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2: <head>
3: <title>pcrepattern specification</title>
4: </head>
5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6: <h1>pcrepattern man page</h1>
7: <p>
8: Return to the <a href="index.html">PCRE index page</a>.
9: </p>
10: <p>
11: This page is part of the PCRE HTML documentation. It was generated automatically
12: from the original man page. If there is any nonsense in it, please consult the
13: man page, in case the conversion went wrong.
14: <br>
15: <ul>
16: <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17: <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18: <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19: <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20: <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21: <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
22: <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
23: <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24: <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25: <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26: <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27: <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28: <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29: <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30: <li><a name="TOC15" href="#SEC15">REPETITION</a>
31: <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
32: <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33: <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34: <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35: <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36: <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37: <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38: <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39: <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40: <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41: <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42: <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43: <li><a name="TOC28" href="#SEC28">REVISION</a>
44: </ul>
45: <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46: <P>
47: The syntax and semantics of the regular expressions that are supported by PCRE
48: are described in detail below. There is a quick-reference syntax summary in the
49: <a href="pcresyntax.html"><b>pcresyntax</b></a>
50: page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51: also supports some alternative regular expression syntax (which does not
52: conflict with the Perl syntax) in order to provide some compatibility with
53: regular expressions in Python, .NET, and Oniguruma.
54: </P>
55: <P>
56: Perl's regular expressions are described in its own documentation, and
57: regular expressions in general are covered in a number of books, some of which
58: have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59: published by O'Reilly, covers regular expressions in great detail. This
60: description of PCRE's regular expressions is intended as reference material.
61: </P>
62: <P>
63: The original operation of PCRE was on strings of one-byte characters. However,
64: there is now also support for UTF-8 character strings. To use this,
65: PCRE must be built to include UTF-8 support, and you must call
66: <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There
67: is also a special sequence that can be given at the start of a pattern:
68: <pre>
69: (*UTF8)
70: </pre>
71: Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
72: option. This feature is not Perl-compatible. How setting UTF-8 mode affects
73: pattern matching is mentioned in several places below. There is also a summary
74: of UTF-8 features in the
75: <a href="pcreunicode.html"><b>pcreunicode</b></a>
76: page.
77: </P>
78: <P>
79: Another special sequence that may appear at the start of a pattern or in
80: combination with (*UTF8) is:
81: <pre>
82: (*UCP)
83: </pre>
84: This has the same effect as setting the PCRE_UCP option: it causes sequences
85: such as \d and \w to use Unicode properties to determine character types,
86: instead of recognizing only characters with codes less than 128 via a lookup
87: table.
88: </P>
89: <P>
90: If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
91: PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
92: also some more of these special sequences that are concerned with the handling
93: of newlines; they are described below.
94: </P>
95: <P>
96: The remainder of this document discusses the patterns that are supported by
97: PCRE when its main matching function, <b>pcre_exec()</b>, is used.
98: From release 6.0, PCRE offers a second matching function,
99: <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
100: Perl-compatible. Some of the features discussed below are not available when
101: <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
102: alternative function, and how it differs from the normal function, are
103: discussed in the
104: <a href="pcrematching.html"><b>pcrematching</b></a>
105: page.
106: <a name="newlines"></a></P>
107: <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
108: <P>
109: PCRE supports five different conventions for indicating line breaks in
110: strings: a single CR (carriage return) character, a single LF (linefeed)
111: character, the two-character sequence CRLF, any of the three preceding, or any
112: Unicode newline sequence. The
113: <a href="pcreapi.html"><b>pcreapi</b></a>
114: page has
115: <a href="pcreapi.html#newlines">further discussion</a>
116: about newlines, and shows how to set the newline convention in the
117: <i>options</i> arguments for the compiling and matching functions.
118: </P>
119: <P>
120: It is also possible to specify a newline convention by starting a pattern
121: string with one of the following five sequences:
122: <pre>
123: (*CR) carriage return
124: (*LF) linefeed
125: (*CRLF) carriage return, followed by linefeed
126: (*ANYCRLF) any of the three above
127: (*ANY) all Unicode newline sequences
128: </pre>
129: These override the default and the options given to <b>pcre_compile()</b> or
130: <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default
131: newline sequence, the pattern
132: <pre>
133: (*CR)a.b
134: </pre>
135: changes the convention to CR. That pattern matches "a\nb" because LF is no
136: longer a newline. Note that these special settings, which are not
137: Perl-compatible, are recognized only at the very start of a pattern, and that
138: they must be in upper case. If more than one of them is present, the last one
139: is used.
140: </P>
141: <P>
142: The newline convention affects the interpretation of the dot metacharacter when
143: PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
144: affect what the \R escape sequence matches. By default, this is any Unicode
145: newline sequence, for Perl compatibility. However, this can be changed; see the
146: description of \R in the section entitled
147: <a href="#newlineseq">"Newline sequences"</a>
148: below. A change of \R setting can be combined with a change of newline
149: convention.
150: </P>
151: <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
152: <P>
153: A regular expression is a pattern that is matched against a subject string from
154: left to right. Most characters stand for themselves in a pattern, and match the
155: corresponding characters in the subject. As a trivial example, the pattern
156: <pre>
157: The quick brown fox
158: </pre>
159: matches a portion of a subject string that is identical to itself. When
160: caseless matching is specified (the PCRE_CASELESS option), letters are matched
161: independently of case. In UTF-8 mode, PCRE always understands the concept of
162: case for characters whose values are less than 128, so caseless matching is
163: always possible. For characters with higher values, the concept of case is
164: supported if PCRE is compiled with Unicode property support, but not otherwise.
165: If you want to use caseless matching for characters 128 and above, you must
166: ensure that PCRE is compiled with Unicode property support as well as with
167: UTF-8 support.
168: </P>
169: <P>
170: The power of regular expressions comes from the ability to include alternatives
171: and repetitions in the pattern. These are encoded in the pattern by the use of
172: <i>metacharacters</i>, which do not stand for themselves but instead are
173: interpreted in some special way.
174: </P>
175: <P>
176: There are two different sets of metacharacters: those that are recognized
177: anywhere in the pattern except within square brackets, and those that are
178: recognized within square brackets. Outside square brackets, the metacharacters
179: are as follows:
180: <pre>
181: \ general escape character with several uses
182: ^ assert start of string (or line, in multiline mode)
183: $ assert end of string (or line, in multiline mode)
184: . match any character except newline (by default)
185: [ start character class definition
186: | start of alternative branch
187: ( start subpattern
188: ) end subpattern
189: ? extends the meaning of (
190: also 0 or 1 quantifier
191: also quantifier minimizer
192: * 0 or more quantifier
193: + 1 or more quantifier
194: also "possessive quantifier"
195: { start min/max quantifier
196: </pre>
197: Part of a pattern that is in square brackets is called a "character class". In
198: a character class the only metacharacters are:
199: <pre>
200: \ general escape character
201: ^ negate the class, but only if the first character
202: - indicates character range
203: [ POSIX character class (only if followed by POSIX syntax)
204: ] terminates the character class
205: </pre>
206: The following sections describe the use of each of the metacharacters.
207: </P>
208: <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
209: <P>
210: The backslash character has several uses. Firstly, if it is followed by a
211: character that is not a number or a letter, it takes away any special meaning
212: that character may have. This use of backslash as an escape character applies
213: both inside and outside character classes.
214: </P>
215: <P>
216: For example, if you want to match a * character, you write \* in the pattern.
217: This escaping action applies whether or not the following character would
218: otherwise be interpreted as a metacharacter, so it is always safe to precede a
219: non-alphanumeric with backslash to specify that it stands for itself. In
220: particular, if you want to match a backslash, you write \\.
221: </P>
222: <P>
223: In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
224: backslash. All other characters (in particular, those whose codepoints are
225: greater than 127) are treated as literals.
226: </P>
227: <P>
228: If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
229: pattern (other than in a character class) and characters between a # outside
230: a character class and the next newline are ignored. An escaping backslash can
231: be used to include a whitespace or # character as part of the pattern.
232: </P>
233: <P>
234: If you want to remove the special meaning from a sequence of characters, you
235: can do so by putting them between \Q and \E. This is different from Perl in
236: that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
237: Perl, $ and @ cause variable interpolation. Note the following examples:
238: <pre>
239: Pattern PCRE matches Perl matches
240:
241: \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
242: \Qabc\$xyz\E abc\$xyz abc\$xyz
243: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
244: </pre>
245: The \Q...\E sequence is recognized both inside and outside character classes.
246: An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
247: by \E later in the pattern, the literal interpretation continues to the end of
248: the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
249: a character class, this causes an error, because the character class is not
250: terminated.
251: <a name="digitsafterbackslash"></a></P>
252: <br><b>
253: Non-printing characters
254: </b><br>
255: <P>
256: A second use of backslash provides a way of encoding non-printing characters
257: in patterns in a visible manner. There is no restriction on the appearance of
258: non-printing characters, apart from the binary zero that terminates a pattern,
259: but when a pattern is being prepared by text editing, it is often easier to use
260: one of the following escape sequences than the binary character it represents:
261: <pre>
262: \a alarm, that is, the BEL character (hex 07)
263: \cx "control-x", where x is any ASCII character
264: \e escape (hex 1B)
265: \f formfeed (hex 0C)
266: \n linefeed (hex 0A)
267: \r carriage return (hex 0D)
268: \t tab (hex 09)
269: \ddd character with octal code ddd, or back reference
270: \xhh character with hex code hh
271: \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
272: \uhhhh character with hex code hhhh (JavaScript mode only)
273: </pre>
274: The precise effect of \cx is as follows: if x is a lower case letter, it
275: is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
276: Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
277: \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
278: than 127, a compile-time error occurs. This locks out non-ASCII characters in
279: both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
280: values are valid. A lower case letter is converted to upper case, and then the
281: 0xc0 bits are flipped.)
282: </P>
283: <P>
284: By default, after \x, from zero to two hexadecimal digits are read (letters
285: can be in upper or lower case). Any number of hexadecimal digits may appear
286: between \x{ and }, but the value of the character code must be less than 256
287: in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum
288: value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest
289: Unicode code point, which is 10FFFF.
290: </P>
291: <P>
292: If characters other than hexadecimal digits appear between \x{ and }, or if
293: there is no terminating }, this form of escape is not recognized. Instead, the
294: initial \x will be interpreted as a basic hexadecimal escape, with no
295: following digits, giving a character whose value is zero.
296: </P>
297: <P>
298: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
299: as just described only when it is followed by two hexadecimal digits.
300: Otherwise, it matches a literal "x" character. In JavaScript mode, support for
301: code points greater than 256 is provided by \u, which must be followed by
302: four hexadecimal digits; otherwise it matches a literal "u" character.
303: </P>
304: <P>
305: Characters whose value is less than 256 can be defined by either of the two
306: syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
307: way they are handled. For example, \xdc is exactly the same as \x{dc} (or
308: \u00dc in JavaScript mode).
309: </P>
310: <P>
311: After \0 up to two further octal digits are read. If there are fewer than two
312: digits, just those that are present are used. Thus the sequence \0\x\07
313: specifies two binary zeros followed by a BEL character (code value 7). Make
314: sure you supply two digits after the initial zero if the pattern character that
315: follows is itself an octal digit.
316: </P>
317: <P>
318: The handling of a backslash followed by a digit other than 0 is complicated.
319: Outside a character class, PCRE reads it and any following digits as a decimal
320: number. If the number is less than 10, or if there have been at least that many
321: previous capturing left parentheses in the expression, the entire sequence is
322: taken as a <i>back reference</i>. A description of how this works is given
323: <a href="#backreferences">later,</a>
324: following the discussion of
325: <a href="#subpattern">parenthesized subpatterns.</a>
326: </P>
327: <P>
328: Inside a character class, or if the decimal number is greater than 9 and there
329: have not been that many capturing subpatterns, PCRE re-reads up to three octal
330: digits following the backslash, and uses them to generate a data character. Any
331: subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
332: character specified in octal must be less than \400. In UTF-8 mode, values up
333: to \777 are permitted. For example:
334: <pre>
335: \040 is another way of writing a space
336: \40 is the same, provided there are fewer than 40 previous capturing subpatterns
337: \7 is always a back reference
338: \11 might be a back reference, or another way of writing a tab
339: \011 is always a tab
340: \0113 is a tab followed by the character "3"
341: \113 might be a back reference, otherwise the character with octal code 113
342: \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
343: \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
344: </pre>
345: Note that octal values of 100 or greater must not be introduced by a leading
346: zero, because no more than three octal digits are ever read.
347: </P>
348: <P>
349: All the sequences that define a single character value can be used both inside
350: and outside character classes. In addition, inside a character class, \b is
351: interpreted as the backspace character (hex 08).
352: </P>
353: <P>
354: \N is not allowed in a character class. \B, \R, and \X are not special
355: inside a character class. Like other unrecognized escape sequences, they are
356: treated as the literal characters "B", "R", and "X" by default, but cause an
357: error if the PCRE_EXTRA option is set. Outside a character class, these
358: sequences have different meanings.
359: </P>
360: <br><b>
361: Unsupported escape sequences
362: </b><br>
363: <P>
364: In Perl, the sequences \l, \L, \u, and \U are recognized by its string
365: handler and used to modify the case of following characters. By default, PCRE
366: does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
367: option is set, \U matches a "U" character, and \u can be used to define a
368: character by code point, as described in the previous section.
