Annotation of embedaddon/pcre/doc/html/pcrepattern.html, revision 1.1.1.3
1.1 misho 1: <html>
2: <head>
3: <title>pcrepattern specification</title>
4: </head>
5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6: <h1>pcrepattern man page</h1>
7: <p>
8: Return to the <a href="index.html">PCRE index page</a>.
9: </p>
10: <p>
11: This page is part of the PCRE HTML documentation. It was generated automatically
12: from the original man page. If there is any nonsense in it, please consult the
13: man page, in case the conversion went wrong.
14: <br>
15: <ul>
16: <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17: <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18: <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19: <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20: <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21: <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
1.1.1.2 misho 22: <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE DATA UNIT</a>
1.1 misho 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,
1.1.1.2 misho 64: there is now also support for UTF-8 strings in the original library, and a
65: second library that supports 16-bit and UTF-16 character strings. To use these
66: features, PCRE must be built to include appropriate support. When using UTF
67: strings you must either call the compiling function with the PCRE_UTF8 or
68: PCRE_UTF16 option, or the pattern must start with one of these special
69: sequences:
1.1 misho 70: <pre>
71: (*UTF8)
1.1.1.2 misho 72: (*UTF16)
1.1 misho 73: </pre>
1.1.1.2 misho 74: Starting a pattern with such a sequence is equivalent to setting the relevant
75: option. This feature is not Perl-compatible. How setting a UTF mode affects
1.1 misho 76: pattern matching is mentioned in several places below. There is also a summary
1.1.1.2 misho 77: of features in the
1.1 misho 78: <a href="pcreunicode.html"><b>pcreunicode</b></a>
79: page.
80: </P>
81: <P>
82: Another special sequence that may appear at the start of a pattern or in
1.1.1.2 misho 83: combination with (*UTF8) or (*UTF16) is:
1.1 misho 84: <pre>
85: (*UCP)
86: </pre>
87: This has the same effect as setting the PCRE_UCP option: it causes sequences
88: such as \d and \w to use Unicode properties to determine character types,
89: instead of recognizing only characters with codes less than 128 via a lookup
90: table.
91: </P>
92: <P>
93: If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
94: PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
95: also some more of these special sequences that are concerned with the handling
96: of newlines; they are described below.
97: </P>
98: <P>
99: The remainder of this document discusses the patterns that are supported by
1.1.1.2 misho 100: PCRE when one its main matching functions, <b>pcre_exec()</b> (8-bit) or
101: <b>pcre16_exec()</b> (16-bit), is used. PCRE also has alternative matching
102: functions, <b>pcre_dfa_exec()</b> and <b>pcre16_dfa_exec()</b>, which match using
103: a different algorithm that is not Perl-compatible. Some of the features
104: discussed below are not available when DFA matching is used. The advantages and
105: disadvantages of the alternative functions, and how they differ from the normal
106: functions, are discussed in the
1.1 misho 107: <a href="pcrematching.html"><b>pcrematching</b></a>
108: page.
109: <a name="newlines"></a></P>
110: <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
111: <P>
112: PCRE supports five different conventions for indicating line breaks in
113: strings: a single CR (carriage return) character, a single LF (linefeed)
114: character, the two-character sequence CRLF, any of the three preceding, or any
115: Unicode newline sequence. The
116: <a href="pcreapi.html"><b>pcreapi</b></a>
117: page has
118: <a href="pcreapi.html#newlines">further discussion</a>
119: about newlines, and shows how to set the newline convention in the
120: <i>options</i> arguments for the compiling and matching functions.
121: </P>
122: <P>
123: It is also possible to specify a newline convention by starting a pattern
124: string with one of the following five sequences:
125: <pre>
126: (*CR) carriage return
127: (*LF) linefeed
128: (*CRLF) carriage return, followed by linefeed
129: (*ANYCRLF) any of the three above
130: (*ANY) all Unicode newline sequences
131: </pre>
1.1.1.2 misho 132: These override the default and the options given to the compiling function. For
133: example, on a Unix system where LF is the default newline sequence, the pattern
1.1 misho 134: <pre>
135: (*CR)a.b
136: </pre>
137: changes the convention to CR. That pattern matches "a\nb" because LF is no
138: longer a newline. Note that these special settings, which are not
139: Perl-compatible, are recognized only at the very start of a pattern, and that
140: they must be in upper case. If more than one of them is present, the last one
141: is used.
142: </P>
143: <P>
144: The newline convention affects the interpretation of the dot metacharacter when
145: PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
146: affect what the \R escape sequence matches. By default, this is any Unicode
147: newline sequence, for Perl compatibility. However, this can be changed; see the
148: description of \R in the section entitled
149: <a href="#newlineseq">"Newline sequences"</a>
150: below. A change of \R setting can be combined with a change of newline
151: convention.
152: </P>
153: <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
154: <P>
155: A regular expression is a pattern that is matched against a subject string from
156: left to right. Most characters stand for themselves in a pattern, and match the
157: corresponding characters in the subject. As a trivial example, the pattern
158: <pre>
159: The quick brown fox
160: </pre>
161: matches a portion of a subject string that is identical to itself. When
162: caseless matching is specified (the PCRE_CASELESS option), letters are matched
1.1.1.2 misho 163: independently of case. In a UTF mode, PCRE always understands the concept of
1.1 misho 164: case for characters whose values are less than 128, so caseless matching is
165: always possible. For characters with higher values, the concept of case is
166: supported if PCRE is compiled with Unicode property support, but not otherwise.
167: If you want to use caseless matching for characters 128 and above, you must
168: ensure that PCRE is compiled with Unicode property support as well as with
1.1.1.2 misho 169: UTF support.
1.1 misho 170: </P>
171: <P>
172: The power of regular expressions comes from the ability to include alternatives
173: and repetitions in the pattern. These are encoded in the pattern by the use of
174: <i>metacharacters</i>, which do not stand for themselves but instead are
175: interpreted in some special way.
