentheses (see below).
1950 .P
1951 Another way of avoiding the ambiguity inherent in the use of digits following a
1952 backslash is to use the \eg escape sequence. This escape must be followed by an
1953 unsigned number or a negative number, optionally enclosed in braces. These
1954 examples are all identical:
1955 .sp
1956 (ring), \e1
1957 (ring), \eg1
1958 (ring), \eg{1}
1959 .sp
1960 An unsigned number specifies an absolute reference without the ambiguity that
1961 is present in the older syntax. It is also useful when literal digits follow
1962 the reference. A negative number is a relative reference. Consider this
1963 example:
1964 .sp
1965 (abc(def)ghi)\eg{-1}
1966 .sp
1967 The sequence \eg{-1} is a reference to the most recently started capturing
1968 subpattern before \eg, that is, is it equivalent to \e2 in this example.
1969 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1970 can be helpful in long patterns, and also in patterns that are created by
1971 joining together fragments that contain references within themselves.
1972 .P
1973 A back reference matches whatever actually matched the capturing subpattern in
1974 the current subject string, rather than anything matching the subpattern
1975 itself (see
1976 .\" HTML <a href="#subpatternsassubroutines">
1977 .\" </a>
1978 "Subpatterns as subroutines"
1979 .\"
1980 below for a way of doing that). So the pattern
1981 .sp
1982 (sens|respons)e and \e1ibility
1983 .sp
1984 matches "sense and sensibility" and "response and responsibility", but not
1985 "sense and responsibility". If caseful matching is in force at the time of the
1986 back reference, the case of letters is relevant. For example,
1987 .sp
1988 ((?i)rah)\es+\e1
1989 .sp
1990 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1991 capturing subpattern is matched caselessly.
1992 .P
1993 There are several different ways of writing back references to named
1994 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1995 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1996 back reference syntax, in which \eg can be used for both numeric and named
1997 references, is also supported. We could rewrite the above example in any of
1998 the following ways:
1999 .sp
2000 (?<p1>(?i)rah)\es+\ek<p1>
2001 (?'p1'(?i)rah)\es+\ek{p1}
2002 (?P<p1>(?i)rah)\es+(?P=p1)
2003 (?<p1>(?i)rah)\es+\eg{p1}
2004 .sp
2005 A subpattern that is referenced by name may appear in the pattern before or
2006 after the reference.
2007 .P
2008 There may be more than one back reference to the same subpattern. If a
2009 subpattern has not actually been used in a particular match, any back
2010 references to it always fail by default. For example, the pattern
2011 .sp
2012 (a|(bc))\e2
2013 .sp
2014 always fails if it starts to match "a" rather than "bc". However, if the
2015 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
2016 unset value matches an empty string.
2017 .P
2018 Because there may be many capturing parentheses in a pattern, all digits
2019 following a backslash are taken as part of a potential back reference number.
2020 If the pattern continues with a digit character, some delimiter must be used to
2021 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2022 white space. Otherwise, the \eg{ syntax or an empty comment (see
2023 .\" HTML <a href="#comments">
2024 .\" </a>
2025 "Comments"
2026 .\"
2027 below) can be used.
2028 .
2029 .SS "Recursive back references"
2030 .rs
2031 .sp
2032 A back reference that occurs inside the parentheses to which it refers fails
2033 when the subpattern is first used, so, for example, (a\e1) never matches.
2034 However, such references can be useful inside repeated subpatterns. For
2035 example, the pattern
2036 .sp
2037 (a|b\e1)+
2038 .sp
2039 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
2040 the subpattern, the back reference matches the character string corresponding
2041 to the previous iteration. In order for this to work, the pattern must be such
2042 that the first iteration does not need to match the back reference. This can be
2043 done using alternation, as in the example above, or by a quantifier with a
2044 minimum of zero.
2045 .P
2046 Back references of this type cause the group that they reference to be treated
2047 as an
2048 .\" HTML <a href="#atomicgroup">
2049 .\" </a>
2050 atomic group.
2051 .\"
2052 Once the whole group has been matched, a subsequent matching failure cannot
2053 cause backtracking into the middle of the group.
2054 .
2055 .
2056 .\" HTML <a name="bigassertions"></a>
2057 .SH ASSERTIONS
2058 .rs
2059 .sp
2060 An assertion is a test on the characters following or preceding the current
2061 matching point that does not actually consume any characters. The simple
2062 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
2063 .\" HTML <a href="#smallassertions">
2064 .\" </a>
2065 above.
2066 .\"
2067 .P
2068 More complicated assertions are coded as subpatterns. There are two kinds:
2069 those that look ahead of the current position in the subject string, and those
2070 that look behind it. An assertion subpattern is matched in the normal way,
2071 except that it does not cause the current matching position to be changed.
2072 .P
2073 Assertion subpatterns are not capturing subpatterns. If such an assertion
2074 contains capturing subpatterns within it, these are counted for the purposes of
2075 numbering the capturing subpatterns in the whole pattern. However, substring
2076 capturing is carried out only for positive assertions. (Perl sometimes, but not
2077 always, does do capturing in negative assertions.)
2078 .P
2079 For compatibility with Perl, assertion subpatterns may be repeated; though
2080 it makes no sense to assert the same thing several times, the side effect of
2081 capturing parentheses may occasionally be useful. In practice, there only three
2082 cases:
2083 .sp
2084 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2085 However, it may contain internal capturing parenthesized groups that are called
2086 from elsewhere via the
2087 .\" HTML <a href="#subpatternsassubroutines">
2088 .\" </a>
2089 subroutine mechanism.
2090 .\"
2091 .sp
2092 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2093 were {0,1}. At run time, the rest of the pattern match is tried with and
2094 without the assertion, the order depending on the greediness of the quantifier.
2095 .sp
2096 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2097 The assertion is obeyed just once when encountered during matching.
2098 .
2099 .