369: </P>
370: <br><b>
371: Absolute and relative back references
372: </b><br>
373: <P>
374: The sequence \g followed by an unsigned or a negative number, optionally
375: enclosed in braces, is an absolute or relative back reference. A named back
376: reference can be coded as \g{name}. Back references are discussed
377: <a href="#backreferences">later,</a>
378: following the discussion of
379: <a href="#subpattern">parenthesized subpatterns.</a>
380: </P>
381: <br><b>
382: Absolute and relative subroutine calls
383: </b><br>
384: <P>
385: For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
386: a number enclosed either in angle brackets or single quotes, is an alternative
387: syntax for referencing a subpattern as a "subroutine". Details are discussed
388: <a href="#onigurumasubroutines">later.</a>
389: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
390: synonymous. The former is a back reference; the latter is a
391: <a href="#subpatternsassubroutines">subroutine</a>
392: call.
393: <a name="genericchartypes"></a></P>
394: <br><b>
395: Generic character types
396: </b><br>
397: <P>
398: Another use of backslash is for specifying generic character types:
399: <pre>
400: \d any decimal digit
401: \D any character that is not a decimal digit
402: \h any horizontal whitespace character
403: \H any character that is not a horizontal whitespace character
404: \s any whitespace character
405: \S any character that is not a whitespace character
406: \v any vertical whitespace character
407: \V any character that is not a vertical whitespace character
408: \w any "word" character
409: \W any "non-word" character
410: </pre>
411: There is also the single sequence \N, which matches a non-newline character.
412: This is the same as
413: <a href="#fullstopdot">the "." metacharacter</a>
414: when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
415: PCRE does not support this.
416: </P>
417: <P>
418: Each pair of lower and upper case escape sequences partitions the complete set
419: of characters into two disjoint sets. Any given character matches one, and only
420: one, of each pair. The sequences can appear both inside and outside character
421: classes. They each match one character of the appropriate type. If the current
422: matching point is at the end of the subject string, all of them fail, because
423: there is no character to match.
424: </P>
425: <P>
426: For compatibility with Perl, \s does not match the VT character (code 11).
427: This makes it different from the the POSIX "space" class. The \s characters
428: are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
429: included in a Perl script, \s may match the VT character. In PCRE, it never
430: does.
431: </P>
432: <P>
433: A "word" character is an underscore or any character that is a letter or digit.
434: By default, the definition of letters and digits is controlled by PCRE's
435: low-valued character tables, and may vary if locale-specific matching is taking
436: place (see
437: <a href="pcreapi.html#localesupport">"Locale support"</a>
438: in the
439: <a href="pcreapi.html"><b>pcreapi</b></a>
440: page). For example, in a French locale such as "fr_FR" in Unix-like systems,
441: or "french" in Windows, some character codes greater than 128 are used for
442: accented letters, and these are then matched by \w. The use of locales with
443: Unicode is discouraged.
444: </P>
445: <P>
446: By default, in UTF-8 mode, characters with values greater than 128 never match
447: \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
448: their original meanings from before UTF-8 support was available, mainly for
449: efficiency reasons. However, if PCRE is compiled with Unicode property support,
450: and the PCRE_UCP option is set, the behaviour is changed so that Unicode
451: properties are used to determine character types, as follows:
452: <pre>
453: \d any character that \p{Nd} matches (decimal digit)
454: \s any character that \p{Z} matches, plus HT, LF, FF, CR
455: \w any character that \p{L} or \p{N} matches, plus underscore
456: </pre>
457: The upper case escapes match the inverse sets of characters. Note that \d
458: matches only decimal digits, whereas \w matches any Unicode digit, as well as
459: any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
460: \B because they are defined in terms of \w and \W. Matching these sequences
461: is noticeably slower when PCRE_UCP is set.
462: </P>
463: <P>
464: The sequences \h, \H, \v, and \V are features that were added to Perl at
465: release 5.10. In contrast to the other sequences, which match only ASCII
466: characters by default, these always match certain high-valued codepoints in
467: UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
468: are:
469: <pre>
470: U+0009 Horizontal tab
471: U+0020 Space
472: U+00A0 Non-break space
473: U+1680 Ogham space mark
474: U+180E Mongolian vowel separator
475: U+2000 En quad
476: U+2001 Em quad
477: U+2002 En space
478: U+2003 Em space
479: U+2004 Three-per-em space
480: U+2005 Four-per-em space
481: U+2006 Six-per-em space
482: U+2007 Figure space
483: U+2008 Punctuation space
484: U+2009 Thin space
485: U+200A Hair space
486: U+202F Narrow no-break space
487: U+205F Medium mathematical space
488: U+3000 Ideographic space
489: </pre>
490: The vertical space characters are:
491: <pre>
492: U+000A Linefeed
493: U+000B Vertical tab
494: U+000C Formfeed
495: U+000D Carriage return
496: U+0085 Next line
497: U+2028 Line separator
498: U+2029 Paragraph separator
499: <a name="newlineseq"></a></PRE>
500: </P>
501: <br><b>
502: Newline sequences
503: </b><br>
504: <P>
505: Outside a character class, by default, the escape sequence \R matches any
506: Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following:
507: <pre>
508: (?>\r\n|\n|\x0b|\f|\r|\x85)
509: </pre>
510: This is an example of an "atomic group", details of which are given
511: <a href="#atomicgroup">below.</a>
512: This particular group matches either the two-character sequence CR followed by
513: LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
514: U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
515: line, U+0085). The two-character sequence is treated as a single unit that
516: cannot be split.
517: </P>
518: <P>
519: In UTF-8 mode, two additional characters whose codepoints are greater than 255
520: are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
521: Unicode character property support is not needed for these characters to be
522: recognized.
523: </P>
524: <P>
525: It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
526: complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
527: either at compile time or when the pattern is matched. (BSR is an abbrevation
528: for "backslash R".) This can be made the default when PCRE is built; if this is
529: the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
530: It is also possible to specify these settings by starting a pattern string with
531: one of the following sequences:
532: <pre>
533: (*BSR_ANYCRLF) CR, LF, or CRLF only
534: (*BSR_UNICODE) any Unicode newline sequence
535: </pre>
536: These override the default and the options given to <b>pcre_compile()</b> or
537: <b>pcre_compile2()</b>, but they can be overridden by options given to
538: <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,
539: which are not Perl-compatible, are recognized only at the very start of a
540: pattern, and that they must be in upper case. If more than one of them is
541: present, the last one is used. They can be combined with a change of newline
542: convention; for example, a pattern can start with:
543: <pre>
544: (*ANY)(*BSR_ANYCRLF)
545: </pre>
546: They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
547: a character class, \R is treated as an unrecognized escape sequence, and so
548: matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
549: <a name="uniextseq"></a></P>
550: <br><b>
551: Unicode character properties
552: </b><br>
553: <P>
554: When PCRE is built with Unicode character property support, three additional
555: escape sequences that match characters with specific properties are available.
556: When not in UTF-8 mode, these sequences are of course limited to testing
557: characters whose codepoints are less than 256, but they do work in this mode.
558: The extra escape sequences are:
559: <pre>
560: \p{<i>xx</i>} a character with the <i>xx</i> property
561: \P{<i>xx</i>} a character without the <i>xx</i> property
562: \X an extended Unicode sequence
563: </pre>
564: The property names represented by <i>xx</i> above are limited to the Unicode
565: script names, the general category properties, "Any", which matches any
566: character (including newline), and some special PCRE properties (described
567: in the
568: <a href="#extraprops">next section).</a>
569: Other Perl properties such as "InMusicalSymbols" are not currently supported by
570: PCRE. Note that \P{Any} does not match any characters, so always causes a
571: match failure.
572: </P>
573: <P>
574: Sets of Unicode characters are defined as belonging to certain scripts. A
575: character from one of these sets can be matched using a script name. For
576: example:
577: <pre>
578: \p{Greek}
579: \P{Han}
580: </pre>
581: Those that are not part of an identified script are lumped together as
582: "Common". The current list of scripts is:
583: </P>
584: <P>
585: Arabic,
586: Armenian,
587: Avestan,
588: Balinese,
589: Bamum,
590: Bengali,
591: Bopomofo,
592: Braille,
593: Buginese,
594: Buhid,
595: Canadian_Aboriginal,
596: Carian,
597: Cham,
598: Cherokee,
599: Common,
600: Coptic,
601: Cuneiform,
602: Cypriot,
603: Cyrillic,
604: Deseret,
605: Devanagari,
606: Egyptian_Hieroglyphs,
607: Ethiopic,
608: Georgian,
609: Glagolitic,
610: Gothic,
611: Greek,
612: Gujarati,
613: Gurmukhi,
614: Han,
615: Hangul,
616: Hanunoo,
617: Hebrew,
618: Hiragana,
619: Imperial_Aramaic,
620: Inherited,
621: Inscriptional_Pahlavi,
622: Inscriptional_Parthian,
623: Javanese,
624: Kaithi,
625: Kannada,
626: Katakana,
627: Kayah_Li,
628: Kharoshthi,
629: Khmer,
630: Lao,
631: Latin,
632: Lepcha,
633: Limbu,
634: Linear_B,
635: Lisu,
636: Lycian,
637: Lydian,
638: Malayalam,
639: Meetei_Mayek,
640: Mongolian,
641: Myanmar,
642: New_Tai_Lue,
643: Nko,
644: Ogham,
645: Old_Italic,
646: Old_Persian,
647: Old_South_Arabian,
648: Old_Turkic,
649: Ol_Chiki,
650: Oriya,
651: Osmanya,
652: Phags_Pa,
653: Phoenician,
654: Rejang,
655: Runic,
656: Samaritan,
657: Saurashtra,
658: Shavian,
659: Sinhala,
660: Sundanese,
661: Syloti_Nagri,
662: Syriac,
663: Tagalog,
664: Tagbanwa,
665: Tai_Le,
666: Tai_Tham,
667: Tai_Viet,
668: Tamil,
669: Telugu,
670: Thaana,
671: Thai,
672: Tibetan,
673: Tifinagh,
674: Ugaritic,
675: Vai,
676: Yi.
677: </P>
678: <P>
679: Each character has exactly one Unicode general category property, specified by
680: a two-letter abbreviation. For compatibility with Perl, negation can be
681: specified by including a circumflex between the opening brace and the property
682: name. For example, \p{^Lu} is the same as \P{Lu}.
683: </P>
684: <P>
685: If only one letter is specified with \p or \P, it includes all the general
686: category properties that start with that letter. In this case, in the absence
687: of negation, the curly brackets in the escape sequence are optional; these two
688: examples have the same effect:
689: <pre>
690: \p{L}
691: \pL
692: </pre>
693: The following general category property codes are supported:
694: <pre>
695: C Other
696: Cc Control
697: Cf Format
698: Cn Unassigned
699: Co Private use
700: Cs Surrogate
701:
702: L Letter
703: Ll Lower case letter
704: Lm Modifier letter
705: Lo Other letter
706: Lt Title case letter
707: Lu Upper case letter
708:
709: M Mark
710: Mc Spacing mark
711: Me Enclosing mark
712: Mn Non-spacing mark
713:
714: N Number
715: Nd Decimal number
716: Nl Letter number
717: No Other number
718:
719: P Punctuation
720: Pc Connector punctuation
721: Pd Dash punctuation
722: Pe Close punctuation
723: Pf Final punctuation
724: Pi Initial punctuation
725: Po Other punctuation
726: Ps Open punctuation
727:
728: S Symbol
729: Sc Currency symbol
730: Sk Modifier symbol
731: Sm Mathematical symbol
732: So Other symbol
733:
734: Z Separator
735: Zl Line separator
736: Zp Paragraph separator
737: Zs Space separator
738: </pre>
739: The special property L& is also supported: it matches a character that has
740: the Lu, Ll, or Lt property, in other words, a letter that is not classified as
741: a modifier or "other".
742: </P>
743: <P>
744: The Cs (Surrogate) property applies only to characters in the range U+D800 to
745: U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
746: cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
747: (see the discussion of PCRE_NO_UTF8_CHECK in the
748: <a href="pcreapi.html"><b>pcreapi</b></a>
749: page). Perl does not support the Cs property.
750: </P>
751: <P>
752: The long synonyms for property names that Perl supports (such as \p{Letter})
753: are not supported by PCRE, nor is it permitted to prefix any of these
754: properties with "Is".
755: </P>
756: <P>
757: No character that is in the Unicode table has the Cn (unassigned) property.
758: Instead, this property is assumed for any code point that is not in the
759: Unicode table.
760: </P>
761: <P>
762: Specifying caseless matching does not affect these escape sequences. For
763: example, \p{Lu} always matches only upper case letters.
764: </P>
765: <P>
766: The \X escape matches any number of Unicode characters that form an extended
767: Unicode sequence. \X is equivalent to
768: <pre>
769: (?>\PM\pM*)
770: </pre>
771: That is, it matches a character without the "mark" property, followed by zero
772: or more characters with the "mark" property, and treats the sequence as an
773: atomic group
774: <a href="#atomicgroup">(see below).</a>
775: Characters with the "mark" property are typically accents that affect the
776: preceding character. None of them have codepoints less than 256, so in
777: non-UTF-8 mode \X matches any one character.
778: </P>
779: <P>
780: Note that recent versions of Perl have changed \X to match what Unicode calls
781: an "extended grapheme cluster", which has a more complicated definition.
782: </P>
783: <P>
784: Matching characters by Unicode property is not fast, because PCRE has to search
785: a structure that contains data for over fifteen thousand characters. That is
786: why the traditional escape sequences such as \d and \w do not use Unicode
787: properties in PCRE by default, though you can make them do so by setting the
788: PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with
789: (*UCP).
790: <a name="extraprops"></a></P>
791: <br><b>
792: PCRE's additional properties
793: </b><br>
794: <P>
795: As well as the standard Unicode properties described in the previous
796: section, PCRE supports four more that make it possible to convert traditional
797: escape sequences such as \w and \s and POSIX character classes to use Unicode
798: properties. PCRE uses these non-standard, non-Perl properties internally when
799: PCRE_UCP is set. They are:
800: <pre>
801: Xan Any alphanumeric character
802: Xps Any POSIX space character
803: Xsp Any Perl space character
804: Xwd Any Perl "word" character
805: </pre>
806: Xan matches characters that have either the L (letter) or the N (number)
807: property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
808: carriage return, and any other character that has the Z (separator) property.