176: </P>
177: <P>
178: There are two different sets of metacharacters: those that are recognized
179: anywhere in the pattern except within square brackets, and those that are
180: recognized within square brackets. Outside square brackets, the metacharacters
181: are as follows:
182: <pre>
183: \ general escape character with several uses
184: ^ assert start of string (or line, in multiline mode)
185: $ assert end of string (or line, in multiline mode)
186: . match any character except newline (by default)
187: [ start character class definition
188: | start of alternative branch
189: ( start subpattern
190: ) end subpattern
191: ? extends the meaning of (
192: also 0 or 1 quantifier
193: also quantifier minimizer
194: * 0 or more quantifier
195: + 1 or more quantifier
196: also "possessive quantifier"
197: { start min/max quantifier
198: </pre>
199: Part of a pattern that is in square brackets is called a "character class". In
200: a character class the only metacharacters are:
201: <pre>
202: \ general escape character
203: ^ negate the class, but only if the first character
204: - indicates character range
205: [ POSIX character class (only if followed by POSIX syntax)
206: ] terminates the character class
207: </pre>
208: The following sections describe the use of each of the metacharacters.
209: </P>
210: <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
211: <P>
212: The backslash character has several uses. Firstly, if it is followed by a
213: character that is not a number or a letter, it takes away any special meaning
214: that character may have. This use of backslash as an escape character applies
215: both inside and outside character classes.
216: </P>
217: <P>
218: For example, if you want to match a * character, you write \* in the pattern.
219: This escaping action applies whether or not the following character would
220: otherwise be interpreted as a metacharacter, so it is always safe to precede a
221: non-alphanumeric with backslash to specify that it stands for itself. In
222: particular, if you want to match a backslash, you write \\.
223: </P>
224: <P>
1.1.1.2 misho 225: In a UTF mode, only ASCII numbers and letters have any special meaning after a
1.1 misho 226: backslash. All other characters (in particular, those whose codepoints are
227: greater than 127) are treated as literals.
228: </P>
229: <P>
1.1.1.3 ! misho 230: If a pattern is compiled with the PCRE_EXTENDED option, white space in the
1.1 misho 231: pattern (other than in a character class) and characters between a # outside
232: a character class and the next newline are ignored. An escaping backslash can
1.1.1.3 ! misho 233: be used to include a white space or # character as part of the pattern.
1.1 misho 234: </P>
235: <P>
236: If you want to remove the special meaning from a sequence of characters, you
237: can do so by putting them between \Q and \E. This is different from Perl in
238: that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
239: Perl, $ and @ cause variable interpolation. Note the following examples:
240: <pre>
241: Pattern PCRE matches Perl matches
242:
243: \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
244: \Qabc\$xyz\E abc\$xyz abc\$xyz
245: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
246: </pre>
247: The \Q...\E sequence is recognized both inside and outside character classes.
248: An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
249: by \E later in the pattern, the literal interpretation continues to the end of
250: the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
251: a character class, this causes an error, because the character class is not
252: terminated.
253: <a name="digitsafterbackslash"></a></P>
254: <br><b>
255: Non-printing characters
256: </b><br>
257: <P>
258: A second use of backslash provides a way of encoding non-printing characters
259: in patterns in a visible manner. There is no restriction on the appearance of
260: non-printing characters, apart from the binary zero that terminates a pattern,
261: but when a pattern is being prepared by text editing, it is often easier to use
262: one of the following escape sequences than the binary character it represents:
263: <pre>
264: \a alarm, that is, the BEL character (hex 07)
265: \cx "control-x", where x is any ASCII character
266: \e escape (hex 1B)
1.1.1.3 ! misho 267: \f form feed (hex 0C)
1.1 misho 268: \n linefeed (hex 0A)
269: \r carriage return (hex 0D)
270: \t tab (hex 09)
271: \ddd character with octal code ddd, or back reference
272: \xhh character with hex code hh
273: \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
274: \uhhhh character with hex code hhhh (JavaScript mode only)
275: </pre>
276: The precise effect of \cx is as follows: if x is a lower case letter, it
277: is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
278: Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
279: \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
280: than 127, a compile-time error occurs. This locks out non-ASCII characters in
1.1.1.2 misho 281: all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
282: lower case letter is converted to upper case, and then the 0xc0 bits are
283: flipped.)
1.1 misho 284: </P>
285: <P>
286: By default, after \x, from zero to two hexadecimal digits are read (letters
287: can be in upper or lower case). Any number of hexadecimal digits may appear
1.1.1.2 misho 288: between \x{ and }, but the character code is constrained as follows:
289: <pre>
290: 8-bit non-UTF mode less than 0x100
291: 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
292: 16-bit non-UTF mode less than 0x10000
293: 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
294: </pre>
295: Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
296: "surrogate" codepoints).
1.1 misho 297: </P>
298: <P>
299: If characters other than hexadecimal digits appear between \x{ and }, or if
300: there is no terminating }, this form of escape is not recognized. Instead, the
301: initial \x will be interpreted as a basic hexadecimal escape, with no
302: following digits, giving a character whose value is zero.
303: </P>
304: <P>
305: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
306: as just described only when it is followed by two hexadecimal digits.
307: Otherwise, it matches a literal "x" character. In JavaScript mode, support for
308: code points greater than 256 is provided by \u, which must be followed by
309: four hexadecimal digits; otherwise it matches a literal "u" character.
1.1.1.3 ! misho 310: Character codes specified by \u in JavaScript mode are constrained in the same
! 311: was as those specified by \x in non-JavaScript mode.
1.1 misho 312: </P>
313: <P>
314: Characters whose value is less than 256 can be defined by either of the two
315: syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
316: way they are handled. For example, \xdc is exactly the same as \x{dc} (or
317: \u00dc in JavaScript mode).
318: </P>
319: <P>
320: After \0 up to two further octal digits are read. If there are fewer than two
321: digits, just those that are present are used. Thus the sequence \0\x\07
322: specifies two binary zeros followed by a BEL character (code value 7). Make
323: sure you supply two digits after the initial zero if the pattern character that
324: follows is itself an octal digit.
325: </P>
326: <P>
327: The handling of a backslash followed by a digit other than 0 is complicated.
