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1 nigel 63 <html>
2     <head>
3     <title>pcrepattern specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcrepattern man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10 ph10 111 <p>
11 nigel 75 This page is part of the PCRE HTML documentation. It was generated automatically
12     from the original man page. If there is any nonsense in it, please consult the
13     man page, in case the conversion went wrong.
14 ph10 111 <br>
15 nigel 63 <ul>
16     <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 ph10 227 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18     <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19     <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20     <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21 ph10 518 <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
22 ph10 227 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
23     <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24     <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25     <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26     <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27     <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28     <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29     <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30     <li><a name="TOC15" href="#SEC15">REPETITION</a>
31     <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
32     <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33     <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34     <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35     <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36     <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37     <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38 ph10 345 <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39     <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40     <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41     <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42     <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43     <li><a name="TOC28" href="#SEC28">REVISION</a>
44 nigel 63 </ul>
45     <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46     <P>
47 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE
48     are described in detail below. There is a quick-reference syntax summary in the
49     <a href="pcresyntax.html"><b>pcresyntax</b></a>
50 ph10 345 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51     also supports some alternative regular expression syntax (which does not
52     conflict with the Perl syntax) in order to provide some compatibility with
53     regular expressions in Python, .NET, and Oniguruma.
54     </P>
55     <P>
56     Perl's regular expressions are described in its own documentation, and
57 ph10 208 regular expressions in general are covered in a number of books, some of which
58     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59     published by O'Reilly, covers regular expressions in great detail. This
60     description of PCRE's regular expressions is intended as reference material.
61 nigel 63 </P>
62     <P>
63 nigel 75 The original operation of PCRE was on strings of one-byte characters. However,
64 ph10 461 there is now also support for UTF-8 character strings. To use this,
65     PCRE must be built to include UTF-8 support, and you must call
66     <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There
67     is also a special sequence that can be given at the start of a pattern:
68 ph10 416 <pre>
69     (*UTF8)
70     </pre>
71     Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
72     option. This feature is not Perl-compatible. How setting UTF-8 mode affects
73     pattern matching is mentioned in several places below. There is also a summary
74     of UTF-8 features in the
75 ph10 678 <a href="pcreunicode.html"><b>pcreunicode</b></a>
76 nigel 63 page.
77     </P>
78     <P>
79 ph10 535 Another special sequence that may appear at the start of a pattern or in
80     combination with (*UTF8) is:
81     <pre>
82     (*UCP)
83     </pre>
84     This has the same effect as setting the PCRE_UCP option: it causes sequences
85     such as \d and \w to use Unicode properties to determine character types,
86     instead of recognizing only characters with codes less than 128 via a lookup
87     table.
88     </P>
89     <P>
90 ph10 579 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
91     PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
92     also some more of these special sequences that are concerned with the handling
93     of newlines; they are described below.
94     </P>
95     <P>
96 nigel 77 The remainder of this document discusses the patterns that are supported by
97     PCRE when its main matching function, <b>pcre_exec()</b>, is used.
98     From release 6.0, PCRE offers a second matching function,
99     <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
100 ph10 172 Perl-compatible. Some of the features discussed below are not available when
101     <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
102     alternative function, and how it differs from the normal function, are
103     discussed in the
104 nigel 77 <a href="pcrematching.html"><b>pcrematching</b></a>
105     page.
106 ph10 567 <a name="newlines"></a></P>
107 ph10 227 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
108 nigel 77 <P>
109 ph10 227 PCRE supports five different conventions for indicating line breaks in
110     strings: a single CR (carriage return) character, a single LF (linefeed)
111     character, the two-character sequence CRLF, any of the three preceding, or any
112     Unicode newline sequence. The
113     <a href="pcreapi.html"><b>pcreapi</b></a>
114     page has
115     <a href="pcreapi.html#newlines">further discussion</a>
116     about newlines, and shows how to set the newline convention in the
117     <i>options</i> arguments for the compiling and matching functions.
118     </P>
119     <P>
120     It is also possible to specify a newline convention by starting a pattern
121     string with one of the following five sequences:
122     <pre>
123     (*CR) carriage return
124     (*LF) linefeed
125     (*CRLF) carriage return, followed by linefeed
126     (*ANYCRLF) any of the three above
127     (*ANY) all Unicode newline sequences
128     </pre>
129 ph10 461 These override the default and the options given to <b>pcre_compile()</b> or
130     <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default
131     newline sequence, the pattern
132 ph10 227 <pre>
133     (*CR)a.b
134     </pre>
135     changes the convention to CR. That pattern matches "a\nb" because LF is no
136     longer a newline. Note that these special settings, which are not
137     Perl-compatible, are recognized only at the very start of a pattern, and that
138 ph10 231 they must be in upper case. If more than one of them is present, the last one
139     is used.
140 ph10 227 </P>
141 ph10 231 <P>
142 ph10 518 The newline convention affects the interpretation of the dot metacharacter when
143     PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
144     affect what the \R escape sequence matches. By default, this is any Unicode
145     newline sequence, for Perl compatibility. However, this can be changed; see the
146     description of \R in the section entitled
147 ph10 231 <a href="#newlineseq">"Newline sequences"</a>
148 ph10 247 below. A change of \R setting can be combined with a change of newline
149     convention.
150 ph10 231 </P>
151 ph10 227 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
152     <P>
153 nigel 63 A regular expression is a pattern that is matched against a subject string from
154     left to right. Most characters stand for themselves in a pattern, and match the
155     corresponding characters in the subject. As a trivial example, the pattern
156     <pre>
157     The quick brown fox
158 nigel 75 </pre>
159 nigel 77 matches a portion of a subject string that is identical to itself. When
160     caseless matching is specified (the PCRE_CASELESS option), letters are matched
161     independently of case. In UTF-8 mode, PCRE always understands the concept of
162     case for characters whose values are less than 128, so caseless matching is
163     always possible. For characters with higher values, the concept of case is
164     supported if PCRE is compiled with Unicode property support, but not otherwise.
165     If you want to use caseless matching for characters 128 and above, you must
166     ensure that PCRE is compiled with Unicode property support as well as with
167     UTF-8 support.
168     </P>
169     <P>
170     The power of regular expressions comes from the ability to include alternatives
171     and repetitions in the pattern. These are encoded in the pattern by the use of
172 nigel 75 <i>metacharacters</i>, which do not stand for themselves but instead are
173 nigel 63 interpreted in some special way.
174     </P>
175     <P>
176 nigel 75 There are two different sets of metacharacters: those that are recognized
177 nigel 63 anywhere in the pattern except within square brackets, and those that are
178 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
179     are as follows:
180 nigel 63 <pre>
181     \ general escape character with several uses
182     ^ assert start of string (or line, in multiline mode)
183     $ assert end of string (or line, in multiline mode)
184     . match any character except newline (by default)
185     [ start character class definition
186     | start of alternative branch
187     ( start subpattern
188     ) end subpattern
189     ? extends the meaning of (
190     also 0 or 1 quantifier
191     also quantifier minimizer
192     * 0 or more quantifier
193     + 1 or more quantifier
194     also "possessive quantifier"
195     { start min/max quantifier
196 nigel 75 </pre>
197 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
198 nigel 75 a character class the only metacharacters are:
199 nigel 63 <pre>
200     \ general escape character
201     ^ negate the class, but only if the first character
202     - indicates character range
203 nigel 75 [ POSIX character class (only if followed by POSIX syntax)
204 nigel 63 ] terminates the character class
205 nigel 75 </pre>
206     The following sections describe the use of each of the metacharacters.
207 nigel 63 </P>
208 ph10 227 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
209 nigel 63 <P>
210     The backslash character has several uses. Firstly, if it is followed by a
211 ph10 579 character that is not a number or a letter, it takes away any special meaning
212     that character may have. This use of backslash as an escape character applies
213     both inside and outside character classes.
214 nigel 63 </P>
215     <P>
216     For example, if you want to match a * character, you write \* in the pattern.
217     This escaping action applies whether or not the following character would
218 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
219     non-alphanumeric with backslash to specify that it stands for itself. In
220 nigel 63 particular, if you want to match a backslash, you write \\.
221     </P>
222     <P>
223 ph10 579 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
224     backslash. All other characters (in particular, those whose codepoints are
225     greater than 127) are treated as literals.
226     </P>
227     <P>
228 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
229     pattern (other than in a character class) and characters between a # outside
230 nigel 91 a character class and the next newline are ignored. An escaping backslash can
231     be used to include a whitespace or # character as part of the pattern.
232 nigel 63 </P>
233     <P>
234     If you want to remove the special meaning from a sequence of characters, you
235     can do so by putting them between \Q and \E. This is different from Perl in
236     that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
237     Perl, $ and @ cause variable interpolation. Note the following examples:
238     <pre>
239     Pattern PCRE matches Perl matches
240 nigel 75
241     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
242 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
243     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
244 nigel 75 </pre>
245 nigel 63 The \Q...\E sequence is recognized both inside and outside character classes.
246 ph10 654 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
247     by \E later in the pattern, the literal interpretation continues to the end of
248     the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
249     a character class, this causes an error, because the character class is not
250     terminated.
251 nigel 75 <a name="digitsafterbackslash"></a></P>
252     <br><b>
253     Non-printing characters
254     </b><br>
255 nigel 63 <P>
256     A second use of backslash provides a way of encoding non-printing characters
257     in patterns in a visible manner. There is no restriction on the appearance of
258     non-printing characters, apart from the binary zero that terminates a pattern,
259 ph10 461 but when a pattern is being prepared by text editing, it is often easier to use
260     one of the following escape sequences than the binary character it represents:
261 nigel 63 <pre>
262     \a alarm, that is, the BEL character (hex 07)
263 ph10 579 \cx "control-x", where x is any ASCII character
264 nigel 63 \e escape (hex 1B)
265     \f formfeed (hex 0C)
266 ph10 227 \n linefeed (hex 0A)
267 nigel 63 \r carriage return (hex 0D)
268     \t tab (hex 09)
269 ph10 488 \ddd character with octal code ddd, or back reference
270 nigel 63 \xhh character with hex code hh
271 ph10 835 \x{hhh..} character with hex code hhh..
272 nigel 75 </pre>
273 nigel 63 The precise effect of \cx is as follows: if x is a lower case letter, it
274     is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
275 ph10 579 Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
276     \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
277     than 127, a compile-time error occurs. This locks out non-ASCII characters in
278     both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
279     values are valid. A lower case letter is converted to upper case, and then the
280     0xc0 bits are flipped.)
281 nigel 63 </P>
282     <P>
283 ph10 835 After \x, from zero to two hexadecimal digits are read (letters can be in
284     upper or lower case). Any number of hexadecimal digits may appear between \x{
285     and }, but the value of the character code must be less than 256 in non-UTF-8
286     mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
287     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
288     point, which is 10FFFF.
289 nigel 63 </P>
290     <P>
291 ph10 211 If characters other than hexadecimal digits appear between \x{ and }, or if
292     there is no terminating }, this form of escape is not recognized. Instead, the
293     initial \x will be interpreted as a basic hexadecimal escape, with no
294     following digits, giving a character whose value is zero.
295     </P>
296     <P>
297 nigel 63 Characters whose value is less than 256 can be defined by either of the two
298 ph10 835 syntaxes for \x. There is no difference in the way they are handled. For
299     example, \xdc is exactly the same as \x{dc}.
300 nigel 63 </P>
301     <P>
302 nigel 91 After \0 up to two further octal digits are read. If there are fewer than two
303     digits, just those that are present are used. Thus the sequence \0\x\07
304     specifies two binary zeros followed by a BEL character (code value 7). Make
305     sure you supply two digits after the initial zero if the pattern character that
306     follows is itself an octal digit.
307 nigel 63 </P>
308     <P>
309     The handling of a backslash followed by a digit other than 0 is complicated.
310     Outside a character class, PCRE reads it and any following digits as a decimal
311     number. If the number is less than 10, or if there have been at least that many
312     previous capturing left parentheses in the expression, the entire sequence is
313     taken as a <i>back reference</i>. A description of how this works is given
314 nigel 75 <a href="#backreferences">later,</a>
315     following the discussion of
316     <a href="#subpattern">parenthesized subpatterns.</a>
317 nigel 63 </P>
318     <P>
319     Inside a character class, or if the decimal number is greater than 9 and there
320     have not been that many capturing subpatterns, PCRE re-reads up to three octal
321 nigel 93 digits following the backslash, and uses them to generate a data character. Any
322 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
323     character specified in octal must be less than \400. In UTF-8 mode, values up
324     to \777 are permitted. For example:
325 nigel 63 <pre>
326     \040 is another way of writing a space
327 nigel 75 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
328 nigel 63 \7 is always a back reference
329 nigel 75 \11 might be a back reference, or another way of writing a tab
330 nigel 63 \011 is always a tab
331     \0113 is a tab followed by the character "3"
332 nigel 75 \113 might be a back reference, otherwise the character with octal code 113
333     \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
334     \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
335     </pre>
336 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
337     zero, because no more than three octal digits are ever read.
338     </P>
339     <P>
340 nigel 91 All the sequences that define a single character value can be used both inside
341 ph10 835 and outside character classes. In addition, inside a character class, the
342     sequence \b is interpreted as the backspace character (hex 08). The sequences
343     \B, \N, \R, and \X are not special inside a character class. Like any other
344     unrecognized escape sequences, they are treated as the literal characters "B",
345     "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
346     set. Outside a character class, these sequences have different meanings.
347 nigel 63 </P>
348 nigel 75 <br><b>
349 nigel 93 Absolute and relative back references
350     </b><br>
351     <P>
352 ph10 208 The sequence \g followed by an unsigned or a negative number, optionally
353     enclosed in braces, is an absolute or relative back reference. A named back
354     reference can be coded as \g{name}. Back references are discussed
355 nigel 93 <a href="#backreferences">later,</a>
356     following the discussion of
357     <a href="#subpattern">parenthesized subpatterns.</a>
358     </P>
359     <br><b>
360 ph10 345 Absolute and relative subroutine calls
361     </b><br>
362     <P>
363     For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
364     a number enclosed either in angle brackets or single quotes, is an alternative
365     syntax for referencing a subpattern as a "subroutine". Details are discussed
366     <a href="#onigurumasubroutines">later.</a>
367     Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
368 ph10 461 synonymous. The former is a back reference; the latter is a
369 ph10 454 <a href="#subpatternsassubroutines">subroutine</a>
370     call.