809: Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
810: same characters as Xan, plus underscore.
811: <a name="resetmatchstart"></a></P>
812: <br><b>
813: Resetting the match start
814: </b><br>
815: <P>
816: The escape sequence \K causes any previously matched characters not to be
817: included in the final matched sequence. For example, the pattern:
818: <pre>
819: foo\Kbar
820: </pre>
821: matches "foobar", but reports that it has matched "bar". This feature is
822: similar to a lookbehind assertion
823: <a href="#lookbehind">(described below).</a>
824: However, in this case, the part of the subject before the real match does not
825: have to be of fixed length, as lookbehind assertions do. The use of \K does
826: not interfere with the setting of
827: <a href="#subpattern">captured substrings.</a>
828: For example, when the pattern
829: <pre>
830: (foo)\Kbar
831: </pre>
832: matches "foobar", the first substring is still set to "foo".
833: </P>
834: <P>
835: Perl documents that the use of \K within assertions is "not well defined". In
836: PCRE, \K is acted upon when it occurs inside positive assertions, but is
837: ignored in negative assertions.
838: <a name="smallassertions"></a></P>
839: <br><b>
840: Simple assertions
841: </b><br>
842: <P>
843: The final use of backslash is for certain simple assertions. An assertion
844: specifies a condition that has to be met at a particular point in a match,
845: without consuming any characters from the subject string. The use of
846: subpatterns for more complicated assertions is described
847: <a href="#bigassertions">below.</a>
848: The backslashed assertions are:
849: <pre>
850: \b matches at a word boundary
851: \B matches when not at a word boundary
852: \A matches at the start of the subject
853: \Z matches at the end of the subject
854: also matches before a newline at the end of the subject
855: \z matches only at the end of the subject
856: \G matches at the first matching position in the subject
857: </pre>
858: Inside a character class, \b has a different meaning; it matches the backspace
859: character. If any other of these assertions appears in a character class, by
860: default it matches the corresponding literal character (for example, \B
861: matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
862: escape sequence" error is generated instead.
863: </P>
864: <P>
865: A word boundary is a position in the subject string where the current character
866: and the previous character do not both match \w or \W (i.e. one matches
867: \w and the other matches \W), or the start or end of the string if the
868: first or last character matches \w, respectively. In UTF-8 mode, the meanings
869: of \w and \W can be changed by setting the PCRE_UCP option. When this is
870: done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
871: of word" or "end of word" metasequence. However, whatever follows \b normally
872: determines which it is. For example, the fragment \ba matches "a" at the start
873: of a word.
874: </P>
875: <P>
876: The \A, \Z, and \z assertions differ from the traditional circumflex and
877: dollar (described in the next section) in that they only ever match at the very
878: start and end of the subject string, whatever options are set. Thus, they are
879: independent of multiline mode. These three assertions are not affected by the
880: PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
881: circumflex and dollar metacharacters. However, if the <i>startoffset</i>
882: argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
883: at a point other than the beginning of the subject, \A can never match. The
884: difference between \Z and \z is that \Z matches before a newline at the end
885: of the string as well as at the very end, whereas \z matches only at the end.
886: </P>
887: <P>
888: The \G assertion is true only when the current matching position is at the
889: start point of the match, as specified by the <i>startoffset</i> argument of
890: <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
891: non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
892: arguments, you can mimic Perl's /g option, and it is in this kind of
893: implementation where \G can be useful.
894: </P>
895: <P>
896: Note, however, that PCRE's interpretation of \G, as the start of the current
897: match, is subtly different from Perl's, which defines it as the end of the
898: previous match. In Perl, these can be different when the previously matched
899: string was empty. Because PCRE does just one match at a time, it cannot
900: reproduce this behaviour.
901: </P>
902: <P>
903: If all the alternatives of a pattern begin with \G, the expression is anchored
904: to the starting match position, and the "anchored" flag is set in the compiled
905: regular expression.
906: </P>
907: <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
908: <P>
909: Outside a character class, in the default matching mode, the circumflex
910: character is an assertion that is true only if the current matching point is
911: at the start of the subject string. If the <i>startoffset</i> argument of
912: <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
913: option is unset. Inside a character class, circumflex has an entirely different
914: meaning
915: <a href="#characterclass">(see below).</a>
916: </P>
917: <P>
918: Circumflex need not be the first character of the pattern if a number of
919: alternatives are involved, but it should be the first thing in each alternative
920: in which it appears if the pattern is ever to match that branch. If all
921: possible alternatives start with a circumflex, that is, if the pattern is
922: constrained to match only at the start of the subject, it is said to be an
923: "anchored" pattern. (There are also other constructs that can cause a pattern
924: to be anchored.)
925: </P>
926: <P>
927: A dollar character is an assertion that is true only if the current matching
928: point is at the end of the subject string, or immediately before a newline
929: at the end of the string (by default). Dollar need not be the last character of
930: the pattern if a number of alternatives are involved, but it should be the last
931: item in any branch in which it appears. Dollar has no special meaning in a
932: character class.
933: </P>
934: <P>
935: The meaning of dollar can be changed so that it matches only at the very end of
936: the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
937: does not affect the \Z assertion.
938: </P>
939: <P>
940: The meanings of the circumflex and dollar characters are changed if the
941: PCRE_MULTILINE option is set. When this is the case, a circumflex matches
942: immediately after internal newlines as well as at the start of the subject
943: string. It does not match after a newline that ends the string. A dollar
944: matches before any newlines in the string, as well as at the very end, when
945: PCRE_MULTILINE is set. When newline is specified as the two-character
946: sequence CRLF, isolated CR and LF characters do not indicate newlines.
947: </P>
948: <P>
949: For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
950: \n represents a newline) in multiline mode, but not otherwise. Consequently,
951: patterns that are anchored in single line mode because all branches start with
952: ^ are not anchored in multiline mode, and a match for circumflex is possible
953: when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
954: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
955: </P>
956: <P>
957: Note that the sequences \A, \Z, and \z can be used to match the start and
958: end of the subject in both modes, and if all branches of a pattern start with
959: \A it is always anchored, whether or not PCRE_MULTILINE is set.
960: <a name="fullstopdot"></a></P>
961: <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
962: <P>
963: Outside a character class, a dot in the pattern matches any one character in
964: the subject string except (by default) a character that signifies the end of a
965: line. In UTF-8 mode, the matched character may be more than one byte long.
966: </P>
967: <P>
968: When a line ending is defined as a single character, dot never matches that
969: character; when the two-character sequence CRLF is used, dot does not match CR
970: if it is immediately followed by LF, but otherwise it matches all characters
971: (including isolated CRs and LFs). When any Unicode line endings are being
972: recognized, dot does not match CR or LF or any of the other line ending
973: characters.
974: </P>
975: <P>
976: The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
977: option is set, a dot matches any one character, without exception. If the
978: two-character sequence CRLF is present in the subject string, it takes two dots
979: to match it.
980: </P>
981: <P>
982: The handling of dot is entirely independent of the handling of circumflex and
983: dollar, the only relationship being that they both involve newlines. Dot has no
984: special meaning in a character class.
985: </P>
986: <P>
987: The escape sequence \N behaves like a dot, except that it is not affected by
988: the PCRE_DOTALL option. In other words, it matches any character except one
989: that signifies the end of a line. Perl also uses \N to match characters by
990: name; PCRE does not support this.
991: </P>
992: <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
993: <P>
994: Outside a character class, the escape sequence \C matches any one byte, both
995: in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
996: characters. The feature is provided in Perl in order to match individual bytes
997: in UTF-8 mode, but it is unclear how it can usefully be used. Because \C
998: breaks up characters into individual bytes, matching one byte with \C in UTF-8
999: mode means that the rest of the string may start with a malformed UTF-8
1000: character. This has undefined results, because PCRE assumes that it is dealing
1001: with valid UTF-8 strings (and by default it checks this at the start of
1002: processing unless the PCRE_NO_UTF8_CHECK option is used).
1003: </P>
1004: <P>
1005: PCRE does not allow \C to appear in lookbehind assertions
1006: <a href="#lookbehind">(described below)</a>
1007: in UTF-8 mode, because this would make it impossible to calculate the length of
1008: the lookbehind.
1009: </P>
1010: <P>
1011: In general, the \C escape sequence is best avoided in UTF-8 mode. However, one
1012: way of using it that avoids the problem of malformed UTF-8 characters is to
1013: use a lookahead to check the length of the next character, as in this pattern
1014: (ignore white space and line breaks):
1015: <pre>
1016: (?| (?=[\x00-\x7f])(\C) |
1017: (?=[\x80-\x{7ff}])(\C)(\C) |
1018: (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1019: (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1020: </pre>
1021: A group that starts with (?| resets the capturing parentheses numbers in each
1022: alternative (see
1023: <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1024: below). The assertions at the start of each branch check the next UTF-8
1025: character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1026: character's individual bytes are then captured by the appropriate number of
1027: groups.
1028: <a name="characterclass"></a></P>
1029: <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1030: <P>
1031: An opening square bracket introduces a character class, terminated by a closing
1032: square bracket. A closing square bracket on its own is not special by default.
1033: However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1034: bracket causes a compile-time error. If a closing square bracket is required as
1035: a member of the class, it should be the first data character in the class
1036: (after an initial circumflex, if present) or escaped with a backslash.
1037: </P>
1038: <P>
1039: A character class matches a single character in the subject. In UTF-8 mode, the
1040: character may be more than one byte long. A matched character must be in the
1041: set of characters defined by the class, unless the first character in the class
1042: definition is a circumflex, in which case the subject character must not be in
1043: the set defined by the class. If a circumflex is actually required as a member
1044: of the class, ensure it is not the first character, or escape it with a
1045: backslash.
1046: </P>
1047: <P>
1048: For example, the character class [aeiou] matches any lower case vowel, while
1049: [^aeiou] matches any character that is not a lower case vowel. Note that a
1050: circumflex is just a convenient notation for specifying the characters that
1051: are in the class by enumerating those that are not. A class that starts with a
1052: circumflex is not an assertion; it still consumes a character from the subject
1053: string, and therefore it fails if the current pointer is at the end of the
1054: string.
1055: </P>
1056: <P>
1057: In UTF-8 mode, characters with values greater than 255 can be included in a
1058: class as a literal string of bytes, or by using the \x{ escaping mechanism.
1059: </P>
1060: <P>
1061: When caseless matching is set, any letters in a class represent both their
1062: upper case and lower case versions, so for example, a caseless [aeiou] matches
1063: "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1064: caseful version would. In UTF-8 mode, PCRE always understands the concept of
1065: case for characters whose values are less than 128, so caseless matching is
1066: always possible. For characters with higher values, the concept of case is
1067: supported if PCRE is compiled with Unicode property support, but not otherwise.
1068: If you want to use caseless matching in UTF8-mode for characters 128 and above,
1069: you must ensure that PCRE is compiled with Unicode property support as well as
1070: with UTF-8 support.
1071: </P>
1072: <P>
1073: Characters that might indicate line breaks are never treated in any special way
1074: when matching character classes, whatever line-ending sequence is in use, and
1075: whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1076: such as [^a] always matches one of these characters.
1077: </P>
1078: <P>
1079: The minus (hyphen) character can be used to specify a range of characters in a
1080: character class. For example, [d-m] matches any letter between d and m,
1081: inclusive. If a minus character is required in a class, it must be escaped with
1082: a backslash or appear in a position where it cannot be interpreted as
1083: indicating a range, typically as the first or last character in the class.
1084: </P>
1085: <P>
1086: It is not possible to have the literal character "]" as the end character of a
1087: range. A pattern such as [W-]46] is interpreted as a class of two characters
1088: ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1089: "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1090: the end of range, so [W-\]46] is interpreted as a class containing a range
1091: followed by two other characters. The octal or hexadecimal representation of
1092: "]" can also be used to end a range.
1093: </P>
1094: <P>
1095: Ranges operate in the collating sequence of character values. They can also be
1096: used for characters specified numerically, for example [\000-\037]. In UTF-8
1097: mode, ranges can include characters whose values are greater than 255, for
1098: example [\x{100}-\x{2ff}].
1099: </P>
1100: <P>
1101: If a range that includes letters is used when caseless matching is set, it
1102: matches the letters in either case. For example, [W-c] is equivalent to
1103: [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1104: tables for a French locale are in use, [\xc8-\xcb] matches accented E
1105: characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1106: characters with values greater than 128 only when it is compiled with Unicode
1107: property support.
1108: </P>
1109: <P>
1110: The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1111: \V, \w, and \W may appear in a character class, and add the characters that
1112: they match to the class. For example, [\dABCDEF] matches any hexadecimal
1113: digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w
1114: and their upper case partners, just as it does when they appear outside a
1115: character class, as described in the section entitled
1116: <a href="#genericchartypes">"Generic character types"</a>
1117: above. The escape sequence \b has a different meaning inside a character
1118: class; it matches the backspace character. The sequences \B, \N, \R, and \X
1119: are not special inside a character class. Like any other unrecognized escape
1120: sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1121: default, but cause an error if the PCRE_EXTRA option is set.
1122: </P>
1123: <P>
1124: A circumflex can conveniently be used with the upper case character types to
1125: specify a more restricted set of characters than the matching lower case type.
1126: For example, the class [^\W_] matches any letter or digit, but not underscore,
1127: whereas [\w] includes underscore. A positive character class should be read as
1128: "something OR something OR ..." and a negative class as "NOT something AND NOT
1129: something AND NOT ...".