328: Outside a character class, PCRE reads it and any following digits as a decimal
329: number. If the number is less than 10, or if there have been at least that many
330: previous capturing left parentheses in the expression, the entire sequence is
331: taken as a <i>back reference</i>. A description of how this works is given
332: <a href="#backreferences">later,</a>
333: following the discussion of
334: <a href="#subpattern">parenthesized subpatterns.</a>
335: </P>
336: <P>
337: Inside a character class, or if the decimal number is greater than 9 and there
338: have not been that many capturing subpatterns, PCRE re-reads up to three octal
339: digits following the backslash, and uses them to generate a data character. Any
1.1.1.2 misho 340: subsequent digits stand for themselves. The value of the character is
341: constrained in the same way as characters specified in hexadecimal.
342: For example:
1.1 misho 343: <pre>
344: \040 is another way of writing a space
345: \40 is the same, provided there are fewer than 40 previous capturing subpatterns
346: \7 is always a back reference
347: \11 might be a back reference, or another way of writing a tab
348: \011 is always a tab
349: \0113 is a tab followed by the character "3"
350: \113 might be a back reference, otherwise the character with octal code 113
1.1.1.2 misho 351: \377 might be a back reference, otherwise the value 255 (decimal)
1.1 misho 352: \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
353: </pre>
354: Note that octal values of 100 or greater must not be introduced by a leading
355: zero, because no more than three octal digits are ever read.
356: </P>
357: <P>
358: All the sequences that define a single character value can be used both inside
359: and outside character classes. In addition, inside a character class, \b is
360: interpreted as the backspace character (hex 08).
361: </P>
362: <P>
363: \N is not allowed in a character class. \B, \R, and \X are not special
364: inside a character class. Like other unrecognized escape sequences, they are
365: treated as the literal characters "B", "R", and "X" by default, but cause an
366: error if the PCRE_EXTRA option is set. Outside a character class, these
367: sequences have different meanings.
368: </P>
369: <br><b>
370: Unsupported escape sequences
371: </b><br>
372: <P>
373: In Perl, the sequences \l, \L, \u, and \U are recognized by its string
374: handler and used to modify the case of following characters. By default, PCRE
375: does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
376: option is set, \U matches a "U" character, and \u can be used to define a
377: character by code point, as described in the previous section.
378: </P>
379: <br><b>
380: Absolute and relative back references
381: </b><br>
382: <P>
383: The sequence \g followed by an unsigned or a negative number, optionally
384: enclosed in braces, is an absolute or relative back reference. A named back
385: reference can be coded as \g{name}. Back references are discussed
386: <a href="#backreferences">later,</a>
387: following the discussion of
388: <a href="#subpattern">parenthesized subpatterns.</a>
389: </P>
390: <br><b>
391: Absolute and relative subroutine calls
392: </b><br>
393: <P>
394: For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
395: a number enclosed either in angle brackets or single quotes, is an alternative
396: syntax for referencing a subpattern as a "subroutine". Details are discussed
397: <a href="#onigurumasubroutines">later.</a>
398: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
399: synonymous. The former is a back reference; the latter is a
400: <a href="#subpatternsassubroutines">subroutine</a>
401: call.
402: <a name="genericchartypes"></a></P>
403: <br><b>
404: Generic character types
405: </b><br>
406: <P>
407: Another use of backslash is for specifying generic character types:
408: <pre>
409: \d any decimal digit
410: \D any character that is not a decimal digit
1.1.1.3 ! misho 411: \h any horizontal white space character
! 412: \H any character that is not a horizontal white space character
! 413: \s any white space character
! 414: \S any character that is not a white space character
! 415: \v any vertical white space character
! 416: \V any character that is not a vertical white space character
1.1 misho 417: \w any "word" character
418: \W any "non-word" character
419: </pre>
420: There is also the single sequence \N, which matches a non-newline character.
421: This is the same as
422: <a href="#fullstopdot">the "." metacharacter</a>
423: when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
424: PCRE does not support this.
425: </P>
426: <P>
427: Each pair of lower and upper case escape sequences partitions the complete set
428: of characters into two disjoint sets. Any given character matches one, and only
429: one, of each pair. The sequences can appear both inside and outside character
430: classes. They each match one character of the appropriate type. If the current
431: matching point is at the end of the subject string, all of them fail, because
432: there is no character to match.
433: </P>
434: <P>
435: For compatibility with Perl, \s does not match the VT character (code 11).
436: This makes it different from the the POSIX "space" class. The \s characters
437: are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
438: included in a Perl script, \s may match the VT character. In PCRE, it never
439: does.
440: </P>
441: <P>
442: A "word" character is an underscore or any character that is a letter or digit.
443: By default, the definition of letters and digits is controlled by PCRE's
444: low-valued character tables, and may vary if locale-specific matching is taking
445: place (see
446: <a href="pcreapi.html#localesupport">"Locale support"</a>
447: in the
448: <a href="pcreapi.html"><b>pcreapi</b></a>
449: page). For example, in a French locale such as "fr_FR" in Unix-like systems,
450: or "french" in Windows, some character codes greater than 128 are used for
451: accented letters, and these are then matched by \w. The use of locales with
452: Unicode is discouraged.
453: </P>
454: <P>
1.1.1.2 misho 455: By default, in a UTF mode, characters with values greater than 128 never match
1.1 misho 456: \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
1.1.1.2 misho 457: their original meanings from before UTF support was available, mainly for
1.1 misho 458: efficiency reasons. However, if PCRE is compiled with Unicode property support,
459: and the PCRE_UCP option is set, the behaviour is changed so that Unicode
460: properties are used to determine character types, as follows:
461: <pre>
462: \d any character that \p{Nd} matches (decimal digit)
463: \s any character that \p{Z} matches, plus HT, LF, FF, CR
464: \w any character that \p{L} or \p{N} matches, plus underscore
465: </pre>
466: The upper case escapes match the inverse sets of characters. Note that \d
467: matches only decimal digits, whereas \w matches any Unicode digit, as well as
468: any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
469: \B because they are defined in terms of \w and \W. Matching these sequences
470: is noticeably slower when PCRE_UCP is set.