371 ph10 518 <a name="genericchartypes"></a></P>
372 ph10 345 <br><b>
373 nigel 75 Generic character types
374     </b><br>
375 nigel 63 <P>
376 ph10 518 Another use of backslash is for specifying generic character types:
377 nigel 63 <pre>
378     \d any decimal digit
379     \D any character that is not a decimal digit
380 ph10 182 \h any horizontal whitespace character
381     \H any character that is not a horizontal whitespace character
382 nigel 63 \s any whitespace character
383     \S any character that is not a whitespace character
384 ph10 182 \v any vertical whitespace character
385     \V any character that is not a vertical whitespace character
386 nigel 63 \w any "word" character
387     \W any "non-word" character
388 nigel 75 </pre>
389 ph10 535 There is also the single sequence \N, which matches a non-newline character.
390     This is the same as
391 ph10 518 <a href="#fullstopdot">the "." metacharacter</a>
392 ph10 835 when PCRE_DOTALL is not set.
393 nigel 63 </P>
394     <P>
395 ph10 518 Each pair of lower and upper case escape sequences partitions the complete set
396     of characters into two disjoint sets. Any given character matches one, and only
397 ph10 535 one, of each pair. The sequences can appear both inside and outside character
398 nigel 75 classes. They each match one character of the appropriate type. If the current
399 ph10 535 matching point is at the end of the subject string, all of them fail, because
400 nigel 75 there is no character to match.
401 nigel 63 </P>
402     <P>
403     For compatibility with Perl, \s does not match the VT character (code 11).
404     This makes it different from the the POSIX "space" class. The \s characters
405 ph10 182 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
406 nigel 91 included in a Perl script, \s may match the VT character. In PCRE, it never
407 ph10 182 does.
408 nigel 63 </P>
409     <P>
410 ph10 535 A "word" character is an underscore or any character that is a letter or digit.
411     By default, the definition of letters and digits is controlled by PCRE's
412     low-valued character tables, and may vary if locale-specific matching is taking
413     place (see
414     <a href="pcreapi.html#localesupport">"Locale support"</a>
415     in the
416     <a href="pcreapi.html"><b>pcreapi</b></a>
417     page). For example, in a French locale such as "fr_FR" in Unix-like systems,
418     or "french" in Windows, some character codes greater than 128 are used for
419     accented letters, and these are then matched by \w. The use of locales with
420     Unicode is discouraged.
421 ph10 182 </P>
422     <P>
423 ph10 535 By default, in UTF-8 mode, characters with values greater than 128 never match
424     \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
425     their original meanings from before UTF-8 support was available, mainly for
426     efficiency reasons. However, if PCRE is compiled with Unicode property support,
427     and the PCRE_UCP option is set, the behaviour is changed so that Unicode
428     properties are used to determine character types, as follows:
429     <pre>
430     \d any character that \p{Nd} matches (decimal digit)
431     \s any character that \p{Z} matches, plus HT, LF, FF, CR
432     \w any character that \p{L} or \p{N} matches, plus underscore
433     </pre>
434     The upper case escapes match the inverse sets of characters. Note that \d
435     matches only decimal digits, whereas \w matches any Unicode digit, as well as
436     any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
437     \B because they are defined in terms of \w and \W. Matching these sequences
438     is noticeably slower when PCRE_UCP is set.
439     </P>
440     <P>
441 ph10 579 The sequences \h, \H, \v, and \V are features that were added to Perl at
442 ph10 572 release 5.10. In contrast to the other sequences, which match only ASCII
443     characters by default, these always match certain high-valued codepoints in
444     UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
445     are:
446 ph10 182 <pre>
447     U+0009 Horizontal tab
448     U+0020 Space
449     U+00A0 Non-break space
450     U+1680 Ogham space mark
451     U+180E Mongolian vowel separator
452     U+2000 En quad
453     U+2001 Em quad
454     U+2002 En space
455     U+2003 Em space
456     U+2004 Three-per-em space
457     U+2005 Four-per-em space
458     U+2006 Six-per-em space
459     U+2007 Figure space
460     U+2008 Punctuation space
461     U+2009 Thin space
462     U+200A Hair space
463     U+202F Narrow no-break space
464     U+205F Medium mathematical space
465     U+3000 Ideographic space
466     </pre>
467     The vertical space characters are:
468     <pre>
469     U+000A Linefeed
470     U+000B Vertical tab
471     U+000C Formfeed
472     U+000D Carriage return
473     U+0085 Next line
474     U+2028 Line separator
475     U+2029 Paragraph separator
476 ph10 535 <a name="newlineseq"></a></PRE>
477 ph10 182 </P>
478 nigel 93 <br><b>
479     Newline sequences
480     </b><br>
481     <P>
482 ph10 231 Outside a character class, by default, the escape sequence \R matches any
483 ph10 572 Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following:
484 nigel 93 <pre>
485     (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
486     </pre>
487     This is an example of an "atomic group", details of which are given
488     <a href="#atomicgroup">below.</a>
489     This particular group matches either the two-character sequence CR followed by
490     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
491     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
492     line, U+0085). The two-character sequence is treated as a single unit that
493     cannot be split.
494     </P>
495     <P>
496     In UTF-8 mode, two additional characters whose codepoints are greater than 255
497     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
498     Unicode character property support is not needed for these characters to be
499     recognized.
500     </P>
501     <P>
502 ph10 231 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
503     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
504 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
505     for "backslash R".) This can be made the default when PCRE is built; if this is
506     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
507     It is also possible to specify these settings by starting a pattern string with
508     one of the following sequences:
509 ph10 231 <pre>
510     (*BSR_ANYCRLF) CR, LF, or CRLF only
511     (*BSR_UNICODE) any Unicode newline sequence
512     </pre>
513 ph10 461 These override the default and the options given to <b>pcre_compile()</b> or
514     <b>pcre_compile2()</b>, but they can be overridden by options given to
515     <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,
516     which are not Perl-compatible, are recognized only at the very start of a
517     pattern, and that they must be in upper case. If more than one of them is
518     present, the last one is used. They can be combined with a change of newline
519 ph10 535 convention; for example, a pattern can start with:
520 ph10 247 <pre>
521     (*ANY)(*BSR_ANYCRLF)
522     </pre>
523 ph10 535 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
524     a character class, \R is treated as an unrecognized escape sequence, and so
525     matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
526 nigel 75 <a name="uniextseq"></a></P>
527     <br><b>
528     Unicode character properties
529     </b><br>
530     <P>
531     When PCRE is built with Unicode character property support, three additional
532 ph10 185 escape sequences that match characters with specific properties are available.
533     When not in UTF-8 mode, these sequences are of course limited to testing
534     characters whose codepoints are less than 256, but they do work in this mode.
535     The extra escape sequences are:
536 nigel 75 <pre>
537 nigel 87 \p{<i>xx</i>} a character with the <i>xx</i> property
538     \P{<i>xx</i>} a character without the <i>xx</i> property
539     \X an extended Unicode sequence
540 nigel 75 </pre>
541 nigel 87 The property names represented by <i>xx</i> above are limited to the Unicode
542 ph10 518 script names, the general category properties, "Any", which matches any
543     character (including newline), and some special PCRE properties (described
544 ph10 535 in the
545 ph10 518 <a href="#extraprops">next section).</a>
546     Other Perl properties such as "InMusicalSymbols" are not currently supported by
547     PCRE. Note that \P{Any} does not match any characters, so always causes a
548     match failure.
549 nigel 63 </P>
550     <P>
551 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
552     character from one of these sets can be matched using a script name. For
553     example:
554 nigel 75 <pre>
555 nigel 87 \p{Greek}
556     \P{Han}
557     </pre>
558     Those that are not part of an identified script are lumped together as
559     "Common". The current list of scripts is:
560     </P>
561     <P>
562     Arabic,
563     Armenian,
564 ph10 507 Avestan,
565 nigel 93 Balinese,
566 ph10 507 Bamum,
567 nigel 87 Bengali,
568     Bopomofo,
569     Braille,
570     Buginese,
571     Buhid,
572     Canadian_Aboriginal,
573 ph10 507 Carian,
574     Cham,
575 nigel 87 Cherokee,
576     Common,
577     Coptic,
578 nigel 93 Cuneiform,
579 nigel 87 Cypriot,
580     Cyrillic,
581     Deseret,
582     Devanagari,
583 ph10 507 Egyptian_Hieroglyphs,
584 nigel 87 Ethiopic,
585     Georgian,
586     Glagolitic,
587     Gothic,
588     Greek,
589     Gujarati,
590     Gurmukhi,
591     Han,
592     Hangul,
593     Hanunoo,
594     Hebrew,
595     Hiragana,
596 ph10 507 Imperial_Aramaic,
597 nigel 87 Inherited,
598 ph10 507 Inscriptional_Pahlavi,
599     Inscriptional_Parthian,
600     Javanese,
601     Kaithi,
602 nigel 87 Kannada,
603     Katakana,
604 ph10 507 Kayah_Li,
605 nigel 87 Kharoshthi,
606     Khmer,
607     Lao,
608     Latin,
609 ph10 507 Lepcha,
610 nigel 87 Limbu,
611     Linear_B,
612 ph10 507 Lisu,
613     Lycian,
614     Lydian,
615 nigel 87 Malayalam,
616 ph10 507 Meetei_Mayek,
617 nigel 87 Mongolian,
618     Myanmar,
619     New_Tai_Lue,
620 nigel 93 Nko,
621 nigel 87 Ogham,
622     Old_Italic,
623     Old_Persian,
624 ph10 507 Old_South_Arabian,
625     Old_Turkic,
626     Ol_Chiki,
627 nigel 87 Oriya,
628     Osmanya,
629 nigel 93 Phags_Pa,
630     Phoenician,
631 ph10 507 Rejang,
632 nigel 87 Runic,
633 ph10 507 Samaritan,
634     Saurashtra,
635 nigel 87 Shavian,
636     Sinhala,
637 ph10 507 Sundanese,
638 nigel 87 Syloti_Nagri,
639     Syriac,
640     Tagalog,
641     Tagbanwa,
642     Tai_Le,
643 ph10 507 Tai_Tham,
644     Tai_Viet,
645 nigel 87 Tamil,
646     Telugu,
647     Thaana,
648     Thai,
649     Tibetan,
650     Tifinagh,
651     Ugaritic,
652 ph10 507 Vai,
653 nigel 87 Yi.
654     </P>
655     <P>
656 ph10 518 Each character has exactly one Unicode general category property, specified by
657     a two-letter abbreviation. For compatibility with Perl, negation can be
658     specified by including a circumflex between the opening brace and the property
659     name. For example, \p{^Lu} is the same as \P{Lu}.
660 nigel 87 </P>
661     <P>
662     If only one letter is specified with \p or \P, it includes all the general
663     category properties that start with that letter. In this case, in the absence
664     of negation, the curly brackets in the escape sequence are optional; these two
665     examples have the same effect:
666     <pre>
667 nigel 75 \p{L}
668     \pL
669     </pre>
670 nigel 87 The following general category property codes are supported:
671 nigel 75 <pre>
672     C Other
673     Cc Control
674     Cf Format
675     Cn Unassigned
676     Co Private use
677     Cs Surrogate
678    
679     L Letter
680     Ll Lower case letter
681     Lm Modifier letter
682     Lo Other letter
683     Lt Title case letter
684     Lu Upper case letter
685    
686     M Mark
687     Mc Spacing mark
688     Me Enclosing mark
689     Mn Non-spacing mark
690    
691     N Number
692     Nd Decimal number
693     Nl Letter number
694     No Other number
695    
696     P Punctuation
697     Pc Connector punctuation
698     Pd Dash punctuation
699     Pe Close punctuation
700     Pf Final punctuation
701     Pi Initial punctuation
702     Po Other punctuation
703     Ps Open punctuation
704    
705     S Symbol
706     Sc Currency symbol
707     Sk Modifier symbol
708     Sm Mathematical symbol
709     So Other symbol
710    
711     Z Separator
712     Zl Line separator
713     Zp Paragraph separator
714     Zs Space separator
715     </pre>
716 nigel 87 The special property L& is also supported: it matches a character that has
717     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
718     a modifier or "other".
719 nigel 75 </P>
720     <P>
721 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
722     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
723     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
724     (see the discussion of PCRE_NO_UTF8_CHECK in the
725     <a href="pcreapi.html"><b>pcreapi</b></a>
726 ph10 453 page). Perl does not support the Cs property.
727 ph10 211 </P>
728     <P>
729 ph10 453 The long synonyms for property names that Perl supports (such as \p{Letter})
730 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
731 nigel 87 properties with "Is".
732     </P>
733     <P>
734     No character that is in the Unicode table has the Cn (unassigned) property.
735     Instead, this property is assumed for any code point that is not in the
736     Unicode table.
737     </P>
738     <P>
739 nigel 75 Specifying caseless matching does not affect these escape sequences. For
740     example, \p{Lu} always matches only upper case letters.
741     </P>
742     <P>
743     The \X escape matches any number of Unicode characters that form an extended
744     Unicode sequence. \X is equivalent to
745     <pre>
746     (?&#62;\PM\pM*)
747     </pre>
748     That is, it matches a character without the "mark" property, followed by zero
749     or more characters with the "mark" property, and treats the sequence as an
750     atomic group
751     <a href="#atomicgroup">(see below).</a>
752     Characters with the "mark" property are typically accents that affect the
753 ph10 185 preceding character. None of them have codepoints less than 256, so in
754     non-UTF-8 mode \X matches any one character.
755 nigel 75 </P>
756     <P>
757 ph10 654 Note that recent versions of Perl have changed \X to match what Unicode calls
758     an "extended grapheme cluster", which has a more complicated definition.
759     </P>
760     <P>
761 nigel 75 Matching characters by Unicode property is not fast, because PCRE has to search
762     a structure that contains data for over fifteen thousand characters. That is
763     why the traditional escape sequences such as \d and \w do not use Unicode
764 ph10 535 properties in PCRE by default, though you can make them do so by setting the
765     PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with
766     (*UCP).