2100 .SS "Lookahead assertions"
2101 .rs
2102 .sp
2103 Lookahead assertions start with (?= for positive assertions and (?! for
2104 negative assertions. For example,
2105 .sp
2106 \ew+(?=;)
2107 .sp
2108 matches a word followed by a semicolon, but does not include the semicolon in
2109 the match, and
2110 .sp
2111 foo(?!bar)
2112 .sp
2113 matches any occurrence of "foo" that is not followed by "bar". Note that the
2114 apparently similar pattern
2115 .sp
2116 (?!foo)bar
2117 .sp
2118 does not find an occurrence of "bar" that is preceded by something other than
2119 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2120 (?!foo) is always true when the next three characters are "bar". A
2121 lookbehind assertion is needed to achieve the other effect.
2122 .P
2123 If you want to force a matching failure at some point in a pattern, the most
2124 convenient way to do it is with (?!) because an empty string always matches, so
2125 an assertion that requires there not to be an empty string must always fail.
2126 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2127 .
2128 .
2129 .\" HTML <a name="lookbehind"></a>
2130 .SS "Lookbehind assertions"
2131 .rs
2132 .sp
2133 Lookbehind assertions start with (?<= for positive assertions and (?<! for
2134 negative assertions. For example,
2135 .sp
2136 (?<!foo)bar
2137 .sp
2138 does find an occurrence of "bar" that is not preceded by "foo". The contents of
2139 a lookbehind assertion are restricted such that all the strings it matches must
2140 have a fixed length. However, if there are several top-level alternatives, they
2141 do not all have to have the same fixed length. Thus
2142 .sp
2143 (?<=bullock|donkey)
2144 .sp
2145 is permitted, but
2146 .sp
2147 (?<!dogs?|cats?)
2148 .sp
2149 causes an error at compile time. Branches that match different length strings
2150 are permitted only at the top level of a lookbehind assertion. This is an
2151 extension compared with Perl, which requires all branches to match the same
2152 length of string. An assertion such as
2153 .sp
2154 (?<=ab(c|de))
2155 .sp
2156 is not permitted, because its single top-level branch can match two different
2157 lengths, but it is acceptable to PCRE if rewritten to use two top-level
2158 branches:
2159 .sp
2160 (?<=abc|abde)
2161 .sp
2162 In some cases, the escape sequence \eK
2163 .\" HTML <a href="#resetmatchstart">
2164 .\" </a>
2165 (see above)
2166 .\"
2167 can be used instead of a lookbehind assertion to get round the fixed-length
2168 restriction.
2169 .P
2170 The implementation of lookbehind assertions is, for each alternative, to
2171 temporarily move the current position back by the fixed length and then try to
2172 match. If there are insufficient characters before the current position, the
2173 assertion fails.
2174 .P
2175 In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2176 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2177 it impossible to calculate the length of the lookbehind. The \eX and \eR
2178 escapes, which can match different numbers of data units, are also not
2179 permitted.
2180 .P
2181 .\" HTML <a href="#subpatternsassubroutines">
2182 .\" </a>
2183 "Subroutine"
2184 .\"
2185 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2186 as the subpattern matches a fixed-length string.
2187 .\" HTML <a href="#recursion">
2188 .\" </a>
2189 Recursion,
2190 .\"
2191 however, is not supported.
2192 .P
2193 Possessive quantifiers can be used in conjunction with lookbehind assertions to
2194 specify efficient matching of fixed-length strings at the end of subject
2195 strings. Consider a simple pattern such as
2196 .sp
2197 abcd$
2198 .sp
2199 when applied to a long string that does not match. Because matching proceeds
2200 from left to right, PCRE will look for each "a" in the subject and then see if
2201 what follows matches the rest of the pattern. If the pattern is specified as
2202 .sp
2203 ^.*abcd$
2204 .sp
2205 the initial .* matches the entire string at first, but when this fails (because
2206 there is no following "a"), it backtracks to match all but the last character,
2207 then all but the last two characters, and so on. Once again the search for "a"
2208 covers the entire string, from right to left, so we are no better off. However,
2209 if the pattern is written as
2210 .sp
2211 ^.*+(?<=abcd)
2212 .sp
2213 there can be no backtracking for the .*+ item; it can match only the entire
2214 string. The subsequent lookbehind assertion does a single test on the last four
2215 characters. If it fails, the match fails immediately. For long strings, this
2216 approach makes a significant difference to the processing time.
2217 .
2218 .
2219 .SS "Using multiple assertions"
2220 .rs
2221 .sp
2222 Several assertions (of any sort) may occur in succession. For example,
2223 .sp
2224 (?<=\ed{3})(?<!999)foo
2225 .sp
2226 matches "foo" preceded by three digits that are not "999". Notice that each of
2227 the assertions is applied independently at the same point in the subject
2228 string. First there is a check that the previous three characters are all
2229 digits, and then there is a check that the same three characters are not "999".
2230 This pattern does \fInot\fP match "foo" preceded by six characters, the first
2231 of which are digits and the last three of which are not "999". For example, it
2232 doesn't match "123abcfoo". A pattern to do that is
2233 .sp
2234 (?<=\ed{3}...)(?<!999)foo
2235 .sp
2236 This time the first assertion looks at the preceding six characters, checking
2237 that the first three are digits, and then the second assertion checks that the
2238 preceding three characters are not "999".
2239 .P
2240 Assertions can be nested in any combination. For example,
2241 .sp
2242 (?<=(?<!foo)bar)baz
2243 .sp
2244 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2245 preceded by "foo", while
2246 .sp
2247 (?<=\ed{3}(?!999)...)foo
2248 .sp
2249 is another pattern that matches "foo" preceded by three digits and any three
2250 characters that are not "999".
2251 .
2252 .