471: </P>
472: <P>
473: The sequences \h, \H, \v, and \V are features that were added to Perl at
474: release 5.10. In contrast to the other sequences, which match only ASCII
1.1.1.2 misho 475: characters by default, these always match certain high-valued codepoints,
476: whether or not PCRE_UCP is set. The horizontal space characters are:
1.1 misho 477: <pre>
478: U+0009 Horizontal tab
479: U+0020 Space
480: U+00A0 Non-break space
481: U+1680 Ogham space mark
482: U+180E Mongolian vowel separator
483: U+2000 En quad
484: U+2001 Em quad
485: U+2002 En space
486: U+2003 Em space
487: U+2004 Three-per-em space
488: U+2005 Four-per-em space
489: U+2006 Six-per-em space
490: U+2007 Figure space
491: U+2008 Punctuation space
492: U+2009 Thin space
493: U+200A Hair space
494: U+202F Narrow no-break space
495: U+205F Medium mathematical space
496: U+3000 Ideographic space
497: </pre>
498: The vertical space characters are:
499: <pre>
500: U+000A Linefeed
501: U+000B Vertical tab
1.1.1.3 ! misho 502: U+000C Form feed
1.1 misho 503: U+000D Carriage return
504: U+0085 Next line
505: U+2028 Line separator
506: U+2029 Paragraph separator
1.1.1.2 misho 507: </pre>
508: In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
509: relevant.
510: <a name="newlineseq"></a></P>
1.1 misho 511: <br><b>
512: Newline sequences
513: </b><br>
514: <P>
515: Outside a character class, by default, the escape sequence \R matches any
1.1.1.2 misho 516: Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
517: following:
1.1 misho 518: <pre>
519: (?>\r\n|\n|\x0b|\f|\r|\x85)
520: </pre>
521: This is an example of an "atomic group", details of which are given
522: <a href="#atomicgroup">below.</a>
523: This particular group matches either the two-character sequence CR followed by
524: LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
1.1.1.3 ! misho 525: U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
1.1 misho 526: line, U+0085). The two-character sequence is treated as a single unit that
527: cannot be split.
528: </P>
529: <P>
1.1.1.2 misho 530: In other modes, two additional characters whose codepoints are greater than 255
1.1 misho 531: are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
532: Unicode character property support is not needed for these characters to be
533: recognized.
534: </P>
535: <P>
536: It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
537: complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
538: either at compile time or when the pattern is matched. (BSR is an abbrevation
539: for "backslash R".) This can be made the default when PCRE is built; if this is
540: the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
541: It is also possible to specify these settings by starting a pattern string with
542: one of the following sequences:
543: <pre>
544: (*BSR_ANYCRLF) CR, LF, or CRLF only
545: (*BSR_UNICODE) any Unicode newline sequence
546: </pre>
1.1.1.2 misho 547: These override the default and the options given to the compiling function, but
548: they can themselves be overridden by options given to a matching function. Note
549: that these special settings, which are not Perl-compatible, are recognized only
550: at the very start of a pattern, and that they must be in upper case. If more
551: than one of them is present, the last one is used. They can be combined with a
552: change of newline convention; for example, a pattern can start with:
1.1 misho 553: <pre>
554: (*ANY)(*BSR_ANYCRLF)
555: </pre>
1.1.1.2 misho 556: They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
557: sequences. Inside a character class, \R is treated as an unrecognized escape
558: sequence, and so matches the letter "R" by default, but causes an error if
559: PCRE_EXTRA is set.
1.1 misho 560: <a name="uniextseq"></a></P>
561: <br><b>
562: Unicode character properties
563: </b><br>
564: <P>
565: When PCRE is built with Unicode character property support, three additional
566: escape sequences that match characters with specific properties are available.
1.1.1.2 misho 567: When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
1.1 misho 568: characters whose codepoints are less than 256, but they do work in this mode.
569: The extra escape sequences are:
570: <pre>
571: \p{<i>xx</i>} a character with the <i>xx</i> property
572: \P{<i>xx</i>} a character without the <i>xx</i> property
573: \X an extended Unicode sequence
574: </pre>
575: The property names represented by <i>xx</i> above are limited to the Unicode
576: script names, the general category properties, "Any", which matches any
577: character (including newline), and some special PCRE properties (described
578: in the
579: <a href="#extraprops">next section).</a>
580: Other Perl properties such as "InMusicalSymbols" are not currently supported by
581: PCRE. Note that \P{Any} does not match any characters, so always causes a
582: match failure.
583: </P>
584: <P>
585: Sets of Unicode characters are defined as belonging to certain scripts. A
586: character from one of these sets can be matched using a script name. For
587: example:
588: <pre>
589: \p{Greek}
590: \P{Han}
591: </pre>
592: Those that are not part of an identified script are lumped together as
593: "Common". The current list of scripts is:
594: </P>
595: <P>
596: Arabic,
597: Armenian,
598: Avestan,
599: Balinese,
600: Bamum,
1.1.1.3 ! misho 601: Batak,
1.1 misho 602: Bengali,
603: Bopomofo,
1.1.1.3 ! misho 604: Brahmi,
1.1 misho 605: Braille,
606: Buginese,
607: Buhid,
608: Canadian_Aboriginal,
609: Carian,
1.1.1.3 ! misho 610: Chakma,
1.1 misho 611: Cham,
612: Cherokee,
613: Common,
614: Coptic,
615: Cuneiform,
616: Cypriot,
617: Cyrillic,
618: Deseret,
619: Devanagari,
620: Egyptian_Hieroglyphs,
621: Ethiopic,
622: Georgian,
623: Glagolitic,
624: Gothic,
625: Greek,
626: Gujarati,
627: Gurmukhi,
628: Han,
629: Hangul,
630: Hanunoo,
631: Hebrew,
632: Hiragana,
633: Imperial_Aramaic,
634: Inherited,
635: Inscriptional_Pahlavi,
636: Inscriptional_Parthian,
637: Javanese,
638: Kaithi,
639: Kannada,
640: Katakana,
641: Kayah_Li,
642: Kharoshthi,
643: Khmer,
644: Lao,
645: Latin,
646: Lepcha,
647: Limbu,
648: Linear_B,
649: Lisu,
650: Lycian,
651: Lydian,
652: Malayalam,
1.1.1.3 ! misho 653: Mandaic,
1.1 misho 654: Meetei_Mayek,
1.1.1.3 ! misho 655: Meroitic_Cursive,
! 656: Meroitic_Hieroglyphs,
! 657: Miao,
1.1 misho 658: Mongolian,
659: Myanmar,
660: New_Tai_Lue,
661: Nko,
662: Ogham,
663: Old_Italic,
664: Old_Persian,
665: Old_South_Arabian,
666: Old_Turkic,
667: Ol_Chiki,
668: Oriya,
669: Osmanya,
670: Phags_Pa,
671: Phoenician,
672: Rejang,
673: Runic,
674: Samaritan,
675: Saurashtra,
1.1.1.3 ! misho 676: Sharada,
1.1 misho 677: Shavian,
678: Sinhala,
1.1.1.3 ! misho 679: Sora_Sompeng,
1.1 misho 680: Sundanese,
681: Syloti_Nagri,
682: Syriac,
683: Tagalog,
684: Tagbanwa,
685: Tai_Le,
686: Tai_Tham,
687: Tai_Viet,
1.1.1.3 ! misho 688: Takri,
1.1 misho 689: Tamil,
690: Telugu,
691: Thaana,
692: Thai,
693: Tibetan,
694: Tifinagh,
695: Ugaritic,
696: Vai,
697: Yi.