767 ph10 518 <a name="extraprops"></a></P>
768     <br><b>
769     PCRE's additional properties
770     </b><br>
771     <P>
772 ph10 535 As well as the standard Unicode properties described in the previous
773     section, PCRE supports four more that make it possible to convert traditional
774 ph10 518 escape sequences such as \w and \s and POSIX character classes to use Unicode
775 ph10 535 properties. PCRE uses these non-standard, non-Perl properties internally when
776     PCRE_UCP is set. They are:
777 ph10 518 <pre>
778     Xan Any alphanumeric character
779     Xps Any POSIX space character
780     Xsp Any Perl space character
781     Xwd Any Perl "word" character
782     </pre>
783 ph10 535 Xan matches characters that have either the L (letter) or the N (number)
784     property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
785 ph10 518 carriage return, and any other character that has the Z (separator) property.
786 ph10 535 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
787 ph10 518 same characters as Xan, plus underscore.
788 ph10 172 <a name="resetmatchstart"></a></P>
789     <br><b>
790     Resetting the match start
791     </b><br>
792     <P>
793 ph10 572 The escape sequence \K causes any previously matched characters not to be
794     included in the final matched sequence. For example, the pattern:
795 ph10 172 <pre>
796     foo\Kbar
797     </pre>
798     matches "foobar", but reports that it has matched "bar". This feature is
799     similar to a lookbehind assertion
800     <a href="#lookbehind">(described below).</a>
801     However, in this case, the part of the subject before the real match does not
802     have to be of fixed length, as lookbehind assertions do. The use of \K does
803     not interfere with the setting of
804     <a href="#subpattern">captured substrings.</a>
805     For example, when the pattern
806     <pre>
807     (foo)\Kbar
808     </pre>
809     matches "foobar", the first substring is still set to "foo".
810 ph10 507 </P>
811     <P>
812     Perl documents that the use of \K within assertions is "not well defined". In
813     PCRE, \K is acted upon when it occurs inside positive assertions, but is
814     ignored in negative assertions.
815 nigel 75 <a name="smallassertions"></a></P>
816     <br><b>
817     Simple assertions
818     </b><br>
819     <P>
820 nigel 93 The final use of backslash is for certain simple assertions. An assertion
821 nigel 63 specifies a condition that has to be met at a particular point in a match,
822     without consuming any characters from the subject string. The use of
823 nigel 75 subpatterns for more complicated assertions is described
824     <a href="#bigassertions">below.</a>
825 nigel 91 The backslashed assertions are:
826 nigel 63 <pre>
827     \b matches at a word boundary
828     \B matches when not at a word boundary
829 nigel 93 \A matches at the start of the subject
830     \Z matches at the end of the subject
831     also matches before a newline at the end of the subject
832     \z matches only at the end of the subject
833     \G matches at the first matching position in the subject
834 nigel 75 </pre>
835 ph10 518 Inside a character class, \b has a different meaning; it matches the backspace
836 ph10 535 character. If any other of these assertions appears in a character class, by
837 ph10 518 default it matches the corresponding literal character (for example, \B
838     matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
839     escape sequence" error is generated instead.
840 nigel 63 </P>
841     <P>
842     A word boundary is a position in the subject string where the current character
843     and the previous character do not both match \w or \W (i.e. one matches
844     \w and the other matches \W), or the start or end of the string if the
845 ph10 535 first or last character matches \w, respectively. In UTF-8 mode, the meanings
846     of \w and \W can be changed by setting the PCRE_UCP option. When this is
847     done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
848     of word" or "end of word" metasequence. However, whatever follows \b normally
849     determines which it is. For example, the fragment \ba matches "a" at the start
850     of a word.
851 nigel 63 </P>
852     <P>
853     The \A, \Z, and \z assertions differ from the traditional circumflex and
854 nigel 75 dollar (described in the next section) in that they only ever match at the very
855     start and end of the subject string, whatever options are set. Thus, they are
856     independent of multiline mode. These three assertions are not affected by the
857     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
858     circumflex and dollar metacharacters. However, if the <i>startoffset</i>
859     argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
860     at a point other than the beginning of the subject, \A can never match. The
861 nigel 91 difference between \Z and \z is that \Z matches before a newline at the end
862     of the string as well as at the very end, whereas \z matches only at the end.
863 nigel 63 </P>
864     <P>
865     The \G assertion is true only when the current matching position is at the
866     start point of the match, as specified by the <i>startoffset</i> argument of
867     <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
868     non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
869     arguments, you can mimic Perl's /g option, and it is in this kind of
870     implementation where \G can be useful.
871     </P>
872     <P>
873     Note, however, that PCRE's interpretation of \G, as the start of the current
874     match, is subtly different from Perl's, which defines it as the end of the
875     previous match. In Perl, these can be different when the previously matched
876     string was empty. Because PCRE does just one match at a time, it cannot
877     reproduce this behaviour.
878     </P>
879     <P>
880     If all the alternatives of a pattern begin with \G, the expression is anchored
881     to the starting match position, and the "anchored" flag is set in the compiled
882     regular expression.
883     </P>
884 ph10 227 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
885 nigel 63 <P>
886     Outside a character class, in the default matching mode, the circumflex
887 nigel 75 character is an assertion that is true only if the current matching point is
888 nigel 63 at the start of the subject string. If the <i>startoffset</i> argument of
889     <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
890     option is unset. Inside a character class, circumflex has an entirely different
891 nigel 75 meaning
892     <a href="#characterclass">(see below).</a>
893 nigel 63 </P>
894     <P>
895     Circumflex need not be the first character of the pattern if a number of
896     alternatives are involved, but it should be the first thing in each alternative
897     in which it appears if the pattern is ever to match that branch. If all
898     possible alternatives start with a circumflex, that is, if the pattern is
899     constrained to match only at the start of the subject, it is said to be an
900     "anchored" pattern. (There are also other constructs that can cause a pattern
901     to be anchored.)
902     </P>
903     <P>
904 nigel 75 A dollar character is an assertion that is true only if the current matching
905 nigel 63 point is at the end of the subject string, or immediately before a newline
906 nigel 91 at the end of the string (by default). Dollar need not be the last character of
907     the pattern if a number of alternatives are involved, but it should be the last
908     item in any branch in which it appears. Dollar has no special meaning in a
909     character class.
910 nigel 63 </P>
911     <P>
912     The meaning of dollar can be changed so that it matches only at the very end of
913     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
914     does not affect the \Z assertion.
915     </P>
916     <P>
917     The meanings of the circumflex and dollar characters are changed if the
918 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
919     immediately after internal newlines as well as at the start of the subject
920     string. It does not match after a newline that ends the string. A dollar
921     matches before any newlines in the string, as well as at the very end, when
922     PCRE_MULTILINE is set. When newline is specified as the two-character
923     sequence CRLF, isolated CR and LF characters do not indicate newlines.
924 nigel 63 </P>
925     <P>
926 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
927     \n represents a newline) in multiline mode, but not otherwise. Consequently,
928     patterns that are anchored in single line mode because all branches start with
929     ^ are not anchored in multiline mode, and a match for circumflex is possible
930     when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
931     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
932     </P>
933     <P>
934 nigel 63 Note that the sequences \A, \Z, and \z can be used to match the start and
935     end of the subject in both modes, and if all branches of a pattern start with
936 nigel 91 \A it is always anchored, whether or not PCRE_MULTILINE is set.
937 ph10 518 <a name="fullstopdot"></a></P>
938     <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
939 nigel 63 <P>
940     Outside a character class, a dot in the pattern matches any one character in
941 nigel 91 the subject string except (by default) a character that signifies the end of a
942 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
943 nigel 63 </P>
944 nigel 91 <P>
945 nigel 93 When a line ending is defined as a single character, dot never matches that
946     character; when the two-character sequence CRLF is used, dot does not match CR
947     if it is immediately followed by LF, but otherwise it matches all characters
948     (including isolated CRs and LFs). When any Unicode line endings are being
949     recognized, dot does not match CR or LF or any of the other line ending
950     characters.
951     </P>
952     <P>
953 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
954 nigel 93 option is set, a dot matches any one character, without exception. If the
955     two-character sequence CRLF is present in the subject string, it takes two dots
956     to match it.
957 nigel 91 </P>
958     <P>
959     The handling of dot is entirely independent of the handling of circumflex and
960     dollar, the only relationship being that they both involve newlines. Dot has no
961     special meaning in a character class.
962     </P>
963 ph10 518 <P>
964 ph10 579 The escape sequence \N behaves like a dot, except that it is not affected by
965 ph10 572 the PCRE_DOTALL option. In other words, it matches any character except one
966 ph10 835 that signifies the end of a line.
967 ph10 518 </P>
968 ph10 227 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
969 nigel 63 <P>
970     Outside a character class, the escape sequence \C matches any one byte, both
971 ph10 738 in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
972 nigel 93 characters. The feature is provided in Perl in order to match individual bytes
973 ph10 738 in UTF-8 mode, but it is unclear how it can usefully be used. Because \C
974     breaks up characters into individual bytes, matching one byte with \C in UTF-8
975     mode means that the rest of the string may start with a malformed UTF-8
976     character. This has undefined results, because PCRE assumes that it is dealing
977     with valid UTF-8 strings (and by default it checks this at the start of
978     processing unless the PCRE_NO_UTF8_CHECK option is used).
979 nigel 63 </P>
980     <P>
981 nigel 75 PCRE does not allow \C to appear in lookbehind assertions
982 ph10 835 <a href="#lookbehind">(described below),</a>
983     because in UTF-8 mode this would make it impossible to calculate the length of
984 nigel 75 the lookbehind.
985 ph10 738 </P>
986     <P>
987     In general, the \C escape sequence is best avoided in UTF-8 mode. However, one
988     way of using it that avoids the problem of malformed UTF-8 characters is to
989     use a lookahead to check the length of the next character, as in this pattern
990     (ignore white space and line breaks):
991     <pre>
992     (?| (?=[\x00-\x7f])(\C) |
993     (?=[\x80-\x{7ff}])(\C)(\C) |
994     (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
995     (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
996     </pre>
997     A group that starts with (?| resets the capturing parentheses numbers in each
998     alternative (see
999     <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1000     below). The assertions at the start of each branch check the next UTF-8
1001     character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1002     character's individual bytes are then captured by the appropriate number of
1003     groups.
1004 nigel 75 <a name="characterclass"></a></P>
1005 ph10 227 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1006 nigel 63 <P>
1007     An opening square bracket introduces a character class, terminated by a closing
1008 ph10 461 square bracket. A closing square bracket on its own is not special by default.
1009     However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1010     bracket causes a compile-time error. If a closing square bracket is required as
1011     a member of the class, it should be the first data character in the class
1012     (after an initial circumflex, if present) or escaped with a backslash.
1013 nigel 63 </P>
1014     <P>
1015     A character class matches a single character in the subject. In UTF-8 mode, the
1016 ph10 461 character may be more than one byte long. A matched character must be in the
1017     set of characters defined by the class, unless the first character in the class
1018 nigel 63 definition is a circumflex, in which case the subject character must not be in
1019     the set defined by the class. If a circumflex is actually required as a member
1020     of the class, ensure it is not the first character, or escape it with a
1021     backslash.
1022     </P>
1023     <P>
1024     For example, the character class [aeiou] matches any lower case vowel, while
1025     [^aeiou] matches any character that is not a lower case vowel. Note that a
1026 nigel 75 circumflex is just a convenient notation for specifying the characters that
1027     are in the class by enumerating those that are not. A class that starts with a
1028 ph10 461 circumflex is not an assertion; it still consumes a character from the subject
1029 nigel 75 string, and therefore it fails if the current pointer is at the end of the
1030     string.
1031 nigel 63 </P>
1032     <P>
1033     In UTF-8 mode, characters with values greater than 255 can be included in a
1034     class as a literal string of bytes, or by using the \x{ escaping mechanism.
1035     </P>
1036     <P>
1037     When caseless matching is set, any letters in a class represent both their
1038     upper case and lower case versions, so for example, a caseless [aeiou] matches
1039     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1040 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1041     case for characters whose values are less than 128, so caseless matching is
1042     always possible. For characters with higher values, the concept of case is
1043     supported if PCRE is compiled with Unicode property support, but not otherwise.
1044 ph10 461 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1045     you must ensure that PCRE is compiled with Unicode property support as well as
1046     with UTF-8 support.
1047 nigel 63 </P>
1048     <P>
1049 nigel 93 Characters that might indicate line breaks are never treated in any special way
1050     when matching character classes, whatever line-ending sequence is in use, and
1051     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1052     such as [^a] always matches one of these characters.
1053 nigel 63 </P>
1054     <P>
1055     The minus (hyphen) character can be used to specify a range of characters in a
1056     character class. For example, [d-m] matches any letter between d and m,
1057     inclusive. If a minus character is required in a class, it must be escaped with
1058     a backslash or appear in a position where it cannot be interpreted as
1059     indicating a range, typically as the first or last character in the class.
1060     </P>
1061     <P>
1062     It is not possible to have the literal character "]" as the end character of a
1063     range. A pattern such as [W-]46] is interpreted as a class of two characters
1064     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1065     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1066 nigel 75 the end of range, so [W-\]46] is interpreted as a class containing a range
1067     followed by two other characters. The octal or hexadecimal representation of
1068     "]" can also be used to end a range.
1069 nigel 63 </P>
1070     <P>
1071     Ranges operate in the collating sequence of character values. They can also be
1072     used for characters specified numerically, for example [\000-\037]. In UTF-8
1073     mode, ranges can include characters whose values are greater than 255, for
1074     example [\x{100}-\x{2ff}].
1075     </P>
1076     <P>
1077     If a range that includes letters is used when caseless matching is set, it
1078     matches the letters in either case. For example, [W-c] is equivalent to
1079 nigel 75 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1080 ph10 142 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1081 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1082     characters with values greater than 128 only when it is compiled with Unicode
1083     property support.
1084 nigel 63 </P>
1085     <P>
1086 ph10 579 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1087     \V, \w, and \W may appear in a character class, and add the characters that
1088     they match to the class. For example, [\dABCDEF] matches any hexadecimal
1089     digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w
1090     and their upper case partners, just as it does when they appear outside a
1091     character class, as described in the section entitled
1092     <a href="#genericchartypes">"Generic character types"</a>
1093     above. The escape sequence \b has a different meaning inside a character
1094     class; it matches the backspace character. The sequences \B, \N, \R, and \X
1095     are not special inside a character class. Like any other unrecognized escape
1096     sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1097     default, but cause an error if the PCRE_EXTRA option is set.