1130: </P>
1131: <P>
1132: The only metacharacters that are recognized in character classes are backslash,
1133: hyphen (only where it can be interpreted as specifying a range), circumflex
1134: (only at the start), opening square bracket (only when it can be interpreted as
1135: introducing a POSIX class name - see the next section), and the terminating
1136: closing square bracket. However, escaping other non-alphanumeric characters
1137: does no harm.
1138: </P>
1139: <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1140: <P>
1141: Perl supports the POSIX notation for character classes. This uses names
1142: enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1143: this notation. For example,
1144: <pre>
1145: [01[:alpha:]%]
1146: </pre>
1147: matches "0", "1", any alphabetic character, or "%". The supported class names
1148: are:
1149: <pre>
1150: alnum letters and digits
1151: alpha letters
1152: ascii character codes 0 - 127
1153: blank space or tab only
1154: cntrl control characters
1155: digit decimal digits (same as \d)
1156: graph printing characters, excluding space
1157: lower lower case letters
1158: print printing characters, including space
1159: punct printing characters, excluding letters and digits and space
1160: space white space (not quite the same as \s)
1161: upper upper case letters
1162: word "word" characters (same as \w)
1163: xdigit hexadecimal digits
1164: </pre>
1165: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1166: space (32). Notice that this list includes the VT character (code 11). This
1167: makes "space" different to \s, which does not include VT (for Perl
1168: compatibility).
1169: </P>
1170: <P>
1171: The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1172: 5.8. Another Perl extension is negation, which is indicated by a ^ character
1173: after the colon. For example,
1174: <pre>
1175: [12[:^digit:]]
1176: </pre>
1177: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1178: syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1179: supported, and an error is given if they are encountered.
1180: </P>
1181: <P>
1182: By default, in UTF-8 mode, characters with values greater than 128 do not match
1183: any of the POSIX character classes. However, if the PCRE_UCP option is passed
1184: to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1185: character properties are used. This is achieved by replacing the POSIX classes
1186: by other sequences, as follows:
1187: <pre>
1188: [:alnum:] becomes \p{Xan}
1189: [:alpha:] becomes \p{L}
1190: [:blank:] becomes \h
1191: [:digit:] becomes \p{Nd}
1192: [:lower:] becomes \p{Ll}
1193: [:space:] becomes \p{Xps}
1194: [:upper:] becomes \p{Lu}
1195: [:word:] becomes \p{Xwd}
1196: </pre>
1197: Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1198: classes are unchanged, and match only characters with code points less than
1199: 128.
1200: </P>
1201: <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1202: <P>
1203: Vertical bar characters are used to separate alternative patterns. For example,
1204: the pattern
1205: <pre>
1206: gilbert|sullivan
1207: </pre>
1208: matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1209: and an empty alternative is permitted (matching the empty string). The matching
1210: process tries each alternative in turn, from left to right, and the first one
1211: that succeeds is used. If the alternatives are within a subpattern
1212: <a href="#subpattern">(defined below),</a>
1213: "succeeds" means matching the rest of the main pattern as well as the
1214: alternative in the subpattern.
1215: </P>
1216: <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1217: <P>
1218: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1219: PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1220: the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1221: The option letters are
1222: <pre>
1223: i for PCRE_CASELESS
1224: m for PCRE_MULTILINE
1225: s for PCRE_DOTALL
1226: x for PCRE_EXTENDED
1227: </pre>
1228: For example, (?im) sets caseless, multiline matching. It is also possible to
1229: unset these options by preceding the letter with a hyphen, and a combined
1230: setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1231: PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1232: permitted. If a letter appears both before and after the hyphen, the option is
1233: unset.
1234: </P>
1235: <P>
1236: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1237: changed in the same way as the Perl-compatible options by using the characters
1238: J, U and X respectively.
1239: </P>
1240: <P>
1241: When one of these option changes occurs at top level (that is, not inside
1242: subpattern parentheses), the change applies to the remainder of the pattern
1243: that follows. If the change is placed right at the start of a pattern, PCRE
1244: extracts it into the global options (and it will therefore show up in data
1245: extracted by the <b>pcre_fullinfo()</b> function).
1246: </P>
1247: <P>
1248: An option change within a subpattern (see below for a description of
1249: subpatterns) affects only that part of the subpattern that follows it, so
1250: <pre>
1251: (a(?i)b)c
1252: </pre>
1253: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1254: By this means, options can be made to have different settings in different
1255: parts of the pattern. Any changes made in one alternative do carry on
1256: into subsequent branches within the same subpattern. For example,
1257: <pre>
1258: (a(?i)b|c)
1259: </pre>
1260: matches "ab", "aB", "c", and "C", even though when matching "C" the first
1261: branch is abandoned before the option setting. This is because the effects of
1262: option settings happen at compile time. There would be some very weird
1263: behaviour otherwise.
1264: </P>
1265: <P>
1266: <b>Note:</b> There are other PCRE-specific options that can be set by the
1267: application when the compile or match functions are called. In some cases the
1268: pattern can contain special leading sequences such as (*CRLF) to override what
1269: the application has set or what has been defaulted. Details are given in the
1270: section entitled
1271: <a href="#newlineseq">"Newline sequences"</a>
1272: above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1273: to set UTF-8 and Unicode property modes; they are equivalent to setting the
1274: PCRE_UTF8 and the PCRE_UCP options, respectively.
1275: <a name="subpattern"></a></P>
1276: <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1277: <P>
1278: Subpatterns are delimited by parentheses (round brackets), which can be nested.
1279: Turning part of a pattern into a subpattern does two things:
1280: <br>
1281: <br>
1282: 1. It localizes a set of alternatives. For example, the pattern
1283: <pre>
1284: cat(aract|erpillar|)
1285: </pre>
1286: matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1287: match "cataract", "erpillar" or an empty string.
1288: <br>
1289: <br>
1290: 2. It sets up the subpattern as a capturing subpattern. This means that, when
1291: the whole pattern matches, that portion of the subject string that matched the
1292: subpattern is passed back to the caller via the <i>ovector</i> argument of
1293: <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1294: from 1) to obtain numbers for the capturing subpatterns. For example, if the
1295: string "the red king" is matched against the pattern
1296: <pre>
1297: the ((red|white) (king|queen))
1298: </pre>
1299: the captured substrings are "red king", "red", and "king", and are numbered 1,
1300: 2, and 3, respectively.
1301: </P>
1302: <P>
1303: The fact that plain parentheses fulfil two functions is not always helpful.
1304: There are often times when a grouping subpattern is required without a
1305: capturing requirement. If an opening parenthesis is followed by a question mark
1306: and a colon, the subpattern does not do any capturing, and is not counted when
1307: computing the number of any subsequent capturing subpatterns. For example, if
1308: the string "the white queen" is matched against the pattern
1309: <pre>
1310: the ((?:red|white) (king|queen))
1311: </pre>
1312: the captured substrings are "white queen" and "queen", and are numbered 1 and
1313: 2. The maximum number of capturing subpatterns is 65535.
1314: </P>
1315: <P>
1316: As a convenient shorthand, if any option settings are required at the start of
1317: a non-capturing subpattern, the option letters may appear between the "?" and
1318: the ":". Thus the two patterns
1319: <pre>
1320: (?i:saturday|sunday)
1321: (?:(?i)saturday|sunday)
1322: </pre>
1323: match exactly the same set of strings. Because alternative branches are tried
1324: from left to right, and options are not reset until the end of the subpattern
1325: is reached, an option setting in one branch does affect subsequent branches, so
1326: the above patterns match "SUNDAY" as well as "Saturday".
1327: <a name="dupsubpatternnumber"></a></P>
1328: <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1329: <P>
1330: Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1331: the same numbers for its capturing parentheses. Such a subpattern starts with
1332: (?| and is itself a non-capturing subpattern. For example, consider this
1333: pattern:
1334: <pre>
1335: (?|(Sat)ur|(Sun))day
1336: </pre>
1337: Because the two alternatives are inside a (?| group, both sets of capturing
1338: parentheses are numbered one. Thus, when the pattern matches, you can look
1339: at captured substring number one, whichever alternative matched. This construct
1340: is useful when you want to capture part, but not all, of one of a number of
1341: alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1342: number is reset at the start of each branch. The numbers of any capturing
1343: parentheses that follow the subpattern start after the highest number used in
1344: any branch. The following example is taken from the Perl documentation. The
1345: numbers underneath show in which buffer the captured content will be stored.
1346: <pre>
1347: # before ---------------branch-reset----------- after
1348: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1349: # 1 2 2 3 2 3 4
1350: </pre>
1351: A back reference to a numbered subpattern uses the most recent value that is
1352: set for that number by any subpattern. The following pattern matches "abcabc"
1353: or "defdef":
1354: <pre>
1355: /(?|(abc)|(def))\1/
1356: </pre>
1357: In contrast, a subroutine call to a numbered subpattern always refers to the
1358: first one in the pattern with the given number. The following pattern matches
1359: "abcabc" or "defabc":
1360: <pre>
1361: /(?|(abc)|(def))(?1)/
1362: </pre>
1363: If a
1364: <a href="#conditions">condition test</a>
1365: for a subpattern's having matched refers to a non-unique number, the test is
1366: true if any of the subpatterns of that number have matched.
1367: </P>
1368: <P>
1369: An alternative approach to using this "branch reset" feature is to use
1370: duplicate named subpatterns, as described in the next section.
1371: </P>
1372: <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1373: <P>
1374: Identifying capturing parentheses by number is simple, but it can be very hard
1375: to keep track of the numbers in complicated regular expressions. Furthermore,
1376: if an expression is modified, the numbers may change. To help with this
1377: difficulty, PCRE supports the naming of subpatterns. This feature was not
1378: added to Perl until release 5.10. Python had the feature earlier, and PCRE
1379: introduced it at release 4.0, using the Python syntax. PCRE now supports both
1380: the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1381: have different names, but PCRE does not.
1382: </P>
1383: <P>
1384: In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1385: (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1386: parentheses from other parts of the pattern, such as
1387: <a href="#backreferences">back references,</a>
1388: <a href="#recursion">recursion,</a>
1389: and
1390: <a href="#conditions">conditions,</a>
1391: can be made by name as well as by number.
1392: </P>
1393: <P>
1394: Names consist of up to 32 alphanumeric characters and underscores. Named
1395: capturing parentheses are still allocated numbers as well as names, exactly as
1396: if the names were not present. The PCRE API provides function calls for
1397: extracting the name-to-number translation table from a compiled pattern. There
1398: is also a convenience function for extracting a captured substring by name.
1399: </P>
1400: <P>
1401: By default, a name must be unique within a pattern, but it is possible to relax
1402: this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1403: names are also always permitted for subpatterns with the same number, set up as
1404: described in the previous section.) Duplicate names can be useful for patterns
1405: where only one instance of the named parentheses can match. Suppose you want to
1406: match the name of a weekday, either as a 3-letter abbreviation or as the full
1407: name, and in both cases you want to extract the abbreviation. This pattern
1408: (ignoring the line breaks) does the job:
1409: <pre>
1410: (?<DN>Mon|Fri|Sun)(?:day)?|
1411: (?<DN>Tue)(?:sday)?|
1412: (?<DN>Wed)(?:nesday)?|
1413: (?<DN>Thu)(?:rsday)?|
1414: (?<DN>Sat)(?:urday)?
1415: </pre>
1416: There are five capturing substrings, but only one is ever set after a match.
1417: (An alternative way of solving this problem is to use a "branch reset"
1418: subpattern, as described in the previous section.)
1419: </P>
1420: <P>
1421: The convenience function for extracting the data by name returns the substring
1422: for the first (and in this example, the only) subpattern of that name that
1423: matched. This saves searching to find which numbered subpattern it was.
1424: </P>
1425: <P>
1426: If you make a back reference to a non-unique named subpattern from elsewhere in
1427: the pattern, the one that corresponds to the first occurrence of the name is
1428: used. In the absence of duplicate numbers (see the previous section) this is
1429: the one with the lowest number. If you use a named reference in a condition
1430: test (see the
1431: <a href="#conditions">section about conditions</a>
1432: below), either to check whether a subpattern has matched, or to check for
1433: recursion, all subpatterns with the same name are tested. If the condition is
1434: true for any one of them, the overall condition is true. This is the same
1435: behaviour as testing by number. For further details of the interfaces for
1436: handling named subpatterns, see the
1437: <a href="pcreapi.html"><b>pcreapi</b></a>
1438: documentation.
1439: </P>
1440: <P>
1441: <b>Warning:</b> You cannot use different names to distinguish between two
1442: subpatterns with the same number because PCRE uses only the numbers when
1443: matching. For this reason, an error is given at compile time if different names
1444: are given to subpatterns with the same number. However, you can give the same
1445: name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1446: </P>
1447: <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1448: <P>
1449: Repetition is specified by quantifiers, which can follow any of the following
1450: items:
1451: <pre>
1452: a literal data character
1453: the dot metacharacter
1454: the \C escape sequence
1455: the \X escape sequence (in UTF-8 mode with Unicode properties)
1456: the \R escape sequence
1457: an escape such as \d or \pL that matches a single character
1458: a character class
1459: a back reference (see next section)
1460: a parenthesized subpattern (including assertions)
1461: a subroutine call to a subpattern (recursive or otherwise)
1462: </pre>
1463: The general repetition quantifier specifies a minimum and maximum number of
1464: permitted matches, by giving the two numbers in curly brackets (braces),
1465: separated by a comma. The numbers must be less than 65536, and the first must
1466: be less than or equal to the second. For example:
1467: <pre>
1468: z{2,4}
1469: </pre>
1470: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1471: character. If the second number is omitted, but the comma is present, there is
1472: no upper limit; if the second number and the comma are both omitted, the
1473: quantifier specifies an exact number of required matches. Thus
1474: <pre>
1475: [aeiou]{3,}
1476: </pre>
1477: matches at least 3 successive vowels, but may match many more, while
1478: <pre>
1479: \d{8}
1480: </pre>
1481: matches exactly 8 digits. An opening curly bracket that appears in a position
1482: where a quantifier is not allowed, or one that does not match the syntax of a
1483: quantifier, is taken as a literal character. For example, {,6} is not a
1484: quantifier, but a literal string of four characters.