698: </P>
699: <P>
700: Each character has exactly one Unicode general category property, specified by
701: a two-letter abbreviation. For compatibility with Perl, negation can be
702: specified by including a circumflex between the opening brace and the property
703: name. For example, \p{^Lu} is the same as \P{Lu}.
704: </P>
705: <P>
706: If only one letter is specified with \p or \P, it includes all the general
707: category properties that start with that letter. In this case, in the absence
708: of negation, the curly brackets in the escape sequence are optional; these two
709: examples have the same effect:
710: <pre>
711: \p{L}
712: \pL
713: </pre>
714: The following general category property codes are supported:
715: <pre>
716: C Other
717: Cc Control
718: Cf Format
719: Cn Unassigned
720: Co Private use
721: Cs Surrogate
722:
723: L Letter
724: Ll Lower case letter
725: Lm Modifier letter
726: Lo Other letter
727: Lt Title case letter
728: Lu Upper case letter
729:
730: M Mark
731: Mc Spacing mark
732: Me Enclosing mark
733: Mn Non-spacing mark
734:
735: N Number
736: Nd Decimal number
737: Nl Letter number
738: No Other number
739:
740: P Punctuation
741: Pc Connector punctuation
742: Pd Dash punctuation
743: Pe Close punctuation
744: Pf Final punctuation
745: Pi Initial punctuation
746: Po Other punctuation
747: Ps Open punctuation
748:
749: S Symbol
750: Sc Currency symbol
751: Sk Modifier symbol
752: Sm Mathematical symbol
753: So Other symbol
754:
755: Z Separator
756: Zl Line separator
757: Zp Paragraph separator
758: Zs Space separator
759: </pre>
760: The special property L& is also supported: it matches a character that has
761: the Lu, Ll, or Lt property, in other words, a letter that is not classified as
762: a modifier or "other".
763: </P>
764: <P>
765: The Cs (Surrogate) property applies only to characters in the range U+D800 to
1.1.1.2 misho 766: U+DFFF. Such characters are not valid in Unicode strings and so
767: cannot be tested by PCRE, unless UTF validity checking has been turned off
768: (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
1.1 misho 769: <a href="pcreapi.html"><b>pcreapi</b></a>
770: page). Perl does not support the Cs property.
771: </P>
772: <P>
773: The long synonyms for property names that Perl supports (such as \p{Letter})
774: are not supported by PCRE, nor is it permitted to prefix any of these
775: properties with "Is".
776: </P>
777: <P>
778: No character that is in the Unicode table has the Cn (unassigned) property.
779: Instead, this property is assumed for any code point that is not in the
780: Unicode table.
781: </P>
782: <P>
783: Specifying caseless matching does not affect these escape sequences. For
784: example, \p{Lu} always matches only upper case letters.
785: </P>
786: <P>
787: The \X escape matches any number of Unicode characters that form an extended
788: Unicode sequence. \X is equivalent to
789: <pre>
790: (?>\PM\pM*)
791: </pre>
792: That is, it matches a character without the "mark" property, followed by zero
793: or more characters with the "mark" property, and treats the sequence as an
794: atomic group
795: <a href="#atomicgroup">(see below).</a>
796: Characters with the "mark" property are typically accents that affect the
797: preceding character. None of them have codepoints less than 256, so in
1.1.1.2 misho 798: 8-bit non-UTF-8 mode \X matches any one character.
1.1 misho 799: </P>
800: <P>
801: Note that recent versions of Perl have changed \X to match what Unicode calls
802: an "extended grapheme cluster", which has a more complicated definition.
803: </P>
804: <P>
805: Matching characters by Unicode property is not fast, because PCRE has to search
806: a structure that contains data for over fifteen thousand characters. That is
807: why the traditional escape sequences such as \d and \w do not use Unicode
808: properties in PCRE by default, though you can make them do so by setting the
1.1.1.2 misho 809: PCRE_UCP option or by starting the pattern with (*UCP).
1.1 misho 810: <a name="extraprops"></a></P>
811: <br><b>
812: PCRE's additional properties
813: </b><br>
814: <P>
815: As well as the standard Unicode properties described in the previous
816: section, PCRE supports four more that make it possible to convert traditional
817: escape sequences such as \w and \s and POSIX character classes to use Unicode
818: properties. PCRE uses these non-standard, non-Perl properties internally when
819: PCRE_UCP is set. They are:
820: <pre>
821: Xan Any alphanumeric character
822: Xps Any POSIX space character
823: Xsp Any Perl space character
824: Xwd Any Perl "word" character
825: </pre>
826: Xan matches characters that have either the L (letter) or the N (number)
1.1.1.3 ! misho 827: property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
1.1 misho 828: carriage return, and any other character that has the Z (separator) property.
829: Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
830: same characters as Xan, plus underscore.