1098     </P>
1099     <P>
1100     A circumflex can conveniently be used with the upper case character types to
1101 ph10 535 specify a more restricted set of characters than the matching lower case type.
1102 ph10 579 For example, the class [^\W_] matches any letter or digit, but not underscore,
1103     whereas [\w] includes underscore. A positive character class should be read as
1104     "something OR something OR ..." and a negative class as "NOT something AND NOT
1105     something AND NOT ...".
1106 nigel 63 </P>
1107     <P>
1108 nigel 75 The only metacharacters that are recognized in character classes are backslash,
1109     hyphen (only where it can be interpreted as specifying a range), circumflex
1110     (only at the start), opening square bracket (only when it can be interpreted as
1111     introducing a POSIX class name - see the next section), and the terminating
1112     closing square bracket. However, escaping other non-alphanumeric characters
1113     does no harm.
1114 nigel 63 </P>
1115 ph10 227 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1116 nigel 63 <P>
1117 nigel 75 Perl supports the POSIX notation for character classes. This uses names
1118 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1119     this notation. For example,
1120     <pre>
1121     [01[:alpha:]%]
1122 nigel 75 </pre>
1123 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
1124 ph10 535 are:
1125 nigel 63 <pre>
1126     alnum letters and digits
1127     alpha letters
1128     ascii character codes 0 - 127
1129     blank space or tab only
1130     cntrl control characters
1131     digit decimal digits (same as \d)
1132     graph printing characters, excluding space
1133     lower lower case letters
1134     print printing characters, including space
1135 ph10 535 punct printing characters, excluding letters and digits and space
1136 nigel 63 space white space (not quite the same as \s)
1137     upper upper case letters
1138     word "word" characters (same as \w)
1139     xdigit hexadecimal digits
1140 nigel 75 </pre>
1141 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1142     space (32). Notice that this list includes the VT character (code 11). This
1143     makes "space" different to \s, which does not include VT (for Perl
1144     compatibility).
1145     </P>
1146     <P>
1147     The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1148     5.8. Another Perl extension is negation, which is indicated by a ^ character
1149     after the colon. For example,
1150     <pre>
1151     [12[:^digit:]]
1152 nigel 75 </pre>
1153 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1154     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1155     supported, and an error is given if they are encountered.
1156     </P>
1157     <P>
1158 ph10 535 By default, in UTF-8 mode, characters with values greater than 128 do not match
1159     any of the POSIX character classes. However, if the PCRE_UCP option is passed
1160     to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1161     character properties are used. This is achieved by replacing the POSIX classes
1162     by other sequences, as follows:
1163     <pre>
1164     [:alnum:] becomes \p{Xan}
1165     [:alpha:] becomes \p{L}
1166     [:blank:] becomes \h
1167     [:digit:] becomes \p{Nd}
1168     [:lower:] becomes \p{Ll}
1169     [:space:] becomes \p{Xps}
1170     [:upper:] becomes \p{Lu}
1171     [:word:] becomes \p{Xwd}
1172     </pre>
1173     Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1174     classes are unchanged, and match only characters with code points less than
1175     128.
1176 nigel 63 </P>
1177 ph10 227 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1178 nigel 63 <P>
1179     Vertical bar characters are used to separate alternative patterns. For example,
1180     the pattern
1181     <pre>
1182     gilbert|sullivan
1183 nigel 75 </pre>
1184 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1185 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1186     process tries each alternative in turn, from left to right, and the first one
1187     that succeeds is used. If the alternatives are within a subpattern
1188 nigel 75 <a href="#subpattern">(defined below),</a>
1189     "succeeds" means matching the rest of the main pattern as well as the
1190     alternative in the subpattern.
1191 nigel 63 </P>
1192 ph10 227 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1193 nigel 63 <P>
1194     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1195 ph10 243 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1196     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1197     The option letters are
1198 nigel 63 <pre>
1199     i for PCRE_CASELESS
1200     m for PCRE_MULTILINE
1201     s for PCRE_DOTALL
1202     x for PCRE_EXTENDED
1203 nigel 75 </pre>
1204 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1205     unset these options by preceding the letter with a hyphen, and a combined
1206     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1207     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1208     permitted. If a letter appears both before and after the hyphen, the option is
1209     unset.
1210     </P>
1211     <P>
1212 ph10 243 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1213     changed in the same way as the Perl-compatible options by using the characters
1214     J, U and X respectively.
1215     </P>
1216     <P>
1217 ph10 416 When one of these option changes occurs at top level (that is, not inside
1218     subpattern parentheses), the change applies to the remainder of the pattern
1219     that follows. If the change is placed right at the start of a pattern, PCRE
1220     extracts it into the global options (and it will therefore show up in data
1221     extracted by the <b>pcre_fullinfo()</b> function).
1222 nigel 63 </P>
1223     <P>
1224 nigel 93 An option change within a subpattern (see below for a description of
1225 ph10 572 subpatterns) affects only that part of the subpattern that follows it, so
1226 nigel 63 <pre>
1227     (a(?i)b)c
1228 nigel 75 </pre>
1229 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1230     By this means, options can be made to have different settings in different
1231     parts of the pattern. Any changes made in one alternative do carry on
1232     into subsequent branches within the same subpattern. For example,
1233     <pre>
1234     (a(?i)b|c)
1235 nigel 75 </pre>
1236 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1237     branch is abandoned before the option setting. This is because the effects of
1238     option settings happen at compile time. There would be some very weird
1239     behaviour otherwise.
1240 ph10 254 </P>
1241     <P>
1242     <b>Note:</b> There are other PCRE-specific options that can be set by the
1243     application when the compile or match functions are called. In some cases the
1244 ph10 416 pattern can contain special leading sequences such as (*CRLF) to override what
1245     the application has set or what has been defaulted. Details are given in the
1246     section entitled
1247 ph10 254 <a href="#newlineseq">"Newline sequences"</a>
1248 ph10 535 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1249     to set UTF-8 and Unicode property modes; they are equivalent to setting the
1250     PCRE_UTF8 and the PCRE_UCP options, respectively.
1251 nigel 75 <a name="subpattern"></a></P>
1252 ph10 227 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1253 nigel 63 <P>
1254     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1255 nigel 75 Turning part of a pattern into a subpattern does two things:
1256     <br>
1257     <br>
1258 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1259     <pre>
1260     cat(aract|erpillar|)
1261 nigel 75 </pre>
1262 ph10 572 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1263     match "cataract", "erpillar" or an empty string.
1264 nigel 75 <br>
1265     <br>
1266     2. It sets up the subpattern as a capturing subpattern. This means that, when
1267     the whole pattern matches, that portion of the subject string that matched the
1268     subpattern is passed back to the caller via the <i>ovector</i> argument of
1269 nigel 63 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1270 ph10 572 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1271     string "the red king" is matched against the pattern
1272 nigel 63 <pre>
1273     the ((red|white) (king|queen))
1274 nigel 75 </pre>
1275 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1276     2, and 3, respectively.
1277     </P>
1278     <P>
1279     The fact that plain parentheses fulfil two functions is not always helpful.
1280     There are often times when a grouping subpattern is required without a
1281     capturing requirement. If an opening parenthesis is followed by a question mark
1282     and a colon, the subpattern does not do any capturing, and is not counted when
1283     computing the number of any subsequent capturing subpatterns. For example, if
1284     the string "the white queen" is matched against the pattern
1285     <pre>
1286     the ((?:red|white) (king|queen))
1287 nigel 75 </pre>
1288 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1289 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1290 nigel 63 </P>
1291     <P>
1292     As a convenient shorthand, if any option settings are required at the start of
1293     a non-capturing subpattern, the option letters may appear between the "?" and
1294     the ":". Thus the two patterns
1295     <pre>
1296     (?i:saturday|sunday)
1297     (?:(?i)saturday|sunday)
1298 nigel 75 </pre>
1299 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1300     from left to right, and options are not reset until the end of the subpattern
1301     is reached, an option setting in one branch does affect subsequent branches, so
1302     the above patterns match "SUNDAY" as well as "Saturday".
1303 ph10 461 <a name="dupsubpatternnumber"></a></P>
1304 ph10 227 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1305 nigel 63 <P>
1306 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1307     the same numbers for its capturing parentheses. Such a subpattern starts with
1308     (?| and is itself a non-capturing subpattern. For example, consider this
1309     pattern:
1310     <pre>
1311     (?|(Sat)ur|(Sun))day
1312     </pre>
1313     Because the two alternatives are inside a (?| group, both sets of capturing
1314     parentheses are numbered one. Thus, when the pattern matches, you can look
1315     at captured substring number one, whichever alternative matched. This construct
1316     is useful when you want to capture part, but not all, of one of a number of
1317     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1318     number is reset at the start of each branch. The numbers of any capturing
1319 ph10 572 parentheses that follow the subpattern start after the highest number used in
1320     any branch. The following example is taken from the Perl documentation. The
1321     numbers underneath show in which buffer the captured content will be stored.
1322 ph10 182 <pre>
1323     # before ---------------branch-reset----------- after
1324     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1325     # 1 2 2 3 2 3 4
1326     </pre>
1327 ph10 488 A back reference to a numbered subpattern uses the most recent value that is
1328     set for that number by any subpattern. The following pattern matches "abcabc"
1329     or "defdef":
1330 ph10 461 <pre>
1331     /(?|(abc)|(def))\1/
1332     </pre>
1333 ph10 733 In contrast, a subroutine call to a numbered subpattern always refers to the
1334     first one in the pattern with the given number. The following pattern matches
1335     "abcabc" or "defabc":
1336 ph10 461 <pre>
1337     /(?|(abc)|(def))(?1)/
1338     </pre>
1339     If a
1340     <a href="#conditions">condition test</a>
1341     for a subpattern's having matched refers to a non-unique number, the test is
1342     true if any of the subpatterns of that number have matched.
1343 ph10 182 </P>
1344     <P>
1345     An alternative approach to using this "branch reset" feature is to use
1346     duplicate named subpatterns, as described in the next section.
1347     </P>
1348 ph10 227 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1349 ph10 182 <P>
1350 nigel 63 Identifying capturing parentheses by number is simple, but it can be very hard
1351     to keep track of the numbers in complicated regular expressions. Furthermore,
1352 nigel 75 if an expression is modified, the numbers may change. To help with this
1353 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1354     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1355     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1356 ph10 461 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1357     have different names, but PCRE does not.
1358 nigel 93 </P>
1359     <P>
1360     In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1361     (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1362 nigel 91 parentheses from other parts of the pattern, such as
1363 ph10 488 <a href="#backreferences">back references,</a>
1364 nigel 91 <a href="#recursion">recursion,</a>
1365     and
1366     <a href="#conditions">conditions,</a>
1367     can be made by name as well as by number.
1368 nigel 63 </P>
1369     <P>
1370 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1371 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1372     if the names were not present. The PCRE API provides function calls for
1373     extracting the name-to-number translation table from a compiled pattern. There
1374     is also a convenience function for extracting a captured substring by name.
1375 nigel 91 </P>
1376     <P>
1377     By default, a name must be unique within a pattern, but it is possible to relax
1378 ph10 461 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1379     names are also always permitted for subpatterns with the same number, set up as
1380     described in the previous section.) Duplicate names can be useful for patterns
1381     where only one instance of the named parentheses can match. Suppose you want to
1382     match the name of a weekday, either as a 3-letter abbreviation or as the full
1383     name, and in both cases you want to extract the abbreviation. This pattern
1384     (ignoring the line breaks) does the job:
1385 nigel 91 <pre>
1386 nigel 93 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1387     (?&#60;DN&#62;Tue)(?:sday)?|
1388     (?&#60;DN&#62;Wed)(?:nesday)?|
1389     (?&#60;DN&#62;Thu)(?:rsday)?|
1390     (?&#60;DN&#62;Sat)(?:urday)?
1391 nigel 91 </pre>
1392     There are five capturing substrings, but only one is ever set after a match.
1393 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1394     subpattern, as described in the previous section.)
1395     </P>
1396     <P>
1397 nigel 91 The convenience function for extracting the data by name returns the substring
1398 nigel 93 for the first (and in this example, the only) subpattern of that name that
1399 ph10 461 matched. This saves searching to find which numbered subpattern it was.
1400     </P>
1401     <P>
1402 ph10 488 If you make a back reference to a non-unique named subpattern from elsewhere in
1403 ph10 461 the pattern, the one that corresponds to the first occurrence of the name is
1404     used. In the absence of duplicate numbers (see the previous section) this is
1405     the one with the lowest number. If you use a named reference in a condition
1406     test (see the
1407     <a href="#conditions">section about conditions</a>
1408     below), either to check whether a subpattern has matched, or to check for
1409     recursion, all subpatterns with the same name are tested. If the condition is
1410     true for any one of them, the overall condition is true. This is the same
1411     behaviour as testing by number. For further details of the interfaces for
1412     handling named subpatterns, see the
1413 nigel 63 <a href="pcreapi.html"><b>pcreapi</b></a>
1414     documentation.
1415     </P>
1416 ph10 392 <P>
1417     <b>Warning:</b> You cannot use different names to distinguish between two
1418 ph10 461 subpatterns with the same number because PCRE uses only the numbers when
1419     matching. For this reason, an error is given at compile time if different names
1420     are given to subpatterns with the same number. However, you can give the same
1421     name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1422 ph10 392 </P>
1423 ph10 227 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1424 nigel 63 <P>
1425     Repetition is specified by quantifiers, which can follow any of the following
1426     items:
1427     <pre>
1428     a literal data character
1429 nigel 93 the dot metacharacter
1430 nigel 63 the \C escape sequence
1431 nigel 75 the \X escape sequence (in UTF-8 mode with Unicode properties)
1432 nigel 93 the \R escape sequence
1433 ph10 572 an escape such as \d or \pL that matches a single character
1434 nigel 63 a character class
1435     a back reference (see next section)
1436 ph10 654 a parenthesized subpattern (including assertions)
1437 ph10 733 a subroutine call to a subpattern (recursive or otherwise)
1438 nigel 75 </pre>
1439 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1440     permitted matches, by giving the two numbers in curly brackets (braces),
1441     separated by a comma. The numbers must be less than 65536, and the first must
1442     be less than or equal to the second. For example:
1443     <pre>
1444     z{2,4}
1445 nigel 75 </pre>
1446 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1447     character. If the second number is omitted, but the comma is present, there is
1448     no upper limit; if the second number and the comma are both omitted, the
1449     quantifier specifies an exact number of required matches. Thus
1450     <pre>
1451     [aeiou]{3,}
1452 nigel 75 </pre>
1453 nigel 63 matches at least 3 successive vowels, but may match many more, while
1454     <pre>
1455     \d{8}
1456 nigel 75 </pre>
1457 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1458     where a quantifier is not allowed, or one that does not match the syntax of a
1459     quantifier, is taken as a literal character. For example, {,6} is not a
1460     quantifier, but a literal string of four characters.