2253 .\" HTML <a name="conditions"></a>
2254 .SH "CONDITIONAL SUBPATTERNS"
2255 .rs
2256 .sp
2257 It is possible to cause the matching process to obey a subpattern
2258 conditionally or to choose between two alternative subpatterns, depending on
2259 the result of an assertion, or whether a specific capturing subpattern has
2260 already been matched. The two possible forms of conditional subpattern are:
2261 .sp
2262 (?(condition)yes-pattern)
2263 (?(condition)yes-pattern|no-pattern)
2264 .sp
2265 If the condition is satisfied, the yes-pattern is used; otherwise the
2266 no-pattern (if present) is used. If there are more than two alternatives in the
2267 subpattern, a compile-time error occurs. Each of the two alternatives may
2268 itself contain nested subpatterns of any form, including conditional
2269 subpatterns; the restriction to two alternatives applies only at the level of
2270 the condition. This pattern fragment is an example where the alternatives are
2271 complex:
2272 .sp
2273 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2274 .sp
2275 .P
2276 There are four kinds of condition: references to subpatterns, references to
2277 recursion, a pseudo-condition called DEFINE, and assertions.
2278 .
2279 .SS "Checking for a used subpattern by number"
2280 .rs
2281 .sp
2282 If the text between the parentheses consists of a sequence of digits, the
2283 condition is true if a capturing subpattern of that number has previously
2284 matched. If there is more than one capturing subpattern with the same number
2285 (see the earlier
2286 .\"
2287 .\" HTML <a href="#recursion">
2288 .\" </a>
2289 section about duplicate subpattern numbers),
2290 .\"
2291 the condition is true if any of them have matched. An alternative notation is
2292 to precede the digits with a plus or minus sign. In this case, the subpattern
2293 number is relative rather than absolute. The most recently opened parentheses
2294 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2295 loops it can also make sense to refer to subsequent groups. The next
2296 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2297 zero in any of these forms is not used; it provokes a compile-time error.)
2298 .P
2299 Consider the following pattern, which contains non-significant white space to
2300 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2301 three parts for ease of discussion:
2302 .sp
2303 ( \e( )? [^()]+ (?(1) \e) )
2304 .sp
2305 The first part matches an optional opening parenthesis, and if that
2306 character is present, sets it as the first captured substring. The second part
2307 matches one or more characters that are not parentheses. The third part is a
2308 conditional subpattern that tests whether or not the first set of parentheses
2309 matched. If they did, that is, if subject started with an opening parenthesis,
2310 the condition is true, and so the yes-pattern is executed and a closing
2311 parenthesis is required. Otherwise, since no-pattern is not present, the
2312 subpattern matches nothing. In other words, this pattern matches a sequence of
2313 non-parentheses, optionally enclosed in parentheses.
2314 .P
2315 If you were embedding this pattern in a larger one, you could use a relative
2316 reference:
2317 .sp
2318 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2319 .sp
2320 This makes the fragment independent of the parentheses in the larger pattern.
2321 .
2322 .SS "Checking for a used subpattern by name"
2323 .rs
2324 .sp
2325 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2326 subpattern by name. For compatibility with earlier versions of PCRE, which had
2327 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2328 there is a possible ambiguity with this syntax, because subpattern names may
2329 consist entirely of digits. PCRE looks first for a named subpattern; if it
2330 cannot find one and the name consists entirely of digits, PCRE looks for a
2331 subpattern of that number, which must be greater than zero. Using subpattern
2332 names that consist entirely of digits is not recommended.
2333 .P
2334 Rewriting the above example to use a named subpattern gives this:
2335 .sp
2336 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2337 .sp
2338 If the name used in a condition of this kind is a duplicate, the test is
2339 applied to all subpatterns of the same name, and is true if any one of them has
2340 matched.
2341 .
2342 .SS "Checking for pattern recursion"
2343 .rs
2344 .sp
2345 If the condition is the string (R), and there is no subpattern with the name R,
2346 the condition is true if a recursive call to the whole pattern or any
2347 subpattern has been made. If digits or a name preceded by ampersand follow the
2348 letter R, for example:
2349 .sp
2350 (?(R3)...) or (?(R&name)...)
2351 .sp
2352 the condition is true if the most recent recursion is into a subpattern whose
2353 number or name is given. This condition does not check the entire recursion
2354 stack. If the name used in a condition of this kind is a duplicate, the test is
2355 applied to all subpatterns of the same name, and is true if any one of them is
2356 the most recent recursion.
2357 .P
2358 At "top level", all these recursion test conditions are false.
2359 .\" HTML <a href="#recursion">
2360 .\" </a>
2361 The syntax for recursive patterns
2362 .\"
2363 is described below.
2364 .
2365 .\" HTML <a name="subdefine"></a>
2366 .SS "Defining subpatterns for use by reference only"
2367 .rs
2368 .sp
2369 If the condition is the string (DEFINE), and there is no subpattern with the
2370 name DEFINE, the condition is always false. In this case, there may be only one
2371 alternative in the subpattern. It is always skipped if control reaches this
2372 point in the pattern; the idea of DEFINE is that it can be used to define
2373 subroutines that can be referenced from elsewhere. (The use of
2374 .\" HTML <a href="#subpatternsassubroutines">
2375 .\" </a>
2376 subroutines
2377 .\"
2378 is described below.) For example, a pattern to match an IPv4 address such as
2379 "192.168.23.245" could be written like this (ignore white space and line
2380 breaks):
2381 .sp
2382 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2383 \eb (?&byte) (\e.(?&byte)){3} \eb
2384 .sp
2385 The first part of the pattern is a DEFINE group inside which a another group
2386 named "byte" is defined. This matches an individual component of an IPv4
2387 address (a number less than 256). When matching takes place, this part of the
2388 pattern is skipped because DEFINE acts like a false condition. The rest of the
2389 pattern uses references to the named group to match the four dot-separated
2390 components of an IPv4 address, insisting on a word boundary at each end.
2391 .
2392 .SS "Assertion conditions"
2393 .rs
2394 .sp
2395 If the condition is not in any of the above formats, it must be an assertion.
2396 This may be a positive or negative lookahead or lookbehind assertion. Consider
2397 this pattern, again containing non-significant white space, and with the two
2398 alternatives on the second line:
2399 .sp
2400 (?(?=[^a-z]*[a-z])
2401 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2402 .sp
2403 The condition is a positive lookahead assertion that matches an optional
2404 sequence of non-letters followed by a letter. In other words, it tests for the
2405 presence of at least one letter in the subject. If a letter is found, the
2406 subject is matched against the first alternative; otherwise it is matched
2407 against the second. This pattern matches strings in one of the two forms
2408 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2409 .