831: <a name="resetmatchstart"></a></P>
832: <br><b>
833: Resetting the match start
834: </b><br>
835: <P>
836: The escape sequence \K causes any previously matched characters not to be
837: included in the final matched sequence. For example, the pattern:
838: <pre>
839: foo\Kbar
840: </pre>
841: matches "foobar", but reports that it has matched "bar". This feature is
842: similar to a lookbehind assertion
843: <a href="#lookbehind">(described below).</a>
844: However, in this case, the part of the subject before the real match does not
845: have to be of fixed length, as lookbehind assertions do. The use of \K does
846: not interfere with the setting of
847: <a href="#subpattern">captured substrings.</a>
848: For example, when the pattern
849: <pre>
850: (foo)\Kbar
851: </pre>
852: matches "foobar", the first substring is still set to "foo".
853: </P>
854: <P>
855: Perl documents that the use of \K within assertions is "not well defined". In
856: PCRE, \K is acted upon when it occurs inside positive assertions, but is
857: ignored in negative assertions.
858: <a name="smallassertions"></a></P>
859: <br><b>
860: Simple assertions
861: </b><br>
862: <P>
863: The final use of backslash is for certain simple assertions. An assertion
864: specifies a condition that has to be met at a particular point in a match,
865: without consuming any characters from the subject string. The use of
866: subpatterns for more complicated assertions is described
867: <a href="#bigassertions">below.</a>
868: The backslashed assertions are:
869: <pre>
870: \b matches at a word boundary
871: \B matches when not at a word boundary
872: \A matches at the start of the subject
873: \Z matches at the end of the subject
874: also matches before a newline at the end of the subject
875: \z matches only at the end of the subject
876: \G matches at the first matching position in the subject
877: </pre>
878: Inside a character class, \b has a different meaning; it matches the backspace
879: character. If any other of these assertions appears in a character class, by
880: default it matches the corresponding literal character (for example, \B
881: matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
882: escape sequence" error is generated instead.
883: </P>
884: <P>
885: A word boundary is a position in the subject string where the current character
886: and the previous character do not both match \w or \W (i.e. one matches
887: \w and the other matches \W), or the start or end of the string if the
1.1.1.2 misho 888: first or last character matches \w, respectively. In a UTF mode, the meanings
1.1 misho 889: of \w and \W can be changed by setting the PCRE_UCP option. When this is
890: done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
891: of word" or "end of word" metasequence. However, whatever follows \b normally
892: determines which it is. For example, the fragment \ba matches "a" at the start
893: of a word.
894: </P>
895: <P>
896: The \A, \Z, and \z assertions differ from the traditional circumflex and
897: dollar (described in the next section) in that they only ever match at the very
898: start and end of the subject string, whatever options are set. Thus, they are
899: independent of multiline mode. These three assertions are not affected by the
900: PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
901: circumflex and dollar metacharacters. However, if the <i>startoffset</i>
902: argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
903: at a point other than the beginning of the subject, \A can never match. The
904: difference between \Z and \z is that \Z matches before a newline at the end
905: of the string as well as at the very end, whereas \z matches only at the end.
906: </P>
907: <P>
908: The \G assertion is true only when the current matching position is at the
909: start point of the match, as specified by the <i>startoffset</i> argument of
910: <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
911: non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
912: arguments, you can mimic Perl's /g option, and it is in this kind of
913: implementation where \G can be useful.
914: </P>
915: <P>
916: Note, however, that PCRE's interpretation of \G, as the start of the current
917: match, is subtly different from Perl's, which defines it as the end of the
918: previous match. In Perl, these can be different when the previously matched
919: string was empty. Because PCRE does just one match at a time, it cannot
920: reproduce this behaviour.
921: </P>
922: <P>
923: If all the alternatives of a pattern begin with \G, the expression is anchored
924: to the starting match position, and the "anchored" flag is set in the compiled
925: regular expression.
926: </P>
927: <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
928: <P>
929: Outside a character class, in the default matching mode, the circumflex
930: character is an assertion that is true only if the current matching point is
931: at the start of the subject string. If the <i>startoffset</i> argument of
932: <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
933: option is unset. Inside a character class, circumflex has an entirely different
934: meaning
935: <a href="#characterclass">(see below).</a>
936: </P>
937: <P>
938: Circumflex need not be the first character of the pattern if a number of
939: alternatives are involved, but it should be the first thing in each alternative
940: in which it appears if the pattern is ever to match that branch. If all
941: possible alternatives start with a circumflex, that is, if the pattern is
942: constrained to match only at the start of the subject, it is said to be an
943: "anchored" pattern. (There are also other constructs that can cause a pattern
944: to be anchored.)
945: </P>
946: <P>
947: A dollar character is an assertion that is true only if the current matching
948: point is at the end of the subject string, or immediately before a newline
949: at the end of the string (by default). Dollar need not be the last character of
950: the pattern if a number of alternatives are involved, but it should be the last
951: item in any branch in which it appears. Dollar has no special meaning in a
952: character class.
953: </P>
954: <P>
955: The meaning of dollar can be changed so that it matches only at the very end of
956: the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
957: does not affect the \Z assertion.
958: </P>
959: <P>
960: The meanings of the circumflex and dollar characters are changed if the
961: PCRE_MULTILINE option is set. When this is the case, a circumflex matches
962: immediately after internal newlines as well as at the start of the subject
963: string. It does not match after a newline that ends the string. A dollar
964: matches before any newlines in the string, as well as at the very end, when
965: PCRE_MULTILINE is set. When newline is specified as the two-character
966: sequence CRLF, isolated CR and LF characters do not indicate newlines.
967: </P>
968: <P>
969: For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
970: \n represents a newline) in multiline mode, but not otherwise. Consequently,
971: patterns that are anchored in single line mode because all branches start with
972: ^ are not anchored in multiline mode, and a match for circumflex is possible
973: when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
974: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
975: </P>
976: <P>
977: Note that the sequences \A, \Z, and \z can be used to match the start and
978: end of the subject in both modes, and if all branches of a pattern start with
979: \A it is always anchored, whether or not PCRE_MULTILINE is set.
980: <a name="fullstopdot"></a></P>
981: <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
982: <P>
983: Outside a character class, a dot in the pattern matches any one character in
984: the subject string except (by default) a character that signifies the end of a
1.1.1.2 misho 985: line.