1461     </P>
1462     <P>
1463     In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1464     bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1465 nigel 75 which is represented by a two-byte sequence. Similarly, when Unicode property
1466     support is available, \X{3} matches three Unicode extended sequences, each of
1467     which may be several bytes long (and they may be of different lengths).
1468 nigel 63 </P>
1469     <P>
1470     The quantifier {0} is permitted, causing the expression to behave as if the
1471 ph10 345 previous item and the quantifier were not present. This may be useful for
1472     subpatterns that are referenced as
1473     <a href="#subpatternsassubroutines">subroutines</a>
1474 ph10 572 from elsewhere in the pattern (but see also the section entitled
1475     <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1476     below). Items other than subpatterns that have a {0} quantifier are omitted
1477     from the compiled pattern.
1478 nigel 63 </P>
1479     <P>
1480 nigel 93 For convenience, the three most common quantifiers have single-character
1481     abbreviations:
1482 nigel 63 <pre>
1483     * is equivalent to {0,}
1484     + is equivalent to {1,}
1485     ? is equivalent to {0,1}
1486 nigel 75 </pre>
1487 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1488     match no characters with a quantifier that has no upper limit, for example:
1489     <pre>
1490     (a?)*
1491 nigel 75 </pre>
1492 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1493     such patterns. However, because there are cases where this can be useful, such
1494     patterns are now accepted, but if any repetition of the subpattern does in fact
1495     match no characters, the loop is forcibly broken.
1496     </P>
1497     <P>
1498     By default, the quantifiers are "greedy", that is, they match as much as
1499     possible (up to the maximum number of permitted times), without causing the
1500     rest of the pattern to fail. The classic example of where this gives problems
1501 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1502     and within the comment, individual * and / characters may appear. An attempt to
1503     match C comments by applying the pattern
1504 nigel 63 <pre>
1505     /\*.*\*/
1506 nigel 75 </pre>
1507 nigel 63 to the string
1508     <pre>
1509 nigel 75 /* first comment */ not comment /* second comment */
1510     </pre>
1511 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1512     item.
1513     </P>
1514     <P>
1515     However, if a quantifier is followed by a question mark, it ceases to be
1516     greedy, and instead matches the minimum number of times possible, so the
1517     pattern
1518     <pre>
1519     /\*.*?\*/
1520 nigel 75 </pre>
1521 nigel 63 does the right thing with the C comments. The meaning of the various
1522     quantifiers is not otherwise changed, just the preferred number of matches.
1523     Do not confuse this use of question mark with its use as a quantifier in its
1524     own right. Because it has two uses, it can sometimes appear doubled, as in
1525     <pre>
1526     \d??\d
1527 nigel 75 </pre>
1528 nigel 63 which matches one digit by preference, but can match two if that is the only
1529     way the rest of the pattern matches.
1530     </P>
1531     <P>
1532 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1533 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1534     greedy by following them with a question mark. In other words, it inverts the
1535     default behaviour.
1536     </P>
1537     <P>
1538     When a parenthesized subpattern is quantified with a minimum repeat count that
1539 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1540 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1541     </P>
1542     <P>
1543     If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1544 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1545 nigel 63 implicitly anchored, because whatever follows will be tried against every
1546     character position in the subject string, so there is no point in retrying the
1547     overall match at any position after the first. PCRE normally treats such a
1548     pattern as though it were preceded by \A.
1549     </P>
1550     <P>
1551     In cases where it is known that the subject string contains no newlines, it is
1552     worth setting PCRE_DOTALL in order to obtain this optimization, or
1553     alternatively using ^ to indicate anchoring explicitly.
1554     </P>
1555     <P>
1556     However, there is one situation where the optimization cannot be used. When .*
1557 ph10 488 is inside capturing parentheses that are the subject of a back reference
1558 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1559     succeeds. Consider, for example:
1560 nigel 63 <pre>
1561     (.*)abc\1
1562 nigel 75 </pre>
1563 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1564     this reason, such a pattern is not implicitly anchored.
1565     </P>
1566     <P>
1567     When a capturing subpattern is repeated, the value captured is the substring
1568     that matched the final iteration. For example, after
1569     <pre>
1570     (tweedle[dume]{3}\s*)+
1571 nigel 75 </pre>
1572 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1573     "tweedledee". However, if there are nested capturing subpatterns, the
1574     corresponding captured values may have been set in previous iterations. For
1575     example, after
1576     <pre>
1577     /(a|(b))+/
1578 nigel 75 </pre>
1579 nigel 63 matches "aba" the value of the second captured substring is "b".
1580 nigel 75 <a name="atomicgroup"></a></P>
1581 ph10 227 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1582 nigel 63 <P>
1583 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1584     repetition, failure of what follows normally causes the repeated item to be
1585     re-evaluated to see if a different number of repeats allows the rest of the
1586     pattern to match. Sometimes it is useful to prevent this, either to change the
1587     nature of the match, or to cause it fail earlier than it otherwise might, when
1588     the author of the pattern knows there is no point in carrying on.
1589 nigel 63 </P>
1590     <P>
1591     Consider, for example, the pattern \d+foo when applied to the subject line
1592     <pre>
1593     123456bar
1594 nigel 75 </pre>
1595 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1596     action of the matcher is to try again with only 5 digits matching the \d+
1597     item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1598     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1599     that once a subpattern has matched, it is not to be re-evaluated in this way.
1600     </P>
1601     <P>
1602 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1603 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1604     special parenthesis, starting with (?&#62; as in this example:
1605     <pre>
1606 nigel 73 (?&#62;\d+)foo
1607 nigel 75 </pre>
1608 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1609     it has matched, and a failure further into the pattern is prevented from
1610     backtracking into it. Backtracking past it to previous items, however, works as
1611     normal.
1612     </P>
1613     <P>
1614     An alternative description is that a subpattern of this type matches the string
1615     of characters that an identical standalone pattern would match, if anchored at
1616     the current point in the subject string.
1617     </P>
1618     <P>
1619     Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1620     the above example can be thought of as a maximizing repeat that must swallow
1621     everything it can. So, while both \d+ and \d+? are prepared to adjust the
1622     number of digits they match in order to make the rest of the pattern match,
1623     (?&#62;\d+) can only match an entire sequence of digits.
1624     </P>
1625     <P>
1626     Atomic groups in general can of course contain arbitrarily complicated
1627     subpatterns, and can be nested. However, when the subpattern for an atomic
1628     group is just a single repeated item, as in the example above, a simpler
1629     notation, called a "possessive quantifier" can be used. This consists of an
1630     additional + character following a quantifier. Using this notation, the
1631     previous example can be rewritten as
1632     <pre>
1633 nigel 75 \d++foo
1634     </pre>
1635 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1636     example:
1637     <pre>
1638     (abc|xyz){2,3}+
1639     </pre>
1640 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1641     option is ignored. They are a convenient notation for the simpler forms of
1642 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1643     quantifier and the equivalent atomic group, though there may be a performance
1644     difference; possessive quantifiers should be slightly faster.
1645 nigel 63 </P>
1646     <P>
1647 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1648     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1649     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1650     package, and PCRE copied it from there. It ultimately found its way into Perl
1651     at release 5.10.
1652 nigel 63 </P>
1653     <P>
1654 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1655     pattern constructs. For example, the sequence A+B is treated as A++B because
1656     there is no point in backtracking into a sequence of A's when B must follow.
1657     </P>
1658     <P>
1659 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1660     be repeated an unlimited number of times, the use of an atomic group is the
1661     only way to avoid some failing matches taking a very long time indeed. The
1662     pattern
1663     <pre>
1664     (\D+|&#60;\d+&#62;)*[!?]
1665 nigel 75 </pre>
1666 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1667     digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1668     quickly. However, if it is applied to
1669     <pre>
1670     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1671 nigel 75 </pre>
1672 nigel 63 it takes a long time before reporting failure. This is because the string can
1673 nigel 75 be divided between the internal \D+ repeat and the external * repeat in a
1674     large number of ways, and all have to be tried. (The example uses [!?] rather
1675     than a single character at the end, because both PCRE and Perl have an
1676     optimization that allows for fast failure when a single character is used. They
1677     remember the last single character that is required for a match, and fail early
1678     if it is not present in the string.) If the pattern is changed so that it uses
1679     an atomic group, like this:
1680 nigel 63 <pre>
1681     ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1682 nigel 75 </pre>
1683 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1684 nigel 75 <a name="backreferences"></a></P>
1685 ph10 227 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1686 nigel 63 <P>
1687     Outside a character class, a backslash followed by a digit greater than 0 (and
1688     possibly further digits) is a back reference to a capturing subpattern earlier
1689     (that is, to its left) in the pattern, provided there have been that many
1690     previous capturing left parentheses.
1691     </P>
1692     <P>
1693     However, if the decimal number following the backslash is less than 10, it is
1694     always taken as a back reference, and causes an error only if there are not
1695     that many capturing left parentheses in the entire pattern. In other words, the
1696     parentheses that are referenced need not be to the left of the reference for
1697 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1698     when a repetition is involved and the subpattern to the right has participated
1699     in an earlier iteration.
1700     </P>
1701     <P>
1702 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1703     whose number is 10 or more using this syntax because a sequence such as \50 is
1704     interpreted as a character defined in octal. See the subsection entitled
1705 nigel 91 "Non-printing characters"
1706 nigel 75 <a href="#digitsafterbackslash">above</a>
1707 nigel 93 for further details of the handling of digits following a backslash. There is
1708     no such problem when named parentheses are used. A back reference to any
1709     subpattern is possible using named parentheses (see below).
1710 nigel 63 </P>
1711     <P>
1712 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1713 ph10 572 backslash is to use the \g escape sequence. This escape must be followed by an
1714     unsigned number or a negative number, optionally enclosed in braces. These
1715     examples are all identical:
1716 nigel 93 <pre>
1717     (ring), \1
1718     (ring), \g1
1719     (ring), \g{1}
1720     </pre>
1721 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1722     is present in the older syntax. It is also useful when literal digits follow
1723     the reference. A negative number is a relative reference. Consider this
1724     example:
1725 nigel 93 <pre>
1726     (abc(def)ghi)\g{-1}
1727     </pre>
1728     The sequence \g{-1} is a reference to the most recently started capturing
1729 ph10 579 subpattern before \g, that is, is it equivalent to \2 in this example.
1730     Similarly, \g{-2} would be equivalent to \1. The use of relative references
1731     can be helpful in long patterns, and also in patterns that are created by
1732     joining together fragments that contain references within themselves.
1733 nigel 93 </P>
1734     <P>
1735 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1736     the current subject string, rather than anything matching the subpattern
1737     itself (see
1738     <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1739     below for a way of doing that). So the pattern
1740     <pre>
1741     (sens|respons)e and \1ibility
1742 nigel 75 </pre>
1743 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1744     "sense and responsibility". If caseful matching is in force at the time of the
1745     back reference, the case of letters is relevant. For example,
1746     <pre>
1747     ((?i)rah)\s+\1
1748 nigel 75 </pre>
1749 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1750     capturing subpattern is matched caselessly.
1751     </P>
1752     <P>
1753 ph10 172 There are several different ways of writing back references to named
1754     subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1755     \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1756     back reference syntax, in which \g can be used for both numeric and named
1757     references, is also supported. We could rewrite the above example in any of
1758 nigel 93 the following ways:
1759 nigel 63 <pre>
1760 nigel 93 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1761 ph10 172 (?'p1'(?i)rah)\s+\k{p1}
1762 nigel 91 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1763 ph10 172 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1764 nigel 75 </pre>
1765 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1766     after the reference.
1767     </P>
1768     <P>
1769 nigel 63 There may be more than one back reference to the same subpattern. If a
1770     subpattern has not actually been used in a particular match, any back
1771 ph10 461 references to it always fail by default. For example, the pattern
1772 nigel 63 <pre>
1773     (a|(bc))\2
1774 nigel 75 </pre>
1775 ph10 461 always fails if it starts to match "a" rather than "bc". However, if the
1776     PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1777     unset value matches an empty string.
1778     </P>
1779     <P>
1780     Because there may be many capturing parentheses in a pattern, all digits
1781     following a backslash are taken as part of a potential back reference number.
1782     If the pattern continues with a digit character, some delimiter must be used to
1783     terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1784     whitespace. Otherwise, the \g{ syntax or an empty comment (see
1785 nigel 75 <a href="#comments">"Comments"</a>
1786     below) can be used.
1787 nigel 63 </P>
1788 ph10 488 <br><b>
1789     Recursive back references
1790     </b><br>
1791 nigel 63 <P>
1792     A back reference that occurs inside the parentheses to which it refers fails
1793     when the subpattern is first used, so, for example, (a\1) never matches.
1794     However, such references can be useful inside repeated subpatterns. For
1795     example, the pattern
1796     <pre>
1797     (a|b\1)+
1798 nigel 75 </pre>
1799 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1800     the subpattern, the back reference matches the character string corresponding
1801     to the previous iteration. In order for this to work, the pattern must be such
1802     that the first iteration does not need to match the back reference. This can be
1803     done using alternation, as in the example above, or by a quantifier with a
1804     minimum of zero.
1805 ph10 488 </P>
1806     <P>
1807     Back references of this type cause the group that they reference to be treated
1808     as an
1809     <a href="#atomicgroup">atomic group.</a>
1810     Once the whole group has been matched, a subsequent matching failure cannot
1811     cause backtracking into the middle of the group.
1812 nigel 75 <a name="bigassertions"></a></P>
1813 ph10 227 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1814 nigel 63 <P>
1815     An assertion is a test on the characters following or preceding the current
1816     matching point that does not actually consume any characters. The simple
1817 nigel 75 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1818     <a href="#smallassertions">above.</a>
1819     </P>
1820     <P>
1821 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1822     those that look ahead of the current position in the subject string, and those
1823 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1824     except that it does not cause the current matching position to be changed.