1485: </P>
1486: <P>
1487: In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1488: bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1489: which is represented by a two-byte sequence. Similarly, when Unicode property
1490: support is available, \X{3} matches three Unicode extended sequences, each of
1491: which may be several bytes long (and they may be of different lengths).
1492: </P>
1493: <P>
1494: The quantifier {0} is permitted, causing the expression to behave as if the
1495: previous item and the quantifier were not present. This may be useful for
1496: subpatterns that are referenced as
1497: <a href="#subpatternsassubroutines">subroutines</a>
1498: from elsewhere in the pattern (but see also the section entitled
1499: <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1500: below). Items other than subpatterns that have a {0} quantifier are omitted
1501: from the compiled pattern.
1502: </P>
1503: <P>
1504: For convenience, the three most common quantifiers have single-character
1505: abbreviations:
1506: <pre>
1507: * is equivalent to {0,}
1508: + is equivalent to {1,}
1509: ? is equivalent to {0,1}
1510: </pre>
1511: It is possible to construct infinite loops by following a subpattern that can
1512: match no characters with a quantifier that has no upper limit, for example:
1513: <pre>
1514: (a?)*
1515: </pre>
1516: Earlier versions of Perl and PCRE used to give an error at compile time for
1517: such patterns. However, because there are cases where this can be useful, such
1518: patterns are now accepted, but if any repetition of the subpattern does in fact
1519: match no characters, the loop is forcibly broken.
1520: </P>
1521: <P>
1522: By default, the quantifiers are "greedy", that is, they match as much as
1523: possible (up to the maximum number of permitted times), without causing the
1524: rest of the pattern to fail. The classic example of where this gives problems
1525: is in trying to match comments in C programs. These appear between /* and */
1526: and within the comment, individual * and / characters may appear. An attempt to
1527: match C comments by applying the pattern
1528: <pre>
1529: /\*.*\*/
1530: </pre>
1531: to the string
1532: <pre>
1533: /* first comment */ not comment /* second comment */
1534: </pre>
1535: fails, because it matches the entire string owing to the greediness of the .*
1536: item.
1537: </P>
1538: <P>
1539: However, if a quantifier is followed by a question mark, it ceases to be
1540: greedy, and instead matches the minimum number of times possible, so the
1541: pattern
1542: <pre>
1543: /\*.*?\*/
1544: </pre>
1545: does the right thing with the C comments. The meaning of the various
1546: quantifiers is not otherwise changed, just the preferred number of matches.
1547: Do not confuse this use of question mark with its use as a quantifier in its
1548: own right. Because it has two uses, it can sometimes appear doubled, as in
1549: <pre>
1550: \d??\d
1551: </pre>
1552: which matches one digit by preference, but can match two if that is the only
1553: way the rest of the pattern matches.
1554: </P>
1555: <P>
1556: If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1557: the quantifiers are not greedy by default, but individual ones can be made
1558: greedy by following them with a question mark. In other words, it inverts the
1559: default behaviour.
1560: </P>
1561: <P>
1562: When a parenthesized subpattern is quantified with a minimum repeat count that
1563: is greater than 1 or with a limited maximum, more memory is required for the
1564: compiled pattern, in proportion to the size of the minimum or maximum.
1565: </P>
1566: <P>
1567: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1568: to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1569: implicitly anchored, because whatever follows will be tried against every
1570: character position in the subject string, so there is no point in retrying the
1571: overall match at any position after the first. PCRE normally treats such a
1572: pattern as though it were preceded by \A.
1573: </P>
1574: <P>
1575: In cases where it is known that the subject string contains no newlines, it is
1576: worth setting PCRE_DOTALL in order to obtain this optimization, or
1577: alternatively using ^ to indicate anchoring explicitly.
1578: </P>
1579: <P>
1580: However, there is one situation where the optimization cannot be used. When .*
1581: is inside capturing parentheses that are the subject of a back reference
1582: elsewhere in the pattern, a match at the start may fail where a later one
1583: succeeds. Consider, for example:
1584: <pre>
1585: (.*)abc\1
1586: </pre>
1587: If the subject is "xyz123abc123" the match point is the fourth character. For
1588: this reason, such a pattern is not implicitly anchored.
1589: </P>
1590: <P>
1591: When a capturing subpattern is repeated, the value captured is the substring
1592: that matched the final iteration. For example, after
1593: <pre>
1594: (tweedle[dume]{3}\s*)+
1595: </pre>
1596: has matched "tweedledum tweedledee" the value of the captured substring is
1597: "tweedledee". However, if there are nested capturing subpatterns, the
1598: corresponding captured values may have been set in previous iterations. For
1599: example, after
1600: <pre>
1601: /(a|(b))+/
1602: </pre>
1603: matches "aba" the value of the second captured substring is "b".
1604: <a name="atomicgroup"></a></P>
1605: <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1606: <P>
1607: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1608: repetition, failure of what follows normally causes the repeated item to be
1609: re-evaluated to see if a different number of repeats allows the rest of the
1610: pattern to match. Sometimes it is useful to prevent this, either to change the
1611: nature of the match, or to cause it fail earlier than it otherwise might, when
1612: the author of the pattern knows there is no point in carrying on.
1613: </P>
1614: <P>
1615: Consider, for example, the pattern \d+foo when applied to the subject line
1616: <pre>
1617: 123456bar
1618: </pre>
1619: After matching all 6 digits and then failing to match "foo", the normal
1620: action of the matcher is to try again with only 5 digits matching the \d+
1621: item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1622: (a term taken from Jeffrey Friedl's book) provides the means for specifying
1623: that once a subpattern has matched, it is not to be re-evaluated in this way.
1624: </P>
1625: <P>
1626: If we use atomic grouping for the previous example, the matcher gives up
1627: immediately on failing to match "foo" the first time. The notation is a kind of
1628: special parenthesis, starting with (?> as in this example:
1629: <pre>
1630: (?>\d+)foo
1631: </pre>
1632: This kind of parenthesis "locks up" the part of the pattern it contains once
1633: it has matched, and a failure further into the pattern is prevented from
1634: backtracking into it. Backtracking past it to previous items, however, works as
1635: normal.
1636: </P>
1637: <P>
1638: An alternative description is that a subpattern of this type matches the string
1639: of characters that an identical standalone pattern would match, if anchored at
1640: the current point in the subject string.
1641: </P>
1642: <P>
1643: Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1644: the above example can be thought of as a maximizing repeat that must swallow
1645: everything it can. So, while both \d+ and \d+? are prepared to adjust the
1646: number of digits they match in order to make the rest of the pattern match,
1647: (?>\d+) can only match an entire sequence of digits.
1648: </P>
1649: <P>
1650: Atomic groups in general can of course contain arbitrarily complicated
1651: subpatterns, and can be nested. However, when the subpattern for an atomic
1652: group is just a single repeated item, as in the example above, a simpler
1653: notation, called a "possessive quantifier" can be used. This consists of an
1654: additional + character following a quantifier. Using this notation, the
1655: previous example can be rewritten as
1656: <pre>
1657: \d++foo
1658: </pre>
1659: Note that a possessive quantifier can be used with an entire group, for
1660: example:
1661: <pre>
1662: (abc|xyz){2,3}+
1663: </pre>
1664: Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1665: option is ignored. They are a convenient notation for the simpler forms of
1666: atomic group. However, there is no difference in the meaning of a possessive
1667: quantifier and the equivalent atomic group, though there may be a performance
1668: difference; possessive quantifiers should be slightly faster.
1669: </P>
1670: <P>
1671: The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1672: Jeffrey Friedl originated the idea (and the name) in the first edition of his
1673: book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1674: package, and PCRE copied it from there. It ultimately found its way into Perl
1675: at release 5.10.
1676: </P>
1677: <P>
1678: PCRE has an optimization that automatically "possessifies" certain simple
1679: pattern constructs. For example, the sequence A+B is treated as A++B because
1680: there is no point in backtracking into a sequence of A's when B must follow.
1681: </P>
1682: <P>
1683: When a pattern contains an unlimited repeat inside a subpattern that can itself
1684: be repeated an unlimited number of times, the use of an atomic group is the
1685: only way to avoid some failing matches taking a very long time indeed. The
1686: pattern
1687: <pre>
1688: (\D+|<\d+>)*[!?]
1689: </pre>
1690: matches an unlimited number of substrings that either consist of non-digits, or
1691: digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1692: quickly. However, if it is applied to
1693: <pre>
1694: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1695: </pre>
1696: it takes a long time before reporting failure. This is because the string can
1697: be divided between the internal \D+ repeat and the external * repeat in a
1698: large number of ways, and all have to be tried. (The example uses [!?] rather
1699: than a single character at the end, because both PCRE and Perl have an
1700: optimization that allows for fast failure when a single character is used. They
1701: remember the last single character that is required for a match, and fail early
1702: if it is not present in the string.) If the pattern is changed so that it uses
1703: an atomic group, like this:
1704: <pre>
1705: ((?>\D+)|<\d+>)*[!?]
1706: </pre>
1707: sequences of non-digits cannot be broken, and failure happens quickly.
1708: <a name="backreferences"></a></P>
1709: <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1710: <P>
1711: Outside a character class, a backslash followed by a digit greater than 0 (and
1712: possibly further digits) is a back reference to a capturing subpattern earlier
1713: (that is, to its left) in the pattern, provided there have been that many
1714: previous capturing left parentheses.
1715: </P>
1716: <P>
1717: However, if the decimal number following the backslash is less than 10, it is
1718: always taken as a back reference, and causes an error only if there are not
1719: that many capturing left parentheses in the entire pattern. In other words, the
1720: parentheses that are referenced need not be to the left of the reference for
1721: numbers less than 10. A "forward back reference" of this type can make sense
1722: when a repetition is involved and the subpattern to the right has participated
1723: in an earlier iteration.
1724: </P>
1725: <P>
1726: It is not possible to have a numerical "forward back reference" to a subpattern
1727: whose number is 10 or more using this syntax because a sequence such as \50 is
1728: interpreted as a character defined in octal. See the subsection entitled
1729: "Non-printing characters"
1730: <a href="#digitsafterbackslash">above</a>
1731: for further details of the handling of digits following a backslash. There is
1732: no such problem when named parentheses are used. A back reference to any
1733: subpattern is possible using named parentheses (see below).
1734: </P>
1735: <P>
1736: Another way of avoiding the ambiguity inherent in the use of digits following a
1737: backslash is to use the \g escape sequence. This escape must be followed by an
1738: unsigned number or a negative number, optionally enclosed in braces. These
1739: examples are all identical:
1740: <pre>
1741: (ring), \1
1742: (ring), \g1
1743: (ring), \g{1}
1744: </pre>
1745: An unsigned number specifies an absolute reference without the ambiguity that
1746: is present in the older syntax. It is also useful when literal digits follow
1747: the reference. A negative number is a relative reference. Consider this
1748: example:
1749: <pre>
1750: (abc(def)ghi)\g{-1}
1751: </pre>
1752: The sequence \g{-1} is a reference to the most recently started capturing
1753: subpattern before \g, that is, is it equivalent to \2 in this example.
1754: Similarly, \g{-2} would be equivalent to \1. The use of relative references
1755: can be helpful in long patterns, and also in patterns that are created by
1756: joining together fragments that contain references within themselves.
1757: </P>
1758: <P>
1759: A back reference matches whatever actually matched the capturing subpattern in
1760: the current subject string, rather than anything matching the subpattern
1761: itself (see
1762: <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1763: below for a way of doing that). So the pattern
1764: <pre>
1765: (sens|respons)e and \1ibility
1766: </pre>
1767: matches "sense and sensibility" and "response and responsibility", but not
1768: "sense and responsibility". If caseful matching is in force at the time of the
1769: back reference, the case of letters is relevant. For example,
1770: <pre>
1771: ((?i)rah)\s+\1
1772: </pre>
1773: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1774: capturing subpattern is matched caselessly.
1775: </P>
1776: <P>
1777: There are several different ways of writing back references to named
1778: subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
1779: \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1780: back reference syntax, in which \g can be used for both numeric and named
1781: references, is also supported. We could rewrite the above example in any of
1782: the following ways:
1783: <pre>
1784: (?<p1>(?i)rah)\s+\k<p1>
1785: (?'p1'(?i)rah)\s+\k{p1}
1786: (?P<p1>(?i)rah)\s+(?P=p1)
1787: (?<p1>(?i)rah)\s+\g{p1}
1788: </pre>
1789: A subpattern that is referenced by name may appear in the pattern before or
1790: after the reference.
1791: </P>
1792: <P>
1793: There may be more than one back reference to the same subpattern. If a
1794: subpattern has not actually been used in a particular match, any back
1795: references to it always fail by default. For example, the pattern
1796: <pre>
1797: (a|(bc))\2
1798: </pre>
1799: always fails if it starts to match "a" rather than "bc". However, if the
1800: PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1801: unset value matches an empty string.
1802: </P>
1803: <P>
1804: Because there may be many capturing parentheses in a pattern, all digits
1805: following a backslash are taken as part of a potential back reference number.
1806: If the pattern continues with a digit character, some delimiter must be used to
1807: terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1808: whitespace. Otherwise, the \g{ syntax or an empty comment (see
1809: <a href="#comments">"Comments"</a>
1810: below) can be used.
1811: </P>
1812: <br><b>
1813: Recursive back references
1814: </b><br>
1815: <P>
1816: A back reference that occurs inside the parentheses to which it refers fails
1817: when the subpattern is first used, so, for example, (a\1) never matches.