1.1 misho 986: </P>
987: <P>
988: When a line ending is defined as a single character, dot never matches that
989: character; when the two-character sequence CRLF is used, dot does not match CR
990: if it is immediately followed by LF, but otherwise it matches all characters
991: (including isolated CRs and LFs). When any Unicode line endings are being
992: recognized, dot does not match CR or LF or any of the other line ending
993: characters.
994: </P>
995: <P>
996: The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
997: option is set, a dot matches any one character, without exception. If the
998: two-character sequence CRLF is present in the subject string, it takes two dots
999: to match it.
1000: </P>
1001: <P>
1002: The handling of dot is entirely independent of the handling of circumflex and
1003: dollar, the only relationship being that they both involve newlines. Dot has no
1004: special meaning in a character class.
1005: </P>
1006: <P>
1007: The escape sequence \N behaves like a dot, except that it is not affected by
1008: the PCRE_DOTALL option. In other words, it matches any character except one
1009: that signifies the end of a line. Perl also uses \N to match characters by
1010: name; PCRE does not support this.
1011: </P>
1.1.1.2 misho 1012: <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1.1 misho 1013: <P>
1.1.1.2 misho 1014: Outside a character class, the escape sequence \C matches any one data unit,
1015: whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1016: byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \C always
1017: matches line-ending characters. The feature is provided in Perl in order to
1018: match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1019: used. Because \C breaks up characters into individual data units, matching one
1020: unit with \C in a UTF mode means that the rest of the string may start with a
1021: malformed UTF character. This has undefined results, because PCRE assumes that
1022: it is dealing with valid UTF strings (and by default it checks this at the
1.1.1.3 ! misho 1023: start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
! 1024: is used).
1.1 misho 1025: </P>
1026: <P>
1027: PCRE does not allow \C to appear in lookbehind assertions
1028: <a href="#lookbehind">(described below)</a>
1.1.1.2 misho 1029: in a UTF mode, because this would make it impossible to calculate the length of
1.1 misho 1030: the lookbehind.
1031: </P>
1032: <P>
1.1.1.2 misho 1033: In general, the \C escape sequence is best avoided. However, one
1034: way of using it that avoids the problem of malformed UTF characters is to use a
1035: lookahead to check the length of the next character, as in this pattern, which
1036: could be used with a UTF-8 string (ignore white space and line breaks):
1.1 misho 1037: <pre>
1038: (?| (?=[\x00-\x7f])(\C) |
1039: (?=[\x80-\x{7ff}])(\C)(\C) |
1040: (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1041: (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1042: </pre>
1043: A group that starts with (?| resets the capturing parentheses numbers in each
1044: alternative (see
1045: <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1046: below). The assertions at the start of each branch check the next UTF-8
1047: character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1048: character's individual bytes are then captured by the appropriate number of
1049: groups.
1050: <a name="characterclass"></a></P>
1051: <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1052: <P>
1053: An opening square bracket introduces a character class, terminated by a closing
1054: square bracket. A closing square bracket on its own is not special by default.
1055: However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1056: bracket causes a compile-time error. If a closing square bracket is required as
1057: a member of the class, it should be the first data character in the class
1058: (after an initial circumflex, if present) or escaped with a backslash.
1059: </P>
1060: <P>
1.1.1.2 misho 1061: A character class matches a single character in the subject. In a UTF mode, the
1062: character may be more than one data unit long. A matched character must be in
1063: the set of characters defined by the class, unless the first character in the
1064: class definition is a circumflex, in which case the subject character must not
1065: be in the set defined by the class. If a circumflex is actually required as a
1066: member of the class, ensure it is not the first character, or escape it with a
1.1 misho 1067: backslash.
1068: </P>
1069: <P>
1070: For example, the character class [aeiou] matches any lower case vowel, while
1071: [^aeiou] matches any character that is not a lower case vowel. Note that a
1072: circumflex is just a convenient notation for specifying the characters that
1073: are in the class by enumerating those that are not. A class that starts with a
1074: circumflex is not an assertion; it still consumes a character from the subject
1075: string, and therefore it fails if the current pointer is at the end of the
1076: string.
1077: </P>
1078: <P>
1.1.1.2 misho 1079: In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1080: included in a class as a literal string of data units, or by using the \x{
1081: escaping mechanism.
1.1 misho 1082: </P>
1083: <P>
1084: When caseless matching is set, any letters in a class represent both their
1085: upper case and lower case versions, so for example, a caseless [aeiou] matches
1086: "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1.1.1.2 misho 1087: caseful version would. In a UTF mode, PCRE always understands the concept of
1.1 misho 1088: case for characters whose values are less than 128, so caseless matching is
1089: always possible. For characters with higher values, the concept of case is
1090: supported if PCRE is compiled with Unicode property support, but not otherwise.
1.1.1.2 misho 1091: If you want to use caseless matching in a UTF mode for characters 128 and
1092: above, you must ensure that PCRE is compiled with Unicode property support as
1093: well as with UTF support.
1.1 misho 1094: </P>
1095: <P>
1096: Characters that might indicate line breaks are never treated in any special way
1097: when matching character classes, whatever line-ending sequence is in use, and
1098: whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1099: such as [^a] always matches one of these characters.
1100: </P>
1101: <P>
1102: The minus (hyphen) character can be used to specify a range of characters in a
1103: character class. For example, [d-m] matches any letter between d and m,
1104: inclusive. If a minus character is required in a class, it must be escaped with
1105: a backslash or appear in a position where it cannot be interpreted as
1106: indicating a range, typically as the first or last character in the class.
1107: </P>
1108: <P>
1109: It is not possible to have the literal character "]" as the end character of a
1110: range. A pattern such as [W-]46] is interpreted as a class of two characters
1111: ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1112: "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1113: the end of range, so [W-\]46] is interpreted as a class containing a range
1114: followed by two other characters. The octal or hexadecimal representation of
1115: "]" can also be used to end a range.
1116: </P>
1117: <P>
1118: Ranges operate in the collating sequence of character values. They can also be
1.1.1.2 misho 1119: used for characters specified numerically, for example [\000-\037]. Ranges
1120: can include any characters that are valid for the current mode.