1825 nigel 63 </P>
1826     <P>
1827 ph10 654 Assertion subpatterns are not capturing subpatterns. If such an assertion
1828     contains capturing subpatterns within it, these are counted for the purposes of
1829     numbering the capturing subpatterns in the whole pattern. However, substring
1830     capturing is carried out only for positive assertions, because it does not make
1831     sense for negative assertions.
1832 nigel 63 </P>
1833 ph10 654 <P>
1834     For compatibility with Perl, assertion subpatterns may be repeated; though
1835     it makes no sense to assert the same thing several times, the side effect of
1836     capturing parentheses may occasionally be useful. In practice, there only three
1837     cases:
1838     <br>
1839     <br>
1840     (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1841     However, it may contain internal capturing parenthesized groups that are called
1842     from elsewhere via the
1843     <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1844     <br>
1845     <br>
1846     (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1847     were {0,1}. At run time, the rest of the pattern match is tried with and
1848     without the assertion, the order depending on the greediness of the quantifier.
1849     <br>
1850     <br>
1851     (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1852     The assertion is obeyed just once when encountered during matching.
1853     </P>
1854 nigel 75 <br><b>
1855     Lookahead assertions
1856     </b><br>
1857 nigel 63 <P>
1858 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1859     negative assertions. For example,
1860 nigel 63 <pre>
1861     \w+(?=;)
1862 nigel 75 </pre>
1863 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1864     the match, and
1865     <pre>
1866     foo(?!bar)
1867 nigel 75 </pre>
1868 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1869     apparently similar pattern
1870     <pre>
1871     (?!foo)bar
1872 nigel 75 </pre>
1873 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1874     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1875     (?!foo) is always true when the next three characters are "bar". A
1876 nigel 75 lookbehind assertion is needed to achieve the other effect.
1877 nigel 63 </P>
1878     <P>
1879     If you want to force a matching failure at some point in a pattern, the most
1880     convenient way to do it is with (?!) because an empty string always matches, so
1881     an assertion that requires there not to be an empty string must always fail.
1882 ph10 579 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1883 nigel 75 <a name="lookbehind"></a></P>
1884     <br><b>
1885     Lookbehind assertions
1886     </b><br>
1887 nigel 63 <P>
1888     Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1889     negative assertions. For example,
1890     <pre>
1891     (?&#60;!foo)bar
1892 nigel 75 </pre>
1893 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1894     a lookbehind assertion are restricted such that all the strings it matches must
1895 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1896     do not all have to have the same fixed length. Thus
1897 nigel 63 <pre>
1898     (?&#60;=bullock|donkey)
1899 nigel 75 </pre>
1900 nigel 63 is permitted, but
1901     <pre>
1902     (?&#60;!dogs?|cats?)
1903 nigel 75 </pre>
1904 nigel 63 causes an error at compile time. Branches that match different length strings
1905     are permitted only at the top level of a lookbehind assertion. This is an
1906 ph10 572 extension compared with Perl, which requires all branches to match the same
1907     length of string. An assertion such as
1908 nigel 63 <pre>
1909     (?&#60;=ab(c|de))
1910 nigel 75 </pre>
1911 nigel 63 is not permitted, because its single top-level branch can match two different
1912 ph10 454 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1913     branches:
1914 nigel 63 <pre>
1915     (?&#60;=abc|abde)
1916 nigel 75 </pre>
1917 ph10 572 In some cases, the escape sequence \K
1918 ph10 172 <a href="#resetmatchstart">(see above)</a>
1919 ph10 461 can be used instead of a lookbehind assertion to get round the fixed-length
1920 ph10 454 restriction.
1921 ph10 172 </P>
1922     <P>
1923 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1924 nigel 93 temporarily move the current position back by the fixed length and then try to
1925 nigel 63 match. If there are insufficient characters before the current position, the
1926 nigel 93 assertion fails.
1927 nigel 63 </P>
1928     <P>
1929 ph10 835 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1930     to appear in lookbehind assertions, because it makes it impossible to calculate
1931     the length of the lookbehind. The \X and \R escapes, which can match
1932     different numbers of bytes, are also not permitted.
1933 nigel 63 </P>
1934     <P>
1935 ph10 454 <a href="#subpatternsassubroutines">"Subroutine"</a>
1936     calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1937 ph10 461 as the subpattern matches a fixed-length string.
1938 ph10 454 <a href="#recursion">Recursion,</a>
1939     however, is not supported.
1940     </P>
1941     <P>
1942 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1943 ph10 461 specify efficient matching of fixed-length strings at the end of subject
1944     strings. Consider a simple pattern such as
1945 nigel 63 <pre>
1946     abcd$
1947 nigel 75 </pre>
1948 nigel 63 when applied to a long string that does not match. Because matching proceeds
1949     from left to right, PCRE will look for each "a" in the subject and then see if
1950     what follows matches the rest of the pattern. If the pattern is specified as
1951     <pre>
1952     ^.*abcd$
1953 nigel 75 </pre>
1954 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1955     there is no following "a"), it backtracks to match all but the last character,
1956     then all but the last two characters, and so on. Once again the search for "a"
1957     covers the entire string, from right to left, so we are no better off. However,
1958     if the pattern is written as
1959     <pre>
1960     ^.*+(?&#60;=abcd)
1961 nigel 75 </pre>
1962 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1963 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1964     characters. If it fails, the match fails immediately. For long strings, this
1965     approach makes a significant difference to the processing time.
1966     </P>
1967 nigel 75 <br><b>
1968     Using multiple assertions
1969     </b><br>
1970 nigel 63 <P>
1971     Several assertions (of any sort) may occur in succession. For example,
1972     <pre>
1973     (?&#60;=\d{3})(?&#60;!999)foo
1974 nigel 75 </pre>
1975 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1976     the assertions is applied independently at the same point in the subject
1977     string. First there is a check that the previous three characters are all
1978     digits, and then there is a check that the same three characters are not "999".
1979     This pattern does <i>not</i> match "foo" preceded by six characters, the first
1980     of which are digits and the last three of which are not "999". For example, it
1981     doesn't match "123abcfoo". A pattern to do that is
1982     <pre>
1983     (?&#60;=\d{3}...)(?&#60;!999)foo
1984 nigel 75 </pre>
1985 nigel 63 This time the first assertion looks at the preceding six characters, checking
1986     that the first three are digits, and then the second assertion checks that the
1987     preceding three characters are not "999".
1988     </P>
1989     <P>
1990     Assertions can be nested in any combination. For example,
1991     <pre>
1992     (?&#60;=(?&#60;!foo)bar)baz
1993 nigel 75 </pre>
1994 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1995     preceded by "foo", while
1996     <pre>
1997     (?&#60;=\d{3}(?!999)...)foo
1998 nigel 75 </pre>
1999     is another pattern that matches "foo" preceded by three digits and any three
2000 nigel 63 characters that are not "999".
2001 nigel 91 <a name="conditions"></a></P>
2002 ph10 227 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2003 nigel 63 <P>
2004     It is possible to cause the matching process to obey a subpattern
2005     conditionally or to choose between two alternative subpatterns, depending on
2006 ph10 461 the result of an assertion, or whether a specific capturing subpattern has
2007     already been matched. The two possible forms of conditional subpattern are:
2008 nigel 63 <pre>
2009     (?(condition)yes-pattern)
2010     (?(condition)yes-pattern|no-pattern)
2011 nigel 75 </pre>
2012 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
2013     no-pattern (if present) is used. If there are more than two alternatives in the
2014 ph10 567 subpattern, a compile-time error occurs. Each of the two alternatives may
2015 ph10 579 itself contain nested subpatterns of any form, including conditional
2016 ph10 567 subpatterns; the restriction to two alternatives applies only at the level of
2017 ph10 579 the condition. This pattern fragment is an example where the alternatives are
2018 ph10 567 complex:
2019     <pre>
2020     (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2021    
2022     </PRE>
2023 nigel 63 </P>
2024     <P>
2025 nigel 93 There are four kinds of condition: references to subpatterns, references to
2026     recursion, a pseudo-condition called DEFINE, and assertions.
2027 nigel 91 </P>
2028 nigel 93 <br><b>
2029     Checking for a used subpattern by number
2030     </b><br>
2031 nigel 91 <P>
2032 nigel 93 If the text between the parentheses consists of a sequence of digits, the
2033 ph10 461 condition is true if a capturing subpattern of that number has previously
2034     matched. If there is more than one capturing subpattern with the same number
2035     (see the earlier
2036     <a href="#recursion">section about duplicate subpattern numbers),</a>
2037 ph10 579 the condition is true if any of them have matched. An alternative notation is
2038 ph10 461 to precede the digits with a plus or minus sign. In this case, the subpattern
2039     number is relative rather than absolute. The most recently opened parentheses
2040 ph10 579 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2041     loops it can also make sense to refer to subsequent groups. The next
2042     parentheses to be opened can be referenced as (?(+1), and so on. (The value
2043     zero in any of these forms is not used; it provokes a compile-time error.)
2044 nigel 93 </P>
2045     <P>
2046 nigel 91 Consider the following pattern, which contains non-significant white space to
2047     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2048     three parts for ease of discussion:
2049 nigel 63 <pre>
2050     ( \( )? [^()]+ (?(1) \) )
2051 nigel 75 </pre>
2052 nigel 63 The first part matches an optional opening parenthesis, and if that
2053     character is present, sets it as the first captured substring. The second part
2054     matches one or more characters that are not parentheses. The third part is a
2055 ph10 579 conditional subpattern that tests whether or not the first set of parentheses
2056     matched. If they did, that is, if subject started with an opening parenthesis,
2057 nigel 63 the condition is true, and so the yes-pattern is executed and a closing
2058     parenthesis is required. Otherwise, since no-pattern is not present, the
2059     subpattern matches nothing. In other words, this pattern matches a sequence of
2060 nigel 93 non-parentheses, optionally enclosed in parentheses.
2061     </P>
2062 ph10 172 <P>
2063     If you were embedding this pattern in a larger one, you could use a relative
2064     reference:
2065     <pre>
2066     ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2067     </pre>
2068     This makes the fragment independent of the parentheses in the larger pattern.
2069     </P>
2070 nigel 93 <br><b>
2071     Checking for a used subpattern by name
2072     </b><br>
2073     <P>
2074     Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2075     subpattern by name. For compatibility with earlier versions of PCRE, which had
2076     this facility before Perl, the syntax (?(name)...) is also recognized. However,
2077     there is a possible ambiguity with this syntax, because subpattern names may
2078     consist entirely of digits. PCRE looks first for a named subpattern; if it
2079     cannot find one and the name consists entirely of digits, PCRE looks for a
2080     subpattern of that number, which must be greater than zero. Using subpattern
2081     names that consist entirely of digits is not recommended.
2082     </P>
2083     <P>
2084     Rewriting the above example to use a named subpattern gives this:
2085 nigel 91 <pre>
2086 nigel 93 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
2087 ph10 461 </pre>
2088     If the name used in a condition of this kind is a duplicate, the test is
2089     applied to all subpatterns of the same name, and is true if any one of them has
2090     matched.
2091 nigel 93 </P>
2092     <br><b>
2093     Checking for pattern recursion
2094     </b><br>
2095     <P>
2096     If the condition is the string (R), and there is no subpattern with the name R,
2097     the condition is true if a recursive call to the whole pattern or any
2098     subpattern has been made. If digits or a name preceded by ampersand follow the
2099     letter R, for example:
2100     <pre>
2101     (?(R3)...) or (?(R&name)...)
2102 nigel 91 </pre>
2103 ph10 461 the condition is true if the most recent recursion is into a subpattern whose
2104 nigel 93 number or name is given. This condition does not check the entire recursion
2105 ph10 461 stack. If the name used in a condition of this kind is a duplicate, the test is
2106     applied to all subpatterns of the same name, and is true if any one of them is
2107     the most recent recursion.
2108 nigel 63 </P>
2109     <P>
2110 ph10 461 At "top level", all these recursion test conditions are false.
2111     <a href="#recursion">The syntax for recursive patterns</a>
2112     is described below.
2113 ph10 572 <a name="subdefine"></a></P>
2114 nigel 93 <br><b>
2115     Defining subpatterns for use by reference only
2116     </b><br>
2117     <P>
2118     If the condition is the string (DEFINE), and there is no subpattern with the
2119     name DEFINE, the condition is always false. In this case, there may be only one
2120     alternative in the subpattern. It is always skipped if control reaches this
2121     point in the pattern; the idea of DEFINE is that it can be used to define
2122 ph10 733 subroutines that can be referenced from elsewhere. (The use of
2123     <a href="#subpatternsassubroutines">subroutines</a>
2124 ph10 579 is described below.) For example, a pattern to match an IPv4 address such as
2125     "192.168.23.245" could be written like this (ignore whitespace and line
2126     breaks):
2127 nigel 93 <pre>
2128     (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2129     \b (?&byte) (\.(?&byte)){3} \b
2130     </pre>
2131     The first part of the pattern is a DEFINE group inside which a another group
2132     named "byte" is defined. This matches an individual component of an IPv4
2133     address (a number less than 256). When matching takes place, this part of the
2134 ph10 461 pattern is skipped because DEFINE acts like a false condition. The rest of the
2135     pattern uses references to the named group to match the four dot-separated
2136     components of an IPv4 address, insisting on a word boundary at each end.
2137 nigel 93 </P>
2138     <br><b>
2139     Assertion conditions
2140     </b><br>
2141     <P>
2142     If the condition is not in any of the above formats, it must be an assertion.
2143 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
2144     this pattern, again containing non-significant white space, and with the two
2145     alternatives on the second line:
2146     <pre>
2147     (?(?=[^a-z]*[a-z])
2148     \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2149 nigel 75 </pre>
2150 nigel 63 The condition is a positive lookahead assertion that matches an optional
2151     sequence of non-letters followed by a letter. In other words, it tests for the
2152     presence of at least one letter in the subject. If a letter is found, the
2153     subject is matched against the first alternative; otherwise it is matched
2154     against the second. This pattern matches strings in one of the two forms
2155     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2156 nigel 75 <a name="comments"></a></P>
2157 ph10 227 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2158 nigel 63 <P>
2159 ph10 579 There are two ways of including comments in patterns that are processed by
2160 ph10 567 PCRE. In both cases, the start of the comment must not be in a character class,
2161     nor in the middle of any other sequence of related characters such as (?: or a
2162     subpattern name or number. The characters that make up a comment play no part
2163     in the pattern matching.