2410 .
2411 .\" HTML <a name="comments"></a>
2412 .SH COMMENTS
2413 .rs
2414 .sp
2415 There are two ways of including comments in patterns that are processed by
2416 PCRE. In both cases, the start of the comment must not be in a character class,
2417 nor in the middle of any other sequence of related characters such as (?: or a
2418 subpattern name or number. The characters that make up a comment play no part
2419 in the pattern matching.
2420 .P
2421 The sequence (?# marks the start of a comment that continues up to the next
2422 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2423 option is set, an unescaped # character also introduces a comment, which in
2424 this case continues to immediately after the next newline character or
2425 character sequence in the pattern. Which characters are interpreted as newlines
2426 is controlled by the options passed to a compiling function or by a special
2427 sequence at the start of the pattern, as described in the section entitled
2428 .\" HTML <a href="#newlines">
2429 .\" </a>
2430 "Newline conventions"
2431 .\"
2432 above. Note that the end of this type of comment is a literal newline sequence
2433 in the pattern; escape sequences that happen to represent a newline do not
2434 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2435 default newline convention is in force:
2436 .sp
2437 abc #comment \en still comment
2438 .sp
2439 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2440 a newline in the pattern. The sequence \en is still literal at this stage, so
2441 it does not terminate the comment. Only an actual character with the code value
2442 0x0a (the default newline) does so.
2443 .
2444 .
2445 .\" HTML <a name="recursion"></a>
2446 .SH "RECURSIVE PATTERNS"
2447 .rs
2448 .sp
2449 Consider the problem of matching a string in parentheses, allowing for
2450 unlimited nested parentheses. Without the use of recursion, the best that can
2451 be done is to use a pattern that matches up to some fixed depth of nesting. It
2452 is not possible to handle an arbitrary nesting depth.
2453 .P
2454 For some time, Perl has provided a facility that allows regular expressions to
2455 recurse (amongst other things). It does this by interpolating Perl code in the
2456 expression at run time, and the code can refer to the expression itself. A Perl
2457 pattern using code interpolation to solve the parentheses problem can be
2458 created like this:
2459 .sp
2460 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2461 .sp
2462 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2463 recursively to the pattern in which it appears.
2464 .P
2465 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2466 supports special syntax for recursion of the entire pattern, and also for
2467 individual subpattern recursion. After its introduction in PCRE and Python,
2468 this kind of recursion was subsequently introduced into Perl at release 5.10.
2469 .P
2470 A special item that consists of (? followed by a number greater than zero and a
2471 closing parenthesis is a recursive subroutine call of the subpattern of the
2472 given number, provided that it occurs inside that subpattern. (If not, it is a
2473 .\" HTML <a href="#subpatternsassubroutines">
2474 .\" </a>
2475 non-recursive subroutine
2476 .\"
2477 call, which is described in the next section.) The special item (?R) or (?0) is
2478 a recursive call of the entire regular expression.
2479 .P
2480 This PCRE pattern solves the nested parentheses problem (assume the
2481 PCRE_EXTENDED option is set so that white space is ignored):
2482 .sp
2483 \e( ( [^()]++ | (?R) )* \e)
2484 .sp
2485 First it matches an opening parenthesis. Then it matches any number of
2486 substrings which can either be a sequence of non-parentheses, or a recursive
2487 match of the pattern itself (that is, a correctly parenthesized substring).
2488 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2489 to avoid backtracking into sequences of non-parentheses.
2490 .P
2491 If this were part of a larger pattern, you would not want to recurse the entire
2492 pattern, so instead you could use this:
2493 .sp
2494 ( \e( ( [^()]++ | (?1) )* \e) )
2495 .sp
2496 We have put the pattern into parentheses, and caused the recursion to refer to
2497 them instead of the whole pattern.
2498 .P
2499 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2500 is made easier by the use of relative references. Instead of (?1) in the
2501 pattern above you can write (?-2) to refer to the second most recently opened
2502 parentheses preceding the recursion. In other words, a negative number counts
2503 capturing parentheses leftwards from the point at which it is encountered.
2504 .P
2505 It is also possible to refer to subsequently opened parentheses, by writing
2506 references such as (?+2). However, these cannot be recursive because the
2507 reference is not inside the parentheses that are referenced. They are always
2508 .\" HTML <a href="#subpatternsassubroutines">
2509 .\" </a>
2510 non-recursive subroutine
2511 .\"
2512 calls, as described in the next section.
2513 .P
2514 An alternative approach is to use named parentheses instead. The Perl syntax
2515 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2516 could rewrite the above example as follows:
2517 .sp
2518 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2519 .sp
2520 If there is more than one subpattern with the same name, the earliest one is
2521 used.
2522 .P
2523 This particular example pattern that we have been looking at contains nested
2524 unlimited repeats, and so the use of a possessive quantifier for matching
2525 strings of non-parentheses is important when applying the pattern to strings
2526 that do not match. For example, when this pattern is applied to
2527 .sp
2528 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2529 .sp
2530 it yields "no match" quickly. However, if a possessive quantifier is not used,
2531 the match runs for a very long time indeed because there are so many different
2532 ways the + and * repeats can carve up the subject, and all have to be tested
2533 before failure can be reported.
2534 .P
2535 At the end of a match, the values of capturing parentheses are those from
2536 the outermost level. If you want to obtain intermediate values, a callout
2537 function can be used (see below and the
2538 .\" HREF
2539 \fBpcrecallout\fP
2540 .\"
2541 documentation). If the pattern above is matched against
2542 .sp
2543 (ab(cd)ef)
2544 .sp
2545 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2546 the last value taken on at the top level. If a capturing subpattern is not
2547 matched at the top level, its final captured value is unset, even if it was
2548 (temporarily) set at a deeper level during the matching process.
2549 .P
2550 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2551 obtain extra memory to store data during a recursion, which it does by using
2552 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2553 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2554 .P
2555 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2556 Consider this pattern, which matches text in angle brackets, allowing for
2557 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2558 recursing), whereas any characters are permitted at the outer level.