1818: However, such references can be useful inside repeated subpatterns. For
1819: example, the pattern
1820: <pre>
1821: (a|b\1)+
1822: </pre>
1823: matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1824: the subpattern, the back reference matches the character string corresponding
1825: to the previous iteration. In order for this to work, the pattern must be such
1826: that the first iteration does not need to match the back reference. This can be
1827: done using alternation, as in the example above, or by a quantifier with a
1828: minimum of zero.
1829: </P>
1830: <P>
1831: Back references of this type cause the group that they reference to be treated
1832: as an
1833: <a href="#atomicgroup">atomic group.</a>
1834: Once the whole group has been matched, a subsequent matching failure cannot
1835: cause backtracking into the middle of the group.
1836: <a name="bigassertions"></a></P>
1837: <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1838: <P>
1839: An assertion is a test on the characters following or preceding the current
1840: matching point that does not actually consume any characters. The simple
1841: assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1842: <a href="#smallassertions">above.</a>
1843: </P>
1844: <P>
1845: More complicated assertions are coded as subpatterns. There are two kinds:
1846: those that look ahead of the current position in the subject string, and those
1847: that look behind it. An assertion subpattern is matched in the normal way,
1848: except that it does not cause the current matching position to be changed.
1849: </P>
1850: <P>
1851: Assertion subpatterns are not capturing subpatterns. If such an assertion
1852: contains capturing subpatterns within it, these are counted for the purposes of
1853: numbering the capturing subpatterns in the whole pattern. However, substring
1854: capturing is carried out only for positive assertions, because it does not make
1855: sense for negative assertions.
1856: </P>
1857: <P>
1858: For compatibility with Perl, assertion subpatterns may be repeated; though
1859: it makes no sense to assert the same thing several times, the side effect of
1860: capturing parentheses may occasionally be useful. In practice, there only three
1861: cases:
1862: <br>
1863: <br>
1864: (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1865: However, it may contain internal capturing parenthesized groups that are called
1866: from elsewhere via the
1867: <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1868: <br>
1869: <br>
1870: (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1871: were {0,1}. At run time, the rest of the pattern match is tried with and
1872: without the assertion, the order depending on the greediness of the quantifier.
1873: <br>
1874: <br>
1875: (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1876: The assertion is obeyed just once when encountered during matching.
1877: </P>
1878: <br><b>
1879: Lookahead assertions
1880: </b><br>
1881: <P>
1882: Lookahead assertions start with (?= for positive assertions and (?! for
1883: negative assertions. For example,
1884: <pre>
1885: \w+(?=;)
1886: </pre>
1887: matches a word followed by a semicolon, but does not include the semicolon in
1888: the match, and
1889: <pre>
1890: foo(?!bar)
1891: </pre>
1892: matches any occurrence of "foo" that is not followed by "bar". Note that the
1893: apparently similar pattern
1894: <pre>
1895: (?!foo)bar
1896: </pre>
1897: does not find an occurrence of "bar" that is preceded by something other than
1898: "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1899: (?!foo) is always true when the next three characters are "bar". A
1900: lookbehind assertion is needed to achieve the other effect.
1901: </P>
1902: <P>
1903: If you want to force a matching failure at some point in a pattern, the most
1904: convenient way to do it is with (?!) because an empty string always matches, so
1905: an assertion that requires there not to be an empty string must always fail.
1906: The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1907: <a name="lookbehind"></a></P>
1908: <br><b>
1909: Lookbehind assertions
1910: </b><br>
1911: <P>
1912: Lookbehind assertions start with (?<= for positive assertions and (?<! for
1913: negative assertions. For example,
1914: <pre>
1915: (?<!foo)bar
1916: </pre>
1917: does find an occurrence of "bar" that is not preceded by "foo". The contents of
1918: a lookbehind assertion are restricted such that all the strings it matches must
1919: have a fixed length. However, if there are several top-level alternatives, they
1920: do not all have to have the same fixed length. Thus
1921: <pre>
1922: (?<=bullock|donkey)
1923: </pre>
1924: is permitted, but
1925: <pre>
1926: (?<!dogs?|cats?)
1927: </pre>
1928: causes an error at compile time. Branches that match different length strings
1929: are permitted only at the top level of a lookbehind assertion. This is an
1930: extension compared with Perl, which requires all branches to match the same
1931: length of string. An assertion such as
1932: <pre>
1933: (?<=ab(c|de))
1934: </pre>
1935: is not permitted, because its single top-level branch can match two different
1936: lengths, but it is acceptable to PCRE if rewritten to use two top-level
1937: branches:
1938: <pre>
1939: (?<=abc|abde)
1940: </pre>
1941: In some cases, the escape sequence \K
1942: <a href="#resetmatchstart">(see above)</a>
1943: can be used instead of a lookbehind assertion to get round the fixed-length
1944: restriction.
1945: </P>
1946: <P>
1947: The implementation of lookbehind assertions is, for each alternative, to
1948: temporarily move the current position back by the fixed length and then try to
1949: match. If there are insufficient characters before the current position, the
1950: assertion fails.
1951: </P>
1952: <P>
1953: In UTF-8 mode, PCRE does not allow the \C escape (which matches a single byte,
1954: even in UTF-8 mode) to appear in lookbehind assertions, because it makes it
1955: impossible to calculate the length of the lookbehind. The \X and \R escapes,
1956: which can match different numbers of bytes, are also not permitted.
1957: </P>
1958: <P>
1959: <a href="#subpatternsassubroutines">"Subroutine"</a>
1960: calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1961: as the subpattern matches a fixed-length string.
1962: <a href="#recursion">Recursion,</a>
1963: however, is not supported.
1964: </P>
1965: <P>
1966: Possessive quantifiers can be used in conjunction with lookbehind assertions to
1967: specify efficient matching of fixed-length strings at the end of subject
1968: strings. Consider a simple pattern such as
1969: <pre>
1970: abcd$
1971: </pre>
1972: when applied to a long string that does not match. Because matching proceeds
1973: from left to right, PCRE will look for each "a" in the subject and then see if
1974: what follows matches the rest of the pattern. If the pattern is specified as
1975: <pre>
1976: ^.*abcd$
1977: </pre>
1978: the initial .* matches the entire string at first, but when this fails (because
1979: there is no following "a"), it backtracks to match all but the last character,
1980: then all but the last two characters, and so on. Once again the search for "a"
1981: covers the entire string, from right to left, so we are no better off. However,
1982: if the pattern is written as
1983: <pre>
1984: ^.*+(?<=abcd)
1985: </pre>
1986: there can be no backtracking for the .*+ item; it can match only the entire
1987: string. The subsequent lookbehind assertion does a single test on the last four
1988: characters. If it fails, the match fails immediately. For long strings, this
1989: approach makes a significant difference to the processing time.
1990: </P>
1991: <br><b>
1992: Using multiple assertions
1993: </b><br>
1994: <P>
1995: Several assertions (of any sort) may occur in succession. For example,
1996: <pre>
1997: (?<=\d{3})(?<!999)foo
1998: </pre>
1999: matches "foo" preceded by three digits that are not "999". Notice that each of
2000: the assertions is applied independently at the same point in the subject
2001: string. First there is a check that the previous three characters are all
2002: digits, and then there is a check that the same three characters are not "999".
2003: This pattern does <i>not</i> match "foo" preceded by six characters, the first
2004: of which are digits and the last three of which are not "999". For example, it
2005: doesn't match "123abcfoo". A pattern to do that is
2006: <pre>
2007: (?<=\d{3}...)(?<!999)foo
2008: </pre>
2009: This time the first assertion looks at the preceding six characters, checking
2010: that the first three are digits, and then the second assertion checks that the
2011: preceding three characters are not "999".
2012: </P>
2013: <P>
2014: Assertions can be nested in any combination. For example,
2015: <pre>
2016: (?<=(?<!foo)bar)baz
2017: </pre>
2018: matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2019: preceded by "foo", while
2020: <pre>
2021: (?<=\d{3}(?!999)...)foo
2022: </pre>
2023: is another pattern that matches "foo" preceded by three digits and any three
2024: characters that are not "999".
2025: <a name="conditions"></a></P>
2026: <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2027: <P>
2028: It is possible to cause the matching process to obey a subpattern
2029: conditionally or to choose between two alternative subpatterns, depending on
2030: the result of an assertion, or whether a specific capturing subpattern has
2031: already been matched. The two possible forms of conditional subpattern are:
2032: <pre>
2033: (?(condition)yes-pattern)
2034: (?(condition)yes-pattern|no-pattern)
2035: </pre>
2036: If the condition is satisfied, the yes-pattern is used; otherwise the
2037: no-pattern (if present) is used. If there are more than two alternatives in the
2038: subpattern, a compile-time error occurs. Each of the two alternatives may
2039: itself contain nested subpatterns of any form, including conditional
2040: subpatterns; the restriction to two alternatives applies only at the level of
2041: the condition. This pattern fragment is an example where the alternatives are
2042: complex:
2043: <pre>
2044: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2045:
2046: </PRE>
2047: </P>
2048: <P>
2049: There are four kinds of condition: references to subpatterns, references to
2050: recursion, a pseudo-condition called DEFINE, and assertions.
2051: </P>
2052: <br><b>
2053: Checking for a used subpattern by number
2054: </b><br>
2055: <P>
2056: If the text between the parentheses consists of a sequence of digits, the
2057: condition is true if a capturing subpattern of that number has previously
2058: matched. If there is more than one capturing subpattern with the same number
2059: (see the earlier
2060: <a href="#recursion">section about duplicate subpattern numbers),</a>
2061: the condition is true if any of them have matched. An alternative notation is
2062: to precede the digits with a plus or minus sign. In this case, the subpattern
2063: number is relative rather than absolute. The most recently opened parentheses
2064: can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2065: loops it can also make sense to refer to subsequent groups. The next
2066: parentheses to be opened can be referenced as (?(+1), and so on. (The value
2067: zero in any of these forms is not used; it provokes a compile-time error.)
2068: </P>
2069: <P>
2070: Consider the following pattern, which contains non-significant white space to
2071: make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2072: three parts for ease of discussion:
2073: <pre>
2074: ( \( )? [^()]+ (?(1) \) )
2075: </pre>
2076: The first part matches an optional opening parenthesis, and if that
2077: character is present, sets it as the first captured substring. The second part
2078: matches one or more characters that are not parentheses. The third part is a
2079: conditional subpattern that tests whether or not the first set of parentheses
2080: matched. If they did, that is, if subject started with an opening parenthesis,
2081: the condition is true, and so the yes-pattern is executed and a closing
2082: parenthesis is required. Otherwise, since no-pattern is not present, the
2083: subpattern matches nothing. In other words, this pattern matches a sequence of
2084: non-parentheses, optionally enclosed in parentheses.
2085: </P>
2086: <P>
2087: If you were embedding this pattern in a larger one, you could use a relative
2088: reference:
2089: <pre>
2090: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2091: </pre>
2092: This makes the fragment independent of the parentheses in the larger pattern.
2093: </P>
2094: <br><b>
2095: Checking for a used subpattern by name
2096: </b><br>
2097: <P>
2098: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2099: subpattern by name. For compatibility with earlier versions of PCRE, which had
2100: this facility before Perl, the syntax (?(name)...) is also recognized. However,
2101: there is a possible ambiguity with this syntax, because subpattern names may
2102: consist entirely of digits. PCRE looks first for a named subpattern; if it
2103: cannot find one and the name consists entirely of digits, PCRE looks for a
2104: subpattern of that number, which must be greater than zero. Using subpattern
2105: names that consist entirely of digits is not recommended.
2106: </P>
2107: <P>
2108: Rewriting the above example to use a named subpattern gives this:
2109: <pre>
2110: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
2111: </pre>
2112: If the name used in a condition of this kind is a duplicate, the test is
2113: applied to all subpatterns of the same name, and is true if any one of them has
2114: matched.
2115: </P>
2116: <br><b>
2117: Checking for pattern recursion
2118: </b><br>
2119: <P>
2120: If the condition is the string (R), and there is no subpattern with the name R,
2121: the condition is true if a recursive call to the whole pattern or any
2122: subpattern has been made. If digits or a name preceded by ampersand follow the
2123: letter R, for example:
2124: <pre>
2125: (?(R3)...) or (?(R&name)...)
2126: </pre>
2127: the condition is true if the most recent recursion is into a subpattern whose
2128: number or name is given. This condition does not check the entire recursion
2129: stack. If the name used in a condition of this kind is a duplicate, the test is
2130: applied to all subpatterns of the same name, and is true if any one of them is
2131: the most recent recursion.
2132: </P>
2133: <P>
2134: At "top level", all these recursion test conditions are false.
2135: <a href="#recursion">The syntax for recursive patterns</a>
2136: is described below.
2137: <a name="subdefine"></a></P>
2138: <br><b>
2139: Defining subpatterns for use by reference only
2140: </b><br>
2141: <P>
2142: If the condition is the string (DEFINE), and there is no subpattern with the
2143: name DEFINE, the condition is always false. In this case, there may be only one
2144: alternative in the subpattern. It is always skipped if control reaches this
2145: point in the pattern; the idea of DEFINE is that it can be used to define
2146: subroutines that can be referenced from elsewhere. (The use of
2147: <a href="#subpatternsassubroutines">subroutines</a>
2148: is described below.) For example, a pattern to match an IPv4 address such as
2149: "192.168.23.245" could be written like this (ignore whitespace and line
2150: breaks):
2151: <pre>
2152: (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2153: \b (?&byte) (\.(?&byte)){3} \b
2154: </pre>
2155: The first part of the pattern is a DEFINE group inside which a another group
2156: named "byte" is defined. This matches an individual component of an IPv4
2157: address (a number less than 256). When matching takes place, this part of the
2158: pattern is skipped because DEFINE acts like a false condition. The rest of the
2159: pattern uses references to the named group to match the four dot-separated
2160: components of an IPv4 address, insisting on a word boundary at each end.