2164 nigel 63 </P>
2165     <P>
2166 ph10 567 The sequence (?# marks the start of a comment that continues up to the next
2167     closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2168     option is set, an unescaped # character also introduces a comment, which in
2169     this case continues to immediately after the next newline character or
2170     character sequence in the pattern. Which characters are interpreted as newlines
2171     is controlled by the options passed to <b>pcre_compile()</b> or by a special
2172     sequence at the start of the pattern, as described in the section entitled
2173 ph10 572 <a href="#newlines">"Newline conventions"</a>
2174     above. Note that the end of this type of comment is a literal newline sequence
2175     in the pattern; escape sequences that happen to represent a newline do not
2176     count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2177     default newline convention is in force:
2178 ph10 567 <pre>
2179     abc #comment \n still comment
2180     </pre>
2181 ph10 579 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2182 ph10 567 a newline in the pattern. The sequence \n is still literal at this stage, so
2183     it does not terminate the comment. Only an actual character with the code value
2184     0x0a (the default newline) does so.
2185 nigel 91 <a name="recursion"></a></P>
2186 ph10 227 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2187 nigel 63 <P>
2188     Consider the problem of matching a string in parentheses, allowing for
2189     unlimited nested parentheses. Without the use of recursion, the best that can
2190     be done is to use a pattern that matches up to some fixed depth of nesting. It
2191 nigel 93 is not possible to handle an arbitrary nesting depth.
2192     </P>
2193     <P>
2194     For some time, Perl has provided a facility that allows regular expressions to
2195     recurse (amongst other things). It does this by interpolating Perl code in the
2196     expression at run time, and the code can refer to the expression itself. A Perl
2197     pattern using code interpolation to solve the parentheses problem can be
2198     created like this:
2199 nigel 63 <pre>
2200     $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2201 nigel 75 </pre>
2202 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2203 nigel 93 recursively to the pattern in which it appears.
2204 nigel 63 </P>
2205     <P>
2206 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2207     supports special syntax for recursion of the entire pattern, and also for
2208     individual subpattern recursion. After its introduction in PCRE and Python,
2209 ph10 453 this kind of recursion was subsequently introduced into Perl at release 5.10.
2210 nigel 63 </P>
2211     <P>
2212 nigel 93 A special item that consists of (? followed by a number greater than zero and a
2213 ph10 733 closing parenthesis is a recursive subroutine call of the subpattern of the
2214     given number, provided that it occurs inside that subpattern. (If not, it is a
2215     <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2216 nigel 93 call, which is described in the next section.) The special item (?R) or (?0) is
2217     a recursive call of the entire regular expression.
2218 nigel 87 </P>
2219     <P>
2220     This PCRE pattern solves the nested parentheses problem (assume the
2221     PCRE_EXTENDED option is set so that white space is ignored):
2222 nigel 63 <pre>
2223 ph10 461 \( ( [^()]++ | (?R) )* \)
2224 nigel 75 </pre>
2225 nigel 63 First it matches an opening parenthesis. Then it matches any number of
2226     substrings which can either be a sequence of non-parentheses, or a recursive
2227 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
2228 ph10 461 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2229     to avoid backtracking into sequences of non-parentheses.
2230 nigel 63 </P>
2231     <P>
2232     If this were part of a larger pattern, you would not want to recurse the entire
2233     pattern, so instead you could use this:
2234     <pre>
2235 ph10 461 ( \( ( [^()]++ | (?1) )* \) )
2236 nigel 75 </pre>
2237 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
2238 ph10 172 them instead of the whole pattern.
2239     </P>
2240     <P>
2241     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2242 ph10 572 is made easier by the use of relative references. Instead of (?1) in the
2243     pattern above you can write (?-2) to refer to the second most recently opened
2244     parentheses preceding the recursion. In other words, a negative number counts
2245     capturing parentheses leftwards from the point at which it is encountered.
2246 ph10 172 </P>
2247     <P>
2248     It is also possible to refer to subsequently opened parentheses, by writing
2249     references such as (?+2). However, these cannot be recursive because the
2250     reference is not inside the parentheses that are referenced. They are always
2251 ph10 733 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2252 ph10 454 calls, as described in the next section.
2253 ph10 172 </P>
2254     <P>
2255     An alternative approach is to use named parentheses instead. The Perl syntax
2256     for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2257     could rewrite the above example as follows:
2258 nigel 63 <pre>
2259 ph10 461 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2260 nigel 75 </pre>
2261 nigel 93 If there is more than one subpattern with the same name, the earliest one is
2262 ph10 172 used.
2263     </P>
2264     <P>
2265     This particular example pattern that we have been looking at contains nested
2266 ph10 461 unlimited repeats, and so the use of a possessive quantifier for matching
2267     strings of non-parentheses is important when applying the pattern to strings
2268     that do not match. For example, when this pattern is applied to
2269 nigel 63 <pre>
2270     (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2271 nigel 75 </pre>
2272 ph10 461 it yields "no match" quickly. However, if a possessive quantifier is not used,
2273 nigel 63 the match runs for a very long time indeed because there are so many different
2274     ways the + and * repeats can carve up the subject, and all have to be tested
2275     before failure can be reported.
2276     </P>
2277     <P>
2278 ph10 469 At the end of a match, the values of capturing parentheses are those from
2279     the outermost level. If you want to obtain intermediate values, a callout
2280     function can be used (see below and the
2281 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2282     documentation). If the pattern above is matched against
2283     <pre>
2284     (ab(cd)ef)
2285 nigel 75 </pre>
2286 ph10 469 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2287     the last value taken on at the top level. If a capturing subpattern is not
2288 ph10 733 matched at the top level, its final captured value is unset, even if it was
2289     (temporarily) set at a deeper level during the matching process.
2290 nigel 63 </P>
2291     <P>
2292 ph10 469 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2293     obtain extra memory to store data during a recursion, which it does by using
2294     <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2295     be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2296     </P>
2297     <P>
2298 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2299     Consider this pattern, which matches text in angle brackets, allowing for
2300     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2301     recursing), whereas any characters are permitted at the outer level.
2302     <pre>
2303     &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2304 nigel 75 </pre>
2305 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2306     different alternatives for the recursive and non-recursive cases. The (?R) item
2307     is the actual recursive call.
2308 ph10 453 <a name="recursiondifference"></a></P>
2309     <br><b>
2310 ph10 733 Differences in recursion processing between PCRE and Perl
2311 ph10 453 </b><br>
2312     <P>
2313 ph10 733 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2314     (like Python, but unlike Perl), a recursive subpattern call is always treated
2315     as an atomic group. That is, once it has matched some of the subject string, it
2316     is never re-entered, even if it contains untried alternatives and there is a
2317     subsequent matching failure. This can be illustrated by the following pattern,
2318     which purports to match a palindromic string that contains an odd number of
2319     characters (for example, "a", "aba", "abcba", "abcdcba"):
2320 ph10 453 <pre>
2321     ^(.|(.)(?1)\2)$
2322     </pre>
2323 ph10 461 The idea is that it either matches a single character, or two identical
2324     characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2325 ph10 453 it does not if the pattern is longer than three characters. Consider the
2326     subject string "abcba":
2327     </P>
2328     <P>
2329 ph10 461 At the top level, the first character is matched, but as it is not at the end
2330 ph10 453 of the string, the first alternative fails; the second alternative is taken
2331     and the recursion kicks in. The recursive call to subpattern 1 successfully
2332     matches the next character ("b"). (Note that the beginning and end of line
2333     tests are not part of the recursion).
2334     </P>
2335     <P>
2336     Back at the top level, the next character ("c") is compared with what
2337 ph10 461 subpattern 2 matched, which was "a". This fails. Because the recursion is
2338 ph10 453 treated as an atomic group, there are now no backtracking points, and so the
2339     entire match fails. (Perl is able, at this point, to re-enter the recursion and
2340     try the second alternative.) However, if the pattern is written with the
2341     alternatives in the other order, things are different:
2342     <pre>
2343     ^((.)(?1)\2|.)$
2344     </pre>
2345 ph10 461 This time, the recursing alternative is tried first, and continues to recurse
2346     until it runs out of characters, at which point the recursion fails. But this
2347     time we do have another alternative to try at the higher level. That is the big
2348 ph10 453 difference: in the previous case the remaining alternative is at a deeper
2349     recursion level, which PCRE cannot use.
2350     </P>
2351     <P>
2352 ph10 579 To change the pattern so that it matches all palindromic strings, not just
2353     those with an odd number of characters, it is tempting to change the pattern to
2354     this:
2355 ph10 453 <pre>
2356     ^((.)(?1)\2|.?)$
2357     </pre>
2358 ph10 461 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2359     deeper recursion has matched a single character, it cannot be entered again in
2360     order to match an empty string. The solution is to separate the two cases, and
2361 ph10 453 write out the odd and even cases as alternatives at the higher level:
2362     <pre>
2363     ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2364     </pre>
2365 ph10 461 If you want to match typical palindromic phrases, the pattern has to ignore all
2366 ph10 453 non-word characters, which can be done like this:
2367     <pre>
2368     ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2369     </pre>
2370 ph10 461 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2371     man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2372     the use of the possessive quantifier *+ to avoid backtracking into sequences of
2373 ph10 453 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2374     more) to match typical phrases, and Perl takes so long that you think it has
2375     gone into a loop.
2376 ph10 461 </P>
2377     <P>
2378     <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2379     string does not start with a palindrome that is shorter than the entire string.
2380     For example, although "abcba" is correctly matched, if the subject is "ababa",
2381     PCRE finds the palindrome "aba" at the start, then fails at top level because
2382     the end of the string does not follow. Once again, it cannot jump back into the
2383     recursion to try other alternatives, so the entire match fails.
2384 ph10 733 </P>
2385     <P>
2386     The second way in which PCRE and Perl differ in their recursion processing is
2387     in the handling of captured values. In Perl, when a subpattern is called
2388     recursively or as a subpattern (see the next section), it has no access to any
2389     values that were captured outside the recursion, whereas in PCRE these values
2390     can be referenced. Consider this pattern:
2391     <pre>
2392     ^(.)(\1|a(?2))
2393     </pre>
2394     In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2395     then in the second group, when the back reference \1 fails to match "b", the
2396     second alternative matches "a" and then recurses. In the recursion, \1 does
2397     now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2398     match because inside the recursive call \1 cannot access the externally set
2399     value.
2400 nigel 75 <a name="subpatternsassubroutines"></a></P>
2401 ph10 227 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2402 nigel 63 <P>
2403 ph10 733 If the syntax for a recursive subpattern call (either by number or by
2404 nigel 63 name) is used outside the parentheses to which it refers, it operates like a
2405 ph10 733 subroutine in a programming language. The called subpattern may be defined
2406 ph10 172 before or after the reference. A numbered reference can be absolute or
2407     relative, as in these examples:
2408 nigel 63 <pre>
2409 ph10 172 (...(absolute)...)...(?2)...
2410     (...(relative)...)...(?-1)...
2411     (...(?+1)...(relative)...
2412     </pre>
2413     An earlier example pointed out that the pattern
2414     <pre>
2415 nigel 63 (sens|respons)e and \1ibility
2416 nigel 75 </pre>
2417 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2418     "sense and responsibility". If instead the pattern
2419     <pre>
2420     (sens|respons)e and (?1)ibility
2421 nigel 75 </pre>
2422 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2423 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2424 nigel 63 </P>
2425 nigel 87 <P>
2426 ph10 733 All subroutine calls, whether recursive or not, are always treated as atomic
2427     groups. That is, once a subroutine has matched some of the subject string, it
2428     is never re-entered, even if it contains untried alternatives and there is a
2429     subsequent matching failure. Any capturing parentheses that are set during the
2430     subroutine call revert to their previous values afterwards.
2431 nigel 87 </P>
2432 nigel 63 <P>
2433 ph10 733 Processing options such as case-independence are fixed when a subpattern is
2434     defined, so if it is used as a subroutine, such options cannot be changed for
2435     different calls. For example, consider this pattern:
2436 nigel 93 <pre>
2437 ph10 172 (abc)(?i:(?-1))
2438 nigel 93 </pre>
2439     It matches "abcabc". It does not match "abcABC" because the change of
2440     processing option does not affect the called subpattern.
2441 ph10 345 <a name="onigurumasubroutines"></a></P>
2442     <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2443     <P>
2444     For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2445     a number enclosed either in angle brackets or single quotes, is an alternative
2446     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2447     are two of the examples used above, rewritten using this syntax:
2448     <pre>
2449     (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2450     (sens|respons)e and \g'1'ibility
2451     </pre>
2452     PCRE supports an extension to Oniguruma: if a number is preceded by a
2453     plus or a minus sign it is taken as a relative reference. For example:
2454     <pre>
2455     (abc)(?i:\g&#60;-1&#62;)
2456     </pre>
2457     Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2458     synonymous. The former is a back reference; the latter is a subroutine call.
2459 nigel 93 </P>
2460 ph10 345 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2461 nigel 93 <P>
2462 nigel 63 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2463     code to be obeyed in the middle of matching a regular expression. This makes it
2464     possible, amongst other things, to extract different substrings that match the
2465     same pair of parentheses when there is a repetition.
2466     </P>
2467     <P>
2468     PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2469     code. The feature is called "callout". The caller of PCRE provides an external
2470     function by putting its entry point in the global variable <i>pcre_callout</i>.
2471     By default, this variable contains NULL, which disables all calling out.
2472     </P>
2473     <P>
2474     Within a regular expression, (?C) indicates the points at which the external
2475     function is to be called. If you want to identify different callout points, you
2476     can put a number less than 256 after the letter C. The default value is zero.
2477     For example, this pattern has two callout points:
2478     <pre>
2479 ph10 155 (?C1)abc(?C2)def
2480 nigel 75 </pre>
2481     If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2482     automatically installed before each item in the pattern. They are all numbered
2483     255.
2484 nigel 63 </P>
2485     <P>
2486     During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2487     set), the external function is called. It is provided with the number of the
2488 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2489     originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2490     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2491     description of the interface to the callout function is given in the
2492 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2493     documentation.
2494 ph10 512 <a name="backtrackcontrol"></a></P>
2495 ph10 345 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2496 nigel 63 <P>
2497 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2498     are described in the Perl documentation as "experimental and subject to change
2499     or removal in a future version of Perl". It goes on to say: "Their usage in
2500     production code should be noted to avoid problems during upgrades." The same
2501     remarks apply to the PCRE features described in this section.