2559 .sp
2560 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2561 .sp
2562 In this pattern, (?(R) is the start of a conditional subpattern, with two
2563 different alternatives for the recursive and non-recursive cases. The (?R) item
2564 is the actual recursive call.
2565 .
2566 .
2567 .\" HTML <a name="recursiondifference"></a>
2568 .SS "Differences in recursion processing between PCRE and Perl"
2569 .rs
2570 .sp
2571 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2572 (like Python, but unlike Perl), a recursive subpattern call is always treated
2573 as an atomic group. That is, once it has matched some of the subject string, it
2574 is never re-entered, even if it contains untried alternatives and there is a
2575 subsequent matching failure. This can be illustrated by the following pattern,
2576 which purports to match a palindromic string that contains an odd number of
2577 characters (for example, "a", "aba", "abcba", "abcdcba"):
2578 .sp
2579 ^(.|(.)(?1)\e2)$
2580 .sp
2581 The idea is that it either matches a single character, or two identical
2582 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2583 it does not if the pattern is longer than three characters. Consider the
2584 subject string "abcba":
2585 .P
2586 At the top level, the first character is matched, but as it is not at the end
2587 of the string, the first alternative fails; the second alternative is taken
2588 and the recursion kicks in. The recursive call to subpattern 1 successfully
2589 matches the next character ("b"). (Note that the beginning and end of line
2590 tests are not part of the recursion).
2591 .P
2592 Back at the top level, the next character ("c") is compared with what
2593 subpattern 2 matched, which was "a". This fails. Because the recursion is
2594 treated as an atomic group, there are now no backtracking points, and so the
2595 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2596 try the second alternative.) However, if the pattern is written with the
2597 alternatives in the other order, things are different:
2598 .sp
2599 ^((.)(?1)\e2|.)$
2600 .sp
2601 This time, the recursing alternative is tried first, and continues to recurse
2602 until it runs out of characters, at which point the recursion fails. But this
2603 time we do have another alternative to try at the higher level. That is the big
2604 difference: in the previous case the remaining alternative is at a deeper
2605 recursion level, which PCRE cannot use.
2606 .P
2607 To change the pattern so that it matches all palindromic strings, not just
2608 those with an odd number of characters, it is tempting to change the pattern to
2609 this:
2610 .sp
2611 ^((.)(?1)\e2|.?)$
2612 .sp
2613 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2614 deeper recursion has matched a single character, it cannot be entered again in
2615 order to match an empty string. The solution is to separate the two cases, and
2616 write out the odd and even cases as alternatives at the higher level:
2617 .sp
2618 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2619 .sp
2620 If you want to match typical palindromic phrases, the pattern has to ignore all
2621 non-word characters, which can be done like this:
2622 .sp
2623 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2624 .sp
2625 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2626 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2627 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2628 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2629 more) to match typical phrases, and Perl takes so long that you think it has
2630 gone into a loop.
2631 .P
2632 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2633 string does not start with a palindrome that is shorter than the entire string.
2634 For example, although "abcba" is correctly matched, if the subject is "ababa",
2635 PCRE finds the palindrome "aba" at the start, then fails at top level because
2636 the end of the string does not follow. Once again, it cannot jump back into the
2637 recursion to try other alternatives, so the entire match fails.
2638 .P
2639 The second way in which PCRE and Perl differ in their recursion processing is
2640 in the handling of captured values. In Perl, when a subpattern is called
2641 recursively or as a subpattern (see the next section), it has no access to any
2642 values that were captured outside the recursion, whereas in PCRE these values
2643 can be referenced. Consider this pattern:
2644 .sp
2645 ^(.)(\e1|a(?2))
2646 .sp
2647 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2648 then in the second group, when the back reference \e1 fails to match "b", the
2649 second alternative matches "a" and then recurses. In the recursion, \e1 does
2650 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2651 match because inside the recursive call \e1 cannot access the externally set
2652 value.
2653 .
2654 .
2655 .\" HTML <a name="subpatternsassubroutines"></a>
2656 .SH "SUBPATTERNS AS SUBROUTINES"
2657 .rs
2658 .sp
2659 If the syntax for a recursive subpattern call (either by number or by
2660 name) is used outside the parentheses to which it refers, it operates like a
2661 subroutine in a programming language. The called subpattern may be defined
2662 before or after the reference. A numbered reference can be absolute or
2663 relative, as in these examples:
2664 .sp
2665 (...(absolute)...)...(?2)...
2666 (...(relative)...)...(?-1)...
2667 (...(?+1)...(relative)...
2668 .sp
2669 An earlier example pointed out that the pattern
2670 .sp
2671 (sens|respons)e and \e1ibility
2672 .sp
2673 matches "sense and sensibility" and "response and responsibility", but not
2674 "sense and responsibility". If instead the pattern
2675 .sp
2676 (sens|respons)e and (?1)ibility
2677 .sp
2678 is used, it does match "sense and responsibility" as well as the other two
2679 strings. Another example is given in the discussion of DEFINE above.
2680 .P
2681 All subroutine calls, whether recursive or not, are always treated as atomic
2682 groups. That is, once a subroutine has matched some of the subject string, it
2683 is never re-entered, even if it contains untried alternatives and there is a
2684 subsequent matching failure. Any capturing parentheses that are set during the
2685 subroutine call revert to their previous values afterwards.
2686 .P
2687 Processing options such as case-independence are fixed when a subpattern is
2688 defined, so if it is used as a subroutine, such options cannot be changed for
2689 different calls. For example, consider this pattern:
2690 .sp
2691 (abc)(?i:(?-1))
2692 .sp
2693 It matches "abcabc". It does not match "abcABC" because the change of
2694 processing option does not affect the called subpattern.
2695 .
2696 .
2697 .\" HTML <a name="onigurumasubroutines"></a>
2698 .SH "ONIGURUMA SUBROUTINE SYNTAX"
2699 .rs
2700 .sp
2701 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2702 a number enclosed either in angle brackets or single quotes, is an alternative
2703 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2704 are two of the examples used above, rewritten using this syntax:
2705 .sp
2706 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2707 (sens|respons)e and \eg'1'ibility
2708 .sp
2709 PCRE supports an extension to Oniguruma: if a number is preceded by a
2710 plus or a minus sign it is taken as a relative reference. For example:
2711 .sp
2712 (abc)(?i:\eg<-1>)
2713 .sp
2714 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2715 synonymous. The former is a back reference; the latter is a subroutine call.