2161: </P>
2162: <br><b>
2163: Assertion conditions
2164: </b><br>
2165: <P>
2166: If the condition is not in any of the above formats, it must be an assertion.
2167: This may be a positive or negative lookahead or lookbehind assertion. Consider
2168: this pattern, again containing non-significant white space, and with the two
2169: alternatives on the second line:
2170: <pre>
2171: (?(?=[^a-z]*[a-z])
2172: \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2173: </pre>
2174: The condition is a positive lookahead assertion that matches an optional
2175: sequence of non-letters followed by a letter. In other words, it tests for the
2176: presence of at least one letter in the subject. If a letter is found, the
2177: subject is matched against the first alternative; otherwise it is matched
2178: against the second. This pattern matches strings in one of the two forms
2179: dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2180: <a name="comments"></a></P>
2181: <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2182: <P>
2183: There are two ways of including comments in patterns that are processed by
2184: PCRE. In both cases, the start of the comment must not be in a character class,
2185: nor in the middle of any other sequence of related characters such as (?: or a
2186: subpattern name or number. The characters that make up a comment play no part
2187: in the pattern matching.
2188: </P>
2189: <P>
2190: The sequence (?# marks the start of a comment that continues up to the next
2191: closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2192: option is set, an unescaped # character also introduces a comment, which in
2193: this case continues to immediately after the next newline character or
2194: character sequence in the pattern. Which characters are interpreted as newlines
2195: is controlled by the options passed to <b>pcre_compile()</b> or by a special
2196: sequence at the start of the pattern, as described in the section entitled
2197: <a href="#newlines">"Newline conventions"</a>
2198: above. Note that the end of this type of comment is a literal newline sequence
2199: in the pattern; escape sequences that happen to represent a newline do not
2200: count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2201: default newline convention is in force:
2202: <pre>
2203: abc #comment \n still comment
2204: </pre>
2205: On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2206: a newline in the pattern. The sequence \n is still literal at this stage, so
2207: it does not terminate the comment. Only an actual character with the code value
2208: 0x0a (the default newline) does so.
2209: <a name="recursion"></a></P>
2210: <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2211: <P>
2212: Consider the problem of matching a string in parentheses, allowing for
2213: unlimited nested parentheses. Without the use of recursion, the best that can
2214: be done is to use a pattern that matches up to some fixed depth of nesting. It
2215: is not possible to handle an arbitrary nesting depth.
2216: </P>
2217: <P>
2218: For some time, Perl has provided a facility that allows regular expressions to
2219: recurse (amongst other things). It does this by interpolating Perl code in the
2220: expression at run time, and the code can refer to the expression itself. A Perl
2221: pattern using code interpolation to solve the parentheses problem can be
2222: created like this:
2223: <pre>
2224: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
2225: </pre>
2226: The (?p{...}) item interpolates Perl code at run time, and in this case refers
2227: recursively to the pattern in which it appears.
2228: </P>
2229: <P>
2230: Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2231: supports special syntax for recursion of the entire pattern, and also for
2232: individual subpattern recursion. After its introduction in PCRE and Python,
2233: this kind of recursion was subsequently introduced into Perl at release 5.10.
2234: </P>
2235: <P>
2236: A special item that consists of (? followed by a number greater than zero and a
2237: closing parenthesis is a recursive subroutine call of the subpattern of the
2238: given number, provided that it occurs inside that subpattern. (If not, it is a
2239: <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2240: call, which is described in the next section.) The special item (?R) or (?0) is
2241: a recursive call of the entire regular expression.
2242: </P>
2243: <P>
2244: This PCRE pattern solves the nested parentheses problem (assume the
2245: PCRE_EXTENDED option is set so that white space is ignored):
2246: <pre>
2247: \( ( [^()]++ | (?R) )* \)
2248: </pre>
2249: First it matches an opening parenthesis. Then it matches any number of
2250: substrings which can either be a sequence of non-parentheses, or a recursive
2251: match of the pattern itself (that is, a correctly parenthesized substring).
2252: Finally there is a closing parenthesis. Note the use of a possessive quantifier
2253: to avoid backtracking into sequences of non-parentheses.
2254: </P>
2255: <P>
2256: If this were part of a larger pattern, you would not want to recurse the entire
2257: pattern, so instead you could use this:
2258: <pre>
2259: ( \( ( [^()]++ | (?1) )* \) )
2260: </pre>
2261: We have put the pattern into parentheses, and caused the recursion to refer to
2262: them instead of the whole pattern.
2263: </P>
2264: <P>
2265: In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2266: is made easier by the use of relative references. Instead of (?1) in the
2267: pattern above you can write (?-2) to refer to the second most recently opened
2268: parentheses preceding the recursion. In other words, a negative number counts
2269: capturing parentheses leftwards from the point at which it is encountered.
2270: </P>
2271: <P>
2272: It is also possible to refer to subsequently opened parentheses, by writing
2273: references such as (?+2). However, these cannot be recursive because the
2274: reference is not inside the parentheses that are referenced. They are always
2275: <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2276: calls, as described in the next section.
2277: </P>
2278: <P>
2279: An alternative approach is to use named parentheses instead. The Perl syntax
2280: for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2281: could rewrite the above example as follows:
2282: <pre>
2283: (?<pn> \( ( [^()]++ | (?&pn) )* \) )
2284: </pre>
2285: If there is more than one subpattern with the same name, the earliest one is
2286: used.
2287: </P>
2288: <P>
2289: This particular example pattern that we have been looking at contains nested
2290: unlimited repeats, and so the use of a possessive quantifier for matching
2291: strings of non-parentheses is important when applying the pattern to strings
2292: that do not match. For example, when this pattern is applied to
2293: <pre>
2294: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2295: </pre>
2296: it yields "no match" quickly. However, if a possessive quantifier is not used,
2297: the match runs for a very long time indeed because there are so many different
2298: ways the + and * repeats can carve up the subject, and all have to be tested
2299: before failure can be reported.
2300: </P>
2301: <P>
2302: At the end of a match, the values of capturing parentheses are those from
2303: the outermost level. If you want to obtain intermediate values, a callout
2304: function can be used (see below and the
2305: <a href="pcrecallout.html"><b>pcrecallout</b></a>
2306: documentation). If the pattern above is matched against
2307: <pre>
2308: (ab(cd)ef)
2309: </pre>
2310: the value for the inner capturing parentheses (numbered 2) is "ef", which is
2311: the last value taken on at the top level. If a capturing subpattern is not
2312: matched at the top level, its final captured value is unset, even if it was
2313: (temporarily) set at a deeper level during the matching process.
2314: </P>
2315: <P>
2316: If there are more than 15 capturing parentheses in a pattern, PCRE has to
2317: obtain extra memory to store data during a recursion, which it does by using
2318: <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2319: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2320: </P>
2321: <P>
2322: Do not confuse the (?R) item with the condition (R), which tests for recursion.
2323: Consider this pattern, which matches text in angle brackets, allowing for
2324: arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2325: recursing), whereas any characters are permitted at the outer level.
2326: <pre>
2327: < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
2328: </pre>
2329: In this pattern, (?(R) is the start of a conditional subpattern, with two
2330: different alternatives for the recursive and non-recursive cases. The (?R) item
2331: is the actual recursive call.
2332: <a name="recursiondifference"></a></P>
2333: <br><b>
2334: Differences in recursion processing between PCRE and Perl
2335: </b><br>
2336: <P>
2337: Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2338: (like Python, but unlike Perl), a recursive subpattern call is always treated
2339: as an atomic group. That is, once it has matched some of the subject string, it
2340: is never re-entered, even if it contains untried alternatives and there is a
2341: subsequent matching failure. This can be illustrated by the following pattern,
2342: which purports to match a palindromic string that contains an odd number of
2343: characters (for example, "a", "aba", "abcba", "abcdcba"):
2344: <pre>
2345: ^(.|(.)(?1)\2)$
2346: </pre>
2347: The idea is that it either matches a single character, or two identical
2348: characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2349: it does not if the pattern is longer than three characters. Consider the
2350: subject string "abcba":
2351: </P>
2352: <P>
2353: At the top level, the first character is matched, but as it is not at the end
2354: of the string, the first alternative fails; the second alternative is taken
2355: and the recursion kicks in. The recursive call to subpattern 1 successfully
2356: matches the next character ("b"). (Note that the beginning and end of line
2357: tests are not part of the recursion).
2358: </P>
2359: <P>
2360: Back at the top level, the next character ("c") is compared with what
2361: subpattern 2 matched, which was "a". This fails. Because the recursion is
2362: treated as an atomic group, there are now no backtracking points, and so the
2363: entire match fails. (Perl is able, at this point, to re-enter the recursion and
2364: try the second alternative.) However, if the pattern is written with the
2365: alternatives in the other order, things are different:
2366: <pre>
2367: ^((.)(?1)\2|.)$
2368: </pre>
2369: This time, the recursing alternative is tried first, and continues to recurse
2370: until it runs out of characters, at which point the recursion fails. But this
2371: time we do have another alternative to try at the higher level. That is the big
2372: difference: in the previous case the remaining alternative is at a deeper
2373: recursion level, which PCRE cannot use.
2374: </P>
2375: <P>
2376: To change the pattern so that it matches all palindromic strings, not just
2377: those with an odd number of characters, it is tempting to change the pattern to
2378: this:
2379: <pre>
2380: ^((.)(?1)\2|.?)$
2381: </pre>
2382: Again, this works in Perl, but not in PCRE, and for the same reason. When a
2383: deeper recursion has matched a single character, it cannot be entered again in
2384: order to match an empty string. The solution is to separate the two cases, and
2385: write out the odd and even cases as alternatives at the higher level:
2386: <pre>
2387: ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2388: </pre>
2389: If you want to match typical palindromic phrases, the pattern has to ignore all
2390: non-word characters, which can be done like this:
2391: <pre>
2392: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2393: </pre>
2394: If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2395: man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2396: the use of the possessive quantifier *+ to avoid backtracking into sequences of
2397: non-word characters. Without this, PCRE takes a great deal longer (ten times or
2398: more) to match typical phrases, and Perl takes so long that you think it has
2399: gone into a loop.
2400: </P>
2401: <P>
2402: <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2403: string does not start with a palindrome that is shorter than the entire string.
2404: For example, although "abcba" is correctly matched, if the subject is "ababa",
2405: PCRE finds the palindrome "aba" at the start, then fails at top level because
2406: the end of the string does not follow. Once again, it cannot jump back into the
2407: recursion to try other alternatives, so the entire match fails.
2408: </P>
2409: <P>
2410: The second way in which PCRE and Perl differ in their recursion processing is
2411: in the handling of captured values. In Perl, when a subpattern is called
2412: recursively or as a subpattern (see the next section), it has no access to any
2413: values that were captured outside the recursion, whereas in PCRE these values
2414: can be referenced. Consider this pattern:
2415: <pre>
2416: ^(.)(\1|a(?2))
2417: </pre>
2418: In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2419: then in the second group, when the back reference \1 fails to match "b", the
2420: second alternative matches "a" and then recurses. In the recursion, \1 does
2421: now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2422: match because inside the recursive call \1 cannot access the externally set
2423: value.
2424: <a name="subpatternsassubroutines"></a></P>
2425: <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2426: <P>
2427: If the syntax for a recursive subpattern call (either by number or by
2428: name) is used outside the parentheses to which it refers, it operates like a
2429: subroutine in a programming language. The called subpattern may be defined
2430: before or after the reference. A numbered reference can be absolute or
2431: relative, as in these examples:
2432: <pre>
2433: (...(absolute)...)...(?2)...
2434: (...(relative)...)...(?-1)...
2435: (...(?+1)...(relative)...
2436: </pre>
2437: An earlier example pointed out that the pattern
2438: <pre>
2439: (sens|respons)e and \1ibility
2440: </pre>
2441: matches "sense and sensibility" and "response and responsibility", but not
2442: "sense and responsibility". If instead the pattern
2443: <pre>
2444: (sens|respons)e and (?1)ibility
2445: </pre>
2446: is used, it does match "sense and responsibility" as well as the other two
2447: strings. Another example is given in the discussion of DEFINE above.
2448: </P>
2449: <P>
2450: All subroutine calls, whether recursive or not, are always treated as atomic
2451: groups. That is, once a subroutine has matched some of the subject string, it
2452: is never re-entered, even if it contains untried alternatives and there is a
2453: subsequent matching failure. Any capturing parentheses that are set during the
2454: subroutine call revert to their previous values afterwards.
2455: </P>
2456: <P>
2457: Processing options such as case-independence are fixed when a subpattern is
2458: defined, so if it is used as a subroutine, such options cannot be changed for
2459: different calls. For example, consider this pattern:
2460: <pre>
2461: (abc)(?i:(?-1))
2462: </pre>
2463: It matches "abcabc". It does not match "abcABC" because the change of
2464: processing option does not affect the called subpattern.
2465: <a name="onigurumasubroutines"></a></P>
2466: <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2467: <P>
2468: For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2469: a number enclosed either in angle brackets or single quotes, is an alternative
2470: syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2471: are two of the examples used above, rewritten using this syntax:
2472: <pre>
2473: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
2474: (sens|respons)e and \g'1'ibility
2475: </pre>
2476: PCRE supports an extension to Oniguruma: if a number is preceded by a
2477: plus or a minus sign it is taken as a relative reference. For example:
2478: <pre>
2479: (abc)(?i:\g<-1>)
2480: </pre>
2481: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
2482: synonymous. The former is a back reference; the latter is a subroutine call.