2502     </P>
2503     <P>
2504 ph10 345 Since these verbs are specifically related to backtracking, most of them can be
2505     used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2506     a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2507     failing negative assertion, they cause an error if encountered by
2508 ph10 211 <b>pcre_dfa_exec()</b>.
2509     </P>
2510     <P>
2511 ph10 733 If any of these verbs are used in an assertion or in a subpattern that is
2512     called as a subroutine (whether or not recursively), their effect is confined
2513     to that subpattern; it does not extend to the surrounding pattern, with one
2514 ph10 835 exception: a *MARK that is encountered in a positive assertion <i>is</i> passed
2515     back (compare capturing parentheses in assertions). Note that such subpatterns
2516     are processed as anchored at the point where they are tested. Note also that
2517     Perl's treatment of subroutines is different in some cases.
2518 ph10 453 </P>
2519     <P>
2520 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2521 ph10 512 parenthesis followed by an asterisk. They are generally of the form
2522     (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2523 ph10 733 depending on whether or not an argument is present. A name is any sequence of
2524     characters that does not include a closing parenthesis. If the name is empty,
2525     that is, if the closing parenthesis immediately follows the colon, the effect
2526     is as if the colon were not there. Any number of these verbs may occur in a
2527     pattern.
2528 ph10 211 </P>
2529 ph10 512 <P>
2530     PCRE contains some optimizations that are used to speed up matching by running
2531     some checks at the start of each match attempt. For example, it may know the
2532     minimum length of matching subject, or that a particular character must be
2533     present. When one of these optimizations suppresses the running of a match, any
2534     included backtracking verbs will not, of course, be processed. You can suppress
2535     the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2536 ph10 579 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2537     pattern with (*NO_START_OPT).
2538 ph10 512 </P>
2539 ph10 211 <br><b>
2540     Verbs that act immediately
2541     </b><br>
2542     <P>
2543 ph10 512 The following verbs act as soon as they are encountered. They may not be
2544     followed by a name.
2545 ph10 211 <pre>
2546     (*ACCEPT)
2547     </pre>
2548     This verb causes the match to end successfully, skipping the remainder of the
2549 ph10 733 pattern. However, when it is inside a subpattern that is called as a
2550     subroutine, only that subpattern is ended successfully. Matching then continues
2551     at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2552     far is captured. For example:
2553 ph10 211 <pre>
2554 ph10 453 A((?:A|B(*ACCEPT)|C)D)
2555 ph10 211 </pre>
2556 ph10 461 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2557 ph10 453 the outer parentheses.
2558 ph10 211 <pre>
2559     (*FAIL) or (*F)
2560     </pre>
2561 ph10 733 This verb causes a matching failure, forcing backtracking to occur. It is
2562 ph10 211 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2563     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2564     Perl features that are not present in PCRE. The nearest equivalent is the
2565     callout feature, as for example in this pattern:
2566     <pre>
2567     a+(?C)(*FAIL)
2568     </pre>
2569     A match with the string "aaaa" always fails, but the callout is taken before
2570     each backtrack happens (in this example, 10 times).
2571     </P>
2572     <br><b>
2573 ph10 512 Recording which path was taken
2574     </b><br>
2575     <P>
2576     There is one verb whose main purpose is to track how a match was arrived at,
2577     though it also has a secondary use in conjunction with advancing the match
2578     starting point (see (*SKIP) below).
2579     <pre>
2580     (*MARK:NAME) or (*:NAME)
2581     </pre>
2582     A name is always required with this verb. There may be as many instances of
2583     (*MARK) as you like in a pattern, and their names do not have to be unique.
2584     </P>
2585     <P>
2586 ph10 835 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2587     to the caller via the <i>pcre_extra</i> data structure, as described in the
2588 ph10 512 <a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a>
2589     in the
2590     <a href="pcreapi.html"><b>pcreapi</b></a>
2591 ph10 835 documentation. No data is returned for a partial match. Here is an example of
2592     <b>pcretest</b> output, where the /K modifier requests the retrieval and
2593     outputting of (*MARK) data:
2594 ph10 512 <pre>
2595 ph10 835 /X(*MARK:A)Y|X(*MARK:B)Z/K
2596     XY
2597 ph10 512 0: XY
2598     MK: A
2599     XZ
2600     0: XZ
2601     MK: B
2602     </pre>
2603     The (*MARK) name is tagged with "MK:" in this output, and in this example it
2604     indicates which of the two alternatives matched. This is a more efficient way
2605     of obtaining this information than putting each alternative in its own
2606     capturing parentheses.
2607     </P>
2608     <P>
2609 ph10 654 If (*MARK) is encountered in a positive assertion, its name is recorded and
2610     passed back if it is the last-encountered. This does not happen for negative
2611 ph10 733 assertions.
2612 ph10 654 </P>
2613     <P>
2614 ph10 835 A name may also be returned after a failed match if the final path through the
2615     pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2616     (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2617     starting point for matching is advanced, the final check is often with an empty
2618     string, causing a failure before (*MARK) is reached. For example:
2619 ph10 512 <pre>
2620 ph10 835 /X(*MARK:A)Y|X(*MARK:B)Z/K
2621     XP
2622     No match
2623     </pre>
2624     There are three potential starting points for this match (starting with X,
2625     starting with P, and with an empty string). If the pattern is anchored, the
2626     result is different:
2627     <pre>
2628     /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2629     XP
2630 ph10 512 No match, mark = B
2631     </pre>
2632 ph10 835 PCRE's start-of-match optimizations can also interfere with this. For example,
2633     if, as a result of a call to <b>pcre_study()</b>, it knows the minimum
2634     subject length for a match, a shorter subject will not be scanned at all.
2635 ph10 512 </P>
2636 ph10 835 <P>
2637     Note that similar anomalies (though different in detail) exist in Perl, no
2638     doubt for the same reasons. The use of (*MARK) data after a failed match of an
2639     unanchored pattern is not recommended, unless (*COMMIT) is involved.
2640     </P>
2641 ph10 512 <br><b>
2642 ph10 211 Verbs that act after backtracking
2643     </b><br>
2644     <P>
2645     The following verbs do nothing when they are encountered. Matching continues
2646 ph10 512 with what follows, but if there is no subsequent match, causing a backtrack to
2647     the verb, a failure is forced. That is, backtracking cannot pass to the left of
2648     the verb. However, when one of these verbs appears inside an atomic group, its
2649     effect is confined to that group, because once the group has been matched,
2650     there is never any backtracking into it. In this situation, backtracking can
2651     "jump back" to the left of the entire atomic group. (Remember also, as stated
2652     above, that this localization also applies in subroutine calls and assertions.)
2653     </P>
2654     <P>
2655     These verbs differ in exactly what kind of failure occurs when backtracking
2656     reaches them.
2657 ph10 211 <pre>
2658     (*COMMIT)
2659     </pre>
2660 ph10 512 This verb, which may not be followed by a name, causes the whole match to fail
2661     outright if the rest of the pattern does not match. Even if the pattern is
2662     unanchored, no further attempts to find a match by advancing the starting point
2663     take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2664     finding a match at the current starting point, or not at all. For example:
2665 ph10 211 <pre>
2666     a+(*COMMIT)b
2667     </pre>
2668     This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2669 ph10 512 dynamic anchor, or "I've started, so I must finish." The name of the most
2670     recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2671     match failure.
2672     </P>
2673     <P>
2674     Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2675     unless PCRE's start-of-match optimizations are turned off, as shown in this
2676     <b>pcretest</b> example:
2677 ph10 211 <pre>
2678 ph10 835 /(*COMMIT)abc/
2679     xyzabc
2680 ph10 512 0: abc
2681     xyzabc\Y
2682     No match
2683 ph10 211 </pre>
2684 ph10 512 PCRE knows that any match must start with "a", so the optimization skips along
2685     the subject to "a" before running the first match attempt, which succeeds. When
2686     the optimization is disabled by the \Y escape in the second subject, the match
2687     starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2688     starting points.
2689 ph10 211 <pre>
2690 ph10 512 (*PRUNE) or (*PRUNE:NAME)
2691     </pre>
2692     This verb causes the match to fail at the current starting position in the
2693     subject if the rest of the pattern does not match. If the pattern is
2694     unanchored, the normal "bumpalong" advance to the next starting character then
2695     happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2696     reached, or when matching to the right of (*PRUNE), but if there is no match to
2697     the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2698     (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2699     but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2700 ph10 835 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2701     match fails completely; the name is passed back if this is the final attempt.
2702     (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2703     pattern (*PRUNE) has the same effect as (*COMMIT).
2704 ph10 512 <pre>
2705 ph10 211 (*SKIP)
2706     </pre>
2707 ph10 512 This verb, when given without a name, is like (*PRUNE), except that if the
2708     pattern is unanchored, the "bumpalong" advance is not to the next character,
2709     but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2710     signifies that whatever text was matched leading up to it cannot be part of a
2711     successful match. Consider:
2712 ph10 211 <pre>
2713     a+(*SKIP)b
2714     </pre>
2715     If the subject is "aaaac...", after the first match attempt fails (starting at
2716     the first character in the string), the starting point skips on to start the
2717     next attempt at "c". Note that a possessive quantifer does not have the same
2718 ph10 461 effect as this example; although it would suppress backtracking during the
2719 ph10 211 first match attempt, the second attempt would start at the second character
2720     instead of skipping on to "c".
2721     <pre>
2722 ph10 512 (*SKIP:NAME)
2723 ph10 211 </pre>
2724 ph10 512 When (*SKIP) has an associated name, its behaviour is modified. If the
2725     following pattern fails to match, the previous path through the pattern is
2726     searched for the most recent (*MARK) that has the same name. If one is found,
2727     the "bumpalong" advance is to the subject position that corresponds to that
2728     (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2729 ph10 835 matching name is found, normal "bumpalong" of one character happens (that is,
2730     the (*SKIP) is ignored).
2731 ph10 512 <pre>
2732     (*THEN) or (*THEN:NAME)
2733     </pre>
2734 ph10 733 This verb causes a skip to the next innermost alternative if the rest of the
2735     pattern does not match. That is, it cancels pending backtracking, but only
2736     within the current alternative. Its name comes from the observation that it can
2737     be used for a pattern-based if-then-else block:
2738 ph10 211 <pre>
2739     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2740     </pre>
2741     If the COND1 pattern matches, FOO is tried (and possibly further items after
2742 ph10 733 the end of the group if FOO succeeds); on failure, the matcher skips to the
2743 ph10 512 second alternative and tries COND2, without backtracking into COND1. The
2744 ph10 835 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2745     overall match fails. If (*THEN) is not inside an alternation, it acts like
2746     (*PRUNE).
2747 ph10 211 </P>
2748 ph10 567 <P>
2749 ph10 733 Note that a subpattern that does not contain a | character is just a part of
2750     the enclosing alternative; it is not a nested alternation with only one
2751     alternative. The effect of (*THEN) extends beyond such a subpattern to the
2752     enclosing alternative. Consider this pattern, where A, B, etc. are complex
2753     pattern fragments that do not contain any | characters at this level:
2754     <pre>
2755     A (B(*THEN)C) | D
2756     </pre>
2757     If A and B are matched, but there is a failure in C, matching does not
2758     backtrack into A; instead it moves to the next alternative, that is, D.
2759     However, if the subpattern containing (*THEN) is given an alternative, it
2760     behaves differently:
2761     <pre>
2762     A (B(*THEN)C | (*FAIL)) | D
2763     </pre>
2764     The effect of (*THEN) is now confined to the inner subpattern. After a failure
2765     in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2766     because there are no more alternatives to try. In this case, matching does now
2767     backtrack into A.
2768 ph10 567 </P>
2769     <P>
2770 ph10 733 Note also that a conditional subpattern is not considered as having two
2771     alternatives, because only one is ever used. In other words, the | character in
2772     a conditional subpattern has a different meaning. Ignoring white space,
2773     consider:
2774 ph10 567 <pre>
2775 ph10 733 ^.*? (?(?=a) a | b(*THEN)c )
2776     </pre>
2777     If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2778     it initially matches zero characters. The condition (?=a) then fails, the
2779     character "b" is matched, but "c" is not. At this point, matching does not
2780     backtrack to .*? as might perhaps be expected from the presence of the |
2781     character. The conditional subpattern is part of the single alternative that
2782     comprises the whole pattern, and so the match fails. (If there was a backtrack
2783     into .*?, allowing it to match "b", the match would succeed.)
2784     </P>
2785     <P>
2786     The verbs just described provide four different "strengths" of control when
2787     subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2788     next alternative. (*PRUNE) comes next, failing the match at the current
2789     starting position, but allowing an advance to the next character (for an
2790     unanchored pattern). (*SKIP) is similar, except that the advance may be more
2791     than one character. (*COMMIT) is the strongest, causing the entire match to
2792     fail.
2793     </P>
2794     <P>
2795     If more than one such verb is present in a pattern, the "strongest" one wins.
2796     For example, consider this pattern, where A, B, etc. are complex pattern
2797     fragments:
2798     <pre>
2799 ph10 567 (A(*COMMIT)B(*THEN)C|D)
2800     </pre>
2801     Once A has matched, PCRE is committed to this match, at the current starting
2802     position. If subsequently B matches, but C does not, the normal (*THEN) action
2803 ph10 733 of trying the next alternative (that is, D) does not happen because (*COMMIT)
2804 ph10 567 overrides.
2805     </P>
2806 ph10 345 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2807 ph10 211 <P>
2808 ph10 461 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2809     <b>pcresyntax</b>(3), <b>pcre</b>(3).
2810 nigel 93 </P>
2811 ph10 345 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2812 nigel 93 <P>
2813 ph10 99 Philip Hazel
2814 nigel 63 <br>
2815 ph10 99 University Computing Service
2816     <br>
2817     Cambridge CB2 3QH, England.
2818     <br>
2819     </P>
2820 ph10 345 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2821 ph10 99 <P>
2822 ph10 835 Last updated: 19 October 2011
2823 ph10 99 <br>
2824 ph10 654 Copyright &copy; 1997-2011 University of Cambridge.
2825 ph10 99 <br>
2826 nigel 75 <p>
2827     Return to the <a href="index.html">PCRE index page</a>.
2828     </p>

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