2716 .
2717 .
2718 .SH CALLOUTS
2719 .rs
2720 .sp
2721 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2722 code to be obeyed in the middle of matching a regular expression. This makes it
2723 possible, amongst other things, to extract different substrings that match the
2724 same pair of parentheses when there is a repetition.
2725 .P
2726 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2727 code. The feature is called "callout". The caller of PCRE provides an external
2728 function by putting its entry point in the global variable \fIpcre_callout\fP
2729 (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2730 By default, this variable contains NULL, which disables all calling out.
2731 .P
2732 Within a regular expression, (?C) indicates the points at which the external
2733 function is to be called. If you want to identify different callout points, you
2734 can put a number less than 256 after the letter C. The default value is zero.
2735 For example, this pattern has two callout points:
2736 .sp
2737 (?C1)abc(?C2)def
2738 .sp
2739 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2740 automatically installed before each item in the pattern. They are all numbered
2741 255. If there is a conditional group in the pattern whose condition is an
2742 assertion, an additional callout is inserted just before the condition. An
2743 explicit callout may also be set at this position, as in this example:
2744 .sp
2745 (?(?C9)(?=a)abc|def)
2746 .sp
2747 Note that this applies only to assertion conditions, not to other types of
2748 condition.
2749 .P
2750 During matching, when PCRE reaches a callout point, the external function is
2751 called. It is provided with the number of the callout, the position in the
2752 pattern, and, optionally, one item of data originally supplied by the caller of
2753 the matching function. The callout function may cause matching to proceed, to
2754 backtrack, or to fail altogether. A complete description of the interface to
2755 the callout function is given in the
2756 .\" HREF
2757 \fBpcrecallout\fP
2758 .\"
2759 documentation.
2760 .
2761 .
2762 .\" HTML <a name="backtrackcontrol"></a>
2763 .SH "BACKTRACKING CONTROL"
2764 .rs
2765 .sp
2766 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2767 are still described in the Perl documentation as "experimental and subject to
2768 change or removal in a future version of Perl". It goes on to say: "Their usage
2769 in production code should be noted to avoid problems during upgrades." The same
2770 remarks apply to the PCRE features described in this section.
2771 .P
2772 The new verbs make use of what was previously invalid syntax: an opening
2773 parenthesis followed by an asterisk. They are generally of the form
2774 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2775 differently depending on whether or not a name is present. A name is any
2776 sequence of characters that does not include a closing parenthesis. The maximum
2777 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2778 libraries. If the name is empty, that is, if the closing parenthesis
2779 immediately follows the colon, the effect is as if the colon were not there.
2780 Any number of these verbs may occur in a pattern.
2781 .P
2782 Since these verbs are specifically related to backtracking, most of them can be
2783 used only when the pattern is to be matched using one of the traditional
2784 matching functions, because these use a backtracking algorithm. With the
2785 exception of (*FAIL), which behaves like a failing negative assertion, the
2786 backtracking control verbs cause an error if encountered by a DFA matching
2787 function.
2788 .P
2789 The behaviour of these verbs in
2790 .\" HTML <a href="#btrepeat">
2791 .\" </a>
2792 repeated groups,
2793 .\"
2794 .\" HTML <a href="#btassert">
2795 .\" </a>
2796 assertions,
2797 .\"
2798 and in
2799 .\" HTML <a href="#btsub">
2800 .\" </a>
2801 subpatterns called as subroutines
2802 .\"
2803 (whether or not recursively) is documented below.
2804 .
2805 .
2806 .\" HTML <a name="nooptimize"></a>
2807 .SS "Optimizations that affect backtracking verbs"
2808 .rs
2809 .sp
2810 PCRE contains some optimizations that are used to speed up matching by running
2811 some checks at the start of each match attempt. For example, it may know the
2812 minimum length of matching subject, or that a particular character must be
2813 present. When one of these optimizations bypasses the running of a match, any
2814 included backtracking verbs will not, of course, be processed. You can suppress
2815 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2816 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2817 pattern with (*NO_START_OPT). There is more discussion of this option in the
2818 section entitled
2819 .\" HTML <a href="pcreapi.html#execoptions">
2820 .\" </a>
2821 "Option bits for \fBpcre_exec()\fP"
2822 .\"
2823 in the
2824 .\" HREF
2825 \fBpcreapi\fP
2826 .\"
2827 documentation.
2828 .P
2829 Experiments with Perl suggest that it too has similar optimizations, sometimes
2830 leading to anomalous results.
2831 .
2832 .
2833 .SS "Verbs that act immediately"
2834 .rs
2835 .sp
2836 The following verbs act as soon as they are encountered. They may not be
2837 followed by a name.
2838 .sp
2839 (*ACCEPT)
2840 .sp
2841 This verb causes the match to end successfully, skipping the remainder of the
2842 pattern. However, when it is inside a subpattern that is called as a
2843 subroutine, only that subpattern is ended successfully. Matching then continues
2844 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2845 assertion succeeds; in a negative assertion, the assertion fails.
2846 .P
2847 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2848 example:
2849 .sp
2850 A((?:A|B(*ACCEPT)|C)D)
2851 .sp
2852 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2853 the outer parentheses.
2854 .sp
2855 (*FAIL) or (*F)
2856 .sp
2857 This verb causes a matching failure, forcing backtracking to occur. It is
2858 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2859 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2860 Perl features that are not present in PCRE. The nearest equivalent is the
2861 callout feature, as for example in this pattern:
2862 .sp
2863 a+(?C)(*FAIL)
2864 .sp
2865 A match with the string "aaaa" always fails, but the callout is taken before
2866 each backtrack happens (in this example, 10 times).
2867 .
2868 .