2483: </P>
2484: <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2485: <P>
2486: Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2487: code to be obeyed in the middle of matching a regular expression. This makes it
2488: possible, amongst other things, to extract different substrings that match the
2489: same pair of parentheses when there is a repetition.
2490: </P>
2491: <P>
2492: PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2493: code. The feature is called "callout". The caller of PCRE provides an external
2494: function by putting its entry point in the global variable <i>pcre_callout</i>.
2495: By default, this variable contains NULL, which disables all calling out.
2496: </P>
2497: <P>
2498: Within a regular expression, (?C) indicates the points at which the external
2499: function is to be called. If you want to identify different callout points, you
2500: can put a number less than 256 after the letter C. The default value is zero.
2501: For example, this pattern has two callout points:
2502: <pre>
2503: (?C1)abc(?C2)def
2504: </pre>
2505: If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2506: automatically installed before each item in the pattern. They are all numbered
2507: 255.
2508: </P>
2509: <P>
2510: During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2511: set), the external function is called. It is provided with the number of the
2512: callout, the position in the pattern, and, optionally, one item of data
2513: originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2514: may cause matching to proceed, to backtrack, or to fail altogether. A complete
2515: description of the interface to the callout function is given in the
2516: <a href="pcrecallout.html"><b>pcrecallout</b></a>
2517: documentation.
2518: <a name="backtrackcontrol"></a></P>
2519: <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2520: <P>
2521: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2522: are described in the Perl documentation as "experimental and subject to change
2523: or removal in a future version of Perl". It goes on to say: "Their usage in
2524: production code should be noted to avoid problems during upgrades." The same
2525: remarks apply to the PCRE features described in this section.
2526: </P>
2527: <P>
2528: Since these verbs are specifically related to backtracking, most of them can be
2529: used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2530: a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2531: failing negative assertion, they cause an error if encountered by
2532: <b>pcre_dfa_exec()</b>.
2533: </P>
2534: <P>
2535: If any of these verbs are used in an assertion or in a subpattern that is
2536: called as a subroutine (whether or not recursively), their effect is confined
2537: to that subpattern; it does not extend to the surrounding pattern, with one
2538: exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2539: a successful positive assertion <i>is</i> passed back when a match succeeds
2540: (compare capturing parentheses in assertions). Note that such subpatterns are
2541: processed as anchored at the point where they are tested. Note also that Perl's
2542: treatment of subroutines is different in some cases.
2543: </P>
2544: <P>
2545: The new verbs make use of what was previously invalid syntax: an opening
2546: parenthesis followed by an asterisk. They are generally of the form
2547: (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2548: depending on whether or not an argument is present. A name is any sequence of
2549: characters that does not include a closing parenthesis. If the name is empty,
2550: that is, if the closing parenthesis immediately follows the colon, the effect
2551: is as if the colon were not there. Any number of these verbs may occur in a
2552: pattern.
2553: </P>
2554: <P>
2555: PCRE contains some optimizations that are used to speed up matching by running
2556: some checks at the start of each match attempt. For example, it may know the
2557: minimum length of matching subject, or that a particular character must be
2558: present. When one of these optimizations suppresses the running of a match, any
2559: included backtracking verbs will not, of course, be processed. You can suppress
2560: the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2561: when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2562: pattern with (*NO_START_OPT).
2563: </P>
2564: <P>
2565: Experiments with Perl suggest that it too has similar optimizations, sometimes
2566: leading to anomalous results.
2567: </P>
2568: <br><b>
2569: Verbs that act immediately
2570: </b><br>
2571: <P>
2572: The following verbs act as soon as they are encountered. They may not be
2573: followed by a name.
2574: <pre>
2575: (*ACCEPT)
2576: </pre>
2577: This verb causes the match to end successfully, skipping the remainder of the
2578: pattern. However, when it is inside a subpattern that is called as a
2579: subroutine, only that subpattern is ended successfully. Matching then continues
2580: at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2581: far is captured. For example:
2582: <pre>
2583: A((?:A|B(*ACCEPT)|C)D)
2584: </pre>
2585: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2586: the outer parentheses.
2587: <pre>
2588: (*FAIL) or (*F)
2589: </pre>
2590: This verb causes a matching failure, forcing backtracking to occur. It is
2591: equivalent to (?!) but easier to read. The Perl documentation notes that it is
2592: probably useful only when combined with (?{}) or (??{}). Those are, of course,
2593: Perl features that are not present in PCRE. The nearest equivalent is the
2594: callout feature, as for example in this pattern:
2595: <pre>
2596: a+(?C)(*FAIL)
2597: </pre>
2598: A match with the string "aaaa" always fails, but the callout is taken before
2599: each backtrack happens (in this example, 10 times).
2600: </P>
2601: <br><b>
2602: Recording which path was taken
2603: </b><br>
2604: <P>
2605: There is one verb whose main purpose is to track how a match was arrived at,
2606: though it also has a secondary use in conjunction with advancing the match
2607: starting point (see (*SKIP) below).
2608: <pre>
2609: (*MARK:NAME) or (*:NAME)
2610: </pre>
2611: A name is always required with this verb. There may be as many instances of
2612: (*MARK) as you like in a pattern, and their names do not have to be unique.
2613: </P>
2614: <P>
2615: When a match succeeds, the name of the last-encountered (*MARK) on the matching
2616: path is passed back to the caller via the <i>pcre_extra</i> data structure, as
2617: described in the
2618: <a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a>
2619: in the
2620: <a href="pcreapi.html"><b>pcreapi</b></a>
2621: documentation. Here is an example of <b>pcretest</b> output, where the /K
2622: modifier requests the retrieval and outputting of (*MARK) data:
2623: <pre>
2624: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2625: data> XY
2626: 0: XY
2627: MK: A
2628: XZ
2629: 0: XZ
2630: MK: B
2631: </pre>
2632: The (*MARK) name is tagged with "MK:" in this output, and in this example it
2633: indicates which of the two alternatives matched. This is a more efficient way
2634: of obtaining this information than putting each alternative in its own
2635: capturing parentheses.
2636: </P>
2637: <P>
2638: If (*MARK) is encountered in a positive assertion, its name is recorded and
2639: passed back if it is the last-encountered. This does not happen for negative
2640: assertions.
2641: </P>
2642: <P>
2643: After a partial match or a failed match, the name of the last encountered
2644: (*MARK) in the entire match process is returned. For example:
2645: <pre>
2646: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2647: data> XP
2648: No match, mark = B
2649: </pre>
2650: Note that in this unanchored example the mark is retained from the match
2651: attempt that started at the letter "X". Subsequent match attempts starting at
2652: "P" and then with an empty string do not get as far as the (*MARK) item, but
2653: nevertheless do not reset it.
2654: </P>
2655: <br><b>
2656: Verbs that act after backtracking
2657: </b><br>
2658: <P>
2659: The following verbs do nothing when they are encountered. Matching continues
2660: with what follows, but if there is no subsequent match, causing a backtrack to
2661: the verb, a failure is forced. That is, backtracking cannot pass to the left of
2662: the verb. However, when one of these verbs appears inside an atomic group, its
2663: effect is confined to that group, because once the group has been matched,
2664: there is never any backtracking into it. In this situation, backtracking can
2665: "jump back" to the left of the entire atomic group. (Remember also, as stated
2666: above, that this localization also applies in subroutine calls and assertions.)
2667: </P>
2668: <P>
2669: These verbs differ in exactly what kind of failure occurs when backtracking
2670: reaches them.
2671: <pre>
2672: (*COMMIT)
2673: </pre>
2674: This verb, which may not be followed by a name, causes the whole match to fail
2675: outright if the rest of the pattern does not match. Even if the pattern is
2676: unanchored, no further attempts to find a match by advancing the starting point
2677: take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2678: finding a match at the current starting point, or not at all. For example:
2679: <pre>
2680: a+(*COMMIT)b
2681: </pre>
2682: This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2683: dynamic anchor, or "I've started, so I must finish." The name of the most
2684: recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2685: match failure.
2686: </P>
2687: <P>
2688: Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2689: unless PCRE's start-of-match optimizations are turned off, as shown in this
2690: <b>pcretest</b> example:
2691: <pre>
2692: re> /(*COMMIT)abc/
2693: data> xyzabc
2694: 0: abc
2695: xyzabc\Y
2696: No match
2697: </pre>
2698: PCRE knows that any match must start with "a", so the optimization skips along
2699: the subject to "a" before running the first match attempt, which succeeds. When
2700: the optimization is disabled by the \Y escape in the second subject, the match
2701: starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2702: starting points.
2703: <pre>
2704: (*PRUNE) or (*PRUNE:NAME)
2705: </pre>
2706: This verb causes the match to fail at the current starting position in the
2707: subject if the rest of the pattern does not match. If the pattern is
2708: unanchored, the normal "bumpalong" advance to the next starting character then
2709: happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2710: reached, or when matching to the right of (*PRUNE), but if there is no match to
2711: the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2712: (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2713: but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2714: The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2715: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2716: <pre>
2717: (*SKIP)
2718: </pre>
2719: This verb, when given without a name, is like (*PRUNE), except that if the
2720: pattern is unanchored, the "bumpalong" advance is not to the next character,
2721: but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2722: signifies that whatever text was matched leading up to it cannot be part of a
2723: successful match. Consider:
2724: <pre>
2725: a+(*SKIP)b
2726: </pre>
2727: If the subject is "aaaac...", after the first match attempt fails (starting at
2728: the first character in the string), the starting point skips on to start the
2729: next attempt at "c". Note that a possessive quantifer does not have the same
2730: effect as this example; although it would suppress backtracking during the
2731: first match attempt, the second attempt would start at the second character
2732: instead of skipping on to "c".
2733: <pre>
2734: (*SKIP:NAME)
2735: </pre>
2736: When (*SKIP) has an associated name, its behaviour is modified. If the
2737: following pattern fails to match, the previous path through the pattern is
2738: searched for the most recent (*MARK) that has the same name. If one is found,
2739: the "bumpalong" advance is to the subject position that corresponds to that
2740: (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2741: matching name is found, the (*SKIP) is ignored.
2742: <pre>
2743: (*THEN) or (*THEN:NAME)
2744: </pre>
2745: This verb causes a skip to the next innermost alternative if the rest of the
2746: pattern does not match. That is, it cancels pending backtracking, but only
2747: within the current alternative. Its name comes from the observation that it can
2748: be used for a pattern-based if-then-else block:
2749: <pre>
2750: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2751: </pre>
2752: If the COND1 pattern matches, FOO is tried (and possibly further items after
2753: the end of the group if FOO succeeds); on failure, the matcher skips to the
2754: second alternative and tries COND2, without backtracking into COND1. The
2755: behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2756: If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2757: </P>
2758: <P>
2759: Note that a subpattern that does not contain a | character is just a part of
2760: the enclosing alternative; it is not a nested alternation with only one
2761: alternative. The effect of (*THEN) extends beyond such a subpattern to the
2762: enclosing alternative. Consider this pattern, where A, B, etc. are complex
2763: pattern fragments that do not contain any | characters at this level:
2764: <pre>
2765: A (B(*THEN)C) | D
2766: </pre>
2767: If A and B are matched, but there is a failure in C, matching does not
2768: backtrack into A; instead it moves to the next alternative, that is, D.
2769: However, if the subpattern containing (*THEN) is given an alternative, it
2770: behaves differently:
2771: <pre>
2772: A (B(*THEN)C | (*FAIL)) | D
2773: </pre>
2774: The effect of (*THEN) is now confined to the inner subpattern. After a failure
2775: in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2776: because there are no more alternatives to try. In this case, matching does now
2777: backtrack into A.
2778: </P>
2779: <P>
2780: Note also that a conditional subpattern is not considered as having two
2781: alternatives, because only one is ever used. In other words, the | character in
2782: a conditional subpattern has a different meaning. Ignoring white space,
2783: consider:
2784: <pre>
2785: ^.*? (?(?=a) a | b(*THEN)c )
2786: </pre>
2787: If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2788: it initially matches zero characters. The condition (?=a) then fails, the
2789: character "b" is matched, but "c" is not. At this point, matching does not
2790: backtrack to .*? as might perhaps be expected from the presence of the |
2791: character. The conditional subpattern is part of the single alternative that
2792: comprises the whole pattern, and so the match fails. (If there was a backtrack
2793: into .*?, allowing it to match "b", the match would succeed.)
2794: </P>
2795: <P>
2796: The verbs just described provide four different "strengths" of control when
2797: subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2798: next alternative. (*PRUNE) comes next, failing the match at the current
2799: starting position, but allowing an advance to the next character (for an
2800: unanchored pattern). (*SKIP) is similar, except that the advance may be more
2801: than one character. (*COMMIT) is the strongest, causing the entire match to
2802: fail.
2803: </P>
2804: <P>
2805: If more than one such verb is present in a pattern, the "strongest" one wins.
2806: For example, consider this pattern, where A, B, etc. are complex pattern
2807: fragments:
2808: <pre>
2809: (A(*COMMIT)B(*THEN)C|D)
2810: </pre>
2811: Once A has matched, PCRE is committed to this match, at the current starting
2812: position. If subsequently B matches, but C does not, the normal (*THEN) action
2813: of trying the next alternative (that is, D) does not happen because (*COMMIT)
2814: overrides.
2815: </P>
2816: <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2817: <P>
2818: <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2819: <b>pcresyntax</b>(3), <b>pcre</b>(3).
2820: </P>
2821: <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2822: <P>
2823: Philip Hazel
2824: <br>
2825: University Computing Service
2826: <br>
2827: Cambridge CB2 3QH, England.
2828: <br>
2829: </P>
2830: <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2831: <P>
2832: Last updated: 29 November 2011
2833: <br>
2834: Copyright © 1997-2011 University of Cambridge.
2835: <br>
2836: <p>
2837: Return to the <a href="index.html">PCRE index page</a>.
2838: </p>
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