2869 .SS "Recording which path was taken"
2870 .rs
2871 .sp
2872 There is one verb whose main purpose is to track how a match was arrived at,
2873 though it also has a secondary use in conjunction with advancing the match
2874 starting point (see (*SKIP) below).
2875 .sp
2876 (*MARK:NAME) or (*:NAME)
2877 .sp
2878 A name is always required with this verb. There may be as many instances of
2879 (*MARK) as you like in a pattern, and their names do not have to be unique.
2880 .P
2881 When a match succeeds, the name of the last-encountered (*MARK:NAME),
2882 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2883 caller as described in the section entitled
2884 .\" HTML <a href="pcreapi.html#extradata">
2885 .\" </a>
2886 "Extra data for \fBpcre_exec()\fP"
2887 .\"
2888 in the
2889 .\" HREF
2890 \fBpcreapi\fP
2891 .\"
2892 documentation. Here is an example of \fBpcretest\fP output, where the /K
2893 modifier requests the retrieval and outputting of (*MARK) data:
2894 .sp
2895 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2896 data> XY
2897 0: XY
2898 MK: A
2899 XZ
2900 0: XZ
2901 MK: B
2902 .sp
2903 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2904 indicates which of the two alternatives matched. This is a more efficient way
2905 of obtaining this information than putting each alternative in its own
2906 capturing parentheses.
2907 .P
2908 If a verb with a name is encountered in a positive assertion that is true, the
2909 name is recorded and passed back if it is the last-encountered. This does not
2910 happen for negative assertions or failing positive assertions.
2911 .P
2912 After a partial match or a failed match, the last encountered name in the
2913 entire match process is returned. For example:
2914 .sp
2915 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2916 data> XP
2917 No match, mark = B
2918 .sp
2919 Note that in this unanchored example the mark is retained from the match
2920 attempt that started at the letter "X" in the subject. Subsequent match
2921 attempts starting at "P" and then with an empty string do not get as far as the
2922 (*MARK) item, but nevertheless do not reset it.
2923 .P
2924 If you are interested in (*MARK) values after failed matches, you should
2925 probably set the PCRE_NO_START_OPTIMIZE option
2926 .\" HTML <a href="#nooptimize">
2927 .\" </a>
2928 (see above)
2929 .\"
2930 to ensure that the match is always attempted.
2931 .
2932 .
2933 .SS "Verbs that act after backtracking"
2934 .rs
2935 .sp
2936 The following verbs do nothing when they are encountered. Matching continues
2937 with what follows, but if there is no subsequent match, causing a backtrack to
2938 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2939 the verb. However, when one of these verbs appears inside an atomic group or an
2940 assertion that is true, its effect is confined to that group, because once the
2941 group has been matched, there is never any backtracking into it. In this
2942 situation, backtracking can "jump back" to the left of the entire atomic group
2943 or assertion. (Remember also, as stated above, that this localization also
2944 applies in subroutine calls.)
2945 .P
2946 These verbs differ in exactly what kind of failure occurs when backtracking
2947 reaches them. The behaviour described below is what happens when the verb is
2948 not in a subroutine or an assertion. Subsequent sections cover these special
2949 cases.
2950 .sp
2951 (*COMMIT)
2952 .sp
2953 This verb, which may not be followed by a name, causes the whole match to fail
2954 outright if there is a later matching failure that causes backtracking to reach
2955 it. Even if the pattern is unanchored, no further attempts to find a match by
2956 advancing the starting point take place. If (*COMMIT) is the only backtracking
2957 verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2958 committed to finding a match at the current starting point, or not at all. For
2959 example:
2960 .sp
2961 a+(*COMMIT)b
2962 .sp
2963 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2964 dynamic anchor, or "I've started, so I must finish." The name of the most
2965 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2966 match failure.
2967 .P
2968 If there is more than one backtracking verb in a pattern, a different one that
2969 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2970 match does not always guarantee that a match must be at this starting point.
2971 .P
2972 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2973 unless PCRE's start-of-match optimizations are turned off, as shown in this
2974 \fBpcretest\fP example:
2975 .sp
2976 re> /(*COMMIT)abc/
2977 data> xyzabc
2978 0: abc
2979 xyzabc\eY
2980 No match
2981 .sp
2982 PCRE knows that any match must start with "a", so the optimization skips along
2983 the subject to "a" before running the first match attempt, which succeeds. When
2984 the optimization is disabled by the \eY escape in the second subject, the match
2985 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2986 starting points.
2987 .sp
2988 (*PRUNE) or (*PRUNE:NAME)
2989 .sp
2990 This verb causes the match to fail at the current starting position in the
2991 subject if there is a later matching failure that causes backtracking to reach
2992 it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2993 starting character then happens. Backtracking can occur as usual to the left of
2994 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
2995 if there is no match to the right, backtracking cannot cross (*PRUNE). In
2996 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
2997 possessive quantifier, but there are some uses of (*PRUNE) that cannot be
2998 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2999 as (*COMMIT).
3000 .P
3001 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3002 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3003 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3004 .sp
3005 (*SKIP)
3006 .sp
3007 This verb, when given without a name, is like (*PRUNE), except that if the
3008 pattern is unanchored, the "bumpalong" advance is not to the next character,
3009 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3010 signifies that whatever text was matched leading up to it cannot be part of a
3011 successful match. Consider:
3012 .sp
3013 a+(*SKIP)b
3014 .sp
3015 If the subject is "aaaac...", after the first match attempt fails (starting at
3016 the first character in the string), the starting point skips on to start the
3017 next attempt at "c". Note that a possessive quantifer does not have the same
3018 effect as this example; although it would suppress backtracking during the
3019 first match attempt, the second attempt would start at the second character
3020 instead of skipping on to "c".
3021 .sp
3022 (*SKIP:NAME)
3023 .sp
3024 When (*SKIP) has an associated name, its behaviour is modified. When it is
3025 triggered, the previous path through the pattern is searched for the most
3026 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3027 is to the subject position that corresponds to that (*MARK) instead of to where
3028 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3029 (*SKIP) is ignored.
3030 .P
3031 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3032 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3033 .sp
3034 (*THEN) or (*THEN:NAME)
3035 .sp
3036 This verb causes a skip to the next innermost alternative when backtracking
3037 reaches it. That is, it cancels any further backtracking within the current
3038 alternative. Its name comes from the observation that it can be used for a
3039 pattern-based if-then-else block:
3040 .sp
3041 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3042 .sp
3043 If the COND1 pattern matches, FOO is tried (and possibly further items after
3044 the end of the group if FOO succeeds); on failure, the matcher skips to the
3045 second alternative and tries COND2, without backtracking into COND1. If that
3046 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3047 more alternatives, so there is a backtrack to whatever came before the entire
3048 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3049 .P
3050 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3051 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3052 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3053 .P
3054 A subpattern that does not contain a | character is just a part of the
3055 enclosing alternative; it is not a nested alternation with only one
3056 alternative. The effect of (*THEN) extends beyond such a subpattern to the
3057 enclosing alternative. Consider this pattern, where A, B, etc. are complex
3058 pattern fragments that do not contain any | characters at this level:
3059 .sp
3060 A (B(*THEN)C) | D
3061 .sp
3062 If A and B are matched, but there is a failure in C, matching does not
3063 backtrack into A; instead it moves to the next alternative, that is, D.
3064 However, if the subpattern containing (*THEN) is given an alternative, it
3065 behaves differently:
3066 .sp
3067 A (B(*THEN)C | (*FAIL)) | D
3068 .sp
3069 The effect of (*THEN) is now confined to the inner subpattern. After a failure
3070 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3071 because there are no more alternatives to try. In this case, matching does now
3072 backtrack into A.
3073 .P
3074 Note that a conditional subpattern is not considered as having two
3075 alternatives, because only one is ever used. In other words, the | character in
3076 a conditional subpattern has a different meaning. Ignoring white space,
3077 consider:
3078 .sp
3079 ^.*? (?(?=a) a | b(*THEN)c )
3080 .sp
3081 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3082 it initially matches zero characters. The condition (?=a) then fails, the
3083 character "b" is matched, but "c" is not. At this point, matching does not
3084 backtrack to .*? as might perhaps be expected from the presence of the |
3085 character. The conditional subpattern is part of the single alternative that
3086 comprises the whole pattern, and so the match fails. (If there was a backtrack
3087 into .*?, allowing it to match "b", the match would succeed.)
3088 .P
3089 The verbs just described provide four different "strengths" of control when
3090 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3091 next alternative. (*PRUNE) comes next, failing the match at the current
3092 starting position, but allowing an advance to the next character (for an
3093 unanchored pattern). (*SKIP) is similar, except that the advance may be more
3094 than one character. (*COMMIT) is the strongest, causing the entire match to
3095 fail.
3096 .
3097 .
3098 .SS "More than one backtracking verb"
3099 .rs
3100 .sp
3101 If more than one backtracking verb is present in a pattern, the one that is
3102 backtracked onto first acts. For example, consider this pattern, where A, B,
3103 etc. are complex pattern fragments:
3104 .sp
3105 (A(*COMMIT)B(*THEN)C|ABD)
3106 .sp
3107 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3108 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3109 the next alternative (ABD) to be tried. This behaviour is consistent, but is
3110 not always the same as Perl's. It means that if two or more backtracking verbs
3111 appear in succession, all the the last of them has no effect. Consider this
3112 example:
3113 .sp
3114 ...(*COMMIT)(*PRUNE)...
3115 .sp
3116 If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3117 it to be triggered, and its action is taken. There can never be a backtrack
3118 onto (*COMMIT).
3119 .
3120 .
3121 .\" HTML <a name="btrepeat"></a>
3122 .SS "Backtracking verbs in repeated groups"
3123 .rs
3124 .sp
3125 PCRE differs from Perl in its handling of backtracking verbs in repeated
3126 groups. For example, consider:
3127 .sp
3128 /(a(*COMMIT)b)+ac/
3129 .sp
3130 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3131 the second repeat of the group acts.
3132 .
3133 .
3134 .\" HTML <a name="btassert"></a>
3135 .SS "Backtracking verbs in assertions"
3136 .rs
3137 .sp
3138 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3139 .P
3140 (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3141 further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3142 fail without any further processing.
3143 .P
3144 The other backtracking verbs are not treated specially if they appear in a
3145 positive assertion. In particular, (*THEN) skips to the next alternative in the
3146 innermost enclosing group that has alternations, whether or not this is within
3147 the assertion.
3148 .P
3149 Negative assertions are, however, different, in order to ensure that changing a
3150 positive assertion into a negative assertion changes its result. Backtracking
3151 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3152 without considering any further alternative branches in the assertion.
3153 Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3154 within the assertion (the normal behaviour), but if the assertion does not have
3155 such an alternative, (*THEN) behaves like (*PRUNE).
3156 .
3157 .
3158 .\" HTML <a name="btsub"></a>
3159 .SS "Backtracking verbs in subroutines"
3160 .rs
3161 .sp
3162 These behaviours occur whether or not the subpattern is called recursively.
3163 Perl's treatment of subroutines is different in some cases.
3164 .P
3165 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3166 an immediate backtrack.
3167 .P
3168 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3169 succeed without any further processing. Matching then continues after the
3170 subroutine call.
3171 .P
3172 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3173 the subroutine match to fail.
3174 .P
3175 (*THEN) skips to the next alternative in the innermost enclosing group within
3176 the subpattern that has alternatives. If there is no such group within the
3177 subpattern, (*THEN) causes the subroutine match to fail.
3178 .
3179 .
3180 .SH "SEE ALSO"
3181 .rs
3182 .sp
3183 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3184 \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3185 .
3186 .
3187 .SH AUTHOR
3188 .rs
3189 .sp
3190 .nf
3191 Philip Hazel
3192 University Computing Service
3193 Cambridge CB2 3QH, England.
3194 .fi
3195 .
3196 .
3197 .SH REVISION
3198 .rs
3199 .sp
3200 .nf
3201 Last updated: 05 November 2013
3202 Copyright (c) 1997-2013 University of Cambridge.
3203 .fi

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