1.1 misho 1121: </P>
1122: <P>
1123: If a range that includes letters is used when caseless matching is set, it
1124: matches the letters in either case. For example, [W-c] is equivalent to
1.1.1.2 misho 1125: [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1.1 misho 1126: tables for a French locale are in use, [\xc8-\xcb] matches accented E
1.1.1.2 misho 1127: characters in both cases. In UTF modes, PCRE supports the concept of case for
1.1 misho 1128: characters with values greater than 128 only when it is compiled with Unicode
1129: property support.
1130: </P>
1131: <P>
1132: The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1133: \V, \w, and \W may appear in a character class, and add the characters that
1134: they match to the class. For example, [\dABCDEF] matches any hexadecimal
1.1.1.2 misho 1135: digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1.1 misho 1136: and their upper case partners, just as it does when they appear outside a
1137: character class, as described in the section entitled
1138: <a href="#genericchartypes">"Generic character types"</a>
1139: above. The escape sequence \b has a different meaning inside a character
1140: class; it matches the backspace character. The sequences \B, \N, \R, and \X
1141: are not special inside a character class. Like any other unrecognized escape
1142: sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1143: default, but cause an error if the PCRE_EXTRA option is set.
1144: </P>
1145: <P>
1146: A circumflex can conveniently be used with the upper case character types to
1147: specify a more restricted set of characters than the matching lower case type.
1148: For example, the class [^\W_] matches any letter or digit, but not underscore,
1149: whereas [\w] includes underscore. A positive character class should be read as
1150: "something OR something OR ..." and a negative class as "NOT something AND NOT
1151: something AND NOT ...".
1152: </P>
1153: <P>
1154: The only metacharacters that are recognized in character classes are backslash,
1155: hyphen (only where it can be interpreted as specifying a range), circumflex
1156: (only at the start), opening square bracket (only when it can be interpreted as
1157: introducing a POSIX class name - see the next section), and the terminating
1158: closing square bracket. However, escaping other non-alphanumeric characters
1159: does no harm.
1160: </P>
1161: <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1162: <P>
1163: Perl supports the POSIX notation for character classes. This uses names
1164: enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1165: this notation. For example,
1166: <pre>
1167: [01[:alpha:]%]
1168: </pre>
1169: matches "0", "1", any alphabetic character, or "%". The supported class names
1170: are:
1171: <pre>
1172: alnum letters and digits
1173: alpha letters
1174: ascii character codes 0 - 127
1175: blank space or tab only
1176: cntrl control characters
1177: digit decimal digits (same as \d)
1178: graph printing characters, excluding space
1179: lower lower case letters
1180: print printing characters, including space
1181: punct printing characters, excluding letters and digits and space
1182: space white space (not quite the same as \s)
1183: upper upper case letters
1184: word "word" characters (same as \w)
1185: xdigit hexadecimal digits
1186: </pre>
1187: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1188: space (32). Notice that this list includes the VT character (code 11). This
1189: makes "space" different to \s, which does not include VT (for Perl
1190: compatibility).
1191: </P>
1192: <P>
1193: The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1194: 5.8. Another Perl extension is negation, which is indicated by a ^ character
1195: after the colon. For example,
1196: <pre>
1197: [12[:^digit:]]
1198: </pre>
1199: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1200: syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1201: supported, and an error is given if they are encountered.
1202: </P>
1203: <P>
1.1.1.2 misho 1204: By default, in UTF modes, characters with values greater than 128 do not match
1.1 misho 1205: any of the POSIX character classes. However, if the PCRE_UCP option is passed
1206: to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1207: character properties are used. This is achieved by replacing the POSIX classes
1208: by other sequences, as follows:
1209: <pre>
1210: [:alnum:] becomes \p{Xan}
1211: [:alpha:] becomes \p{L}
1212: [:blank:] becomes \h
1213: [:digit:] becomes \p{Nd}
1214: [:lower:] becomes \p{Ll}
1215: [:space:] becomes \p{Xps}
1216: [:upper:] becomes \p{Lu}
1217: [:word:] becomes \p{Xwd}
1218: </pre>
1219: Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1220: classes are unchanged, and match only characters with code points less than
1221: 128.
1222: </P>
1223: <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1224: <P>
1225: Vertical bar characters are used to separate alternative patterns. For example,
1226: the pattern
1227: <pre>
1228: gilbert|sullivan
1229: </pre>
1230: matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1231: and an empty alternative is permitted (matching the empty string). The matching
1232: process tries each alternative in turn, from left to right, and the first one
1233: that succeeds is used. If the alternatives are within a subpattern
1234: <a href="#subpattern">(defined below),</a>
1235: "succeeds" means matching the rest of the main pattern as well as the
1236: alternative in the subpattern.
1237: </P>
1238: <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1239: <P>
1240: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1241: PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1242: the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1243: The option letters are
1244: <pre>
1245: i for PCRE_CASELESS
1246: m for PCRE_MULTILINE
1247: s for PCRE_DOTALL
1248: x for PCRE_EXTENDED
1249: </pre>
1250: For example, (?im) sets caseless, multiline matching. It is also possible to
1251: unset these options by preceding the letter with a hyphen, and a combined
1252: setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1253: PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1254: permitted. If a letter appears both before and after the hyphen, the option is
1255: unset.
1256: </P>
1257: <P>
1258: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1259: changed in the same way as the Perl-compatible options by using the characters
1260: J, U and X respectively.
1261: </P>
1262: <P>
1263: When one of these option changes occurs at top level (that is, not inside
1264: subpattern parentheses), the change applies to the remainder of the pattern
1265: that follows. If the change is placed right at the start of a pattern, PCRE
1266: extracts it into the global options (and it will therefore show up in data
1267: extracted by the <b>pcre_fullinfo()</b> function).
1268: </P>
1269: <P>
1270: An option change within a subpattern (see below for a description of
1271: subpatterns) affects only that part of the subpattern that follows it, so
1272: <pre>
1273: (a(?i)b)c
1274: </pre>
1275: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1276: By this means, options can be made to have different settings in different
1277: parts of the pattern. Any changes made in one alternative do carry on
1278: into subsequent branches within the same subpattern. For example,
1279: <pre>
1280: (a(?i)b|c)
1281: </pre>
1282: matches "ab", "aB", "c", and "C", even though when matching "C" the first
1283: branch is abandoned before the option setting. This is because the effects of
1284: option settings happen at compile time. There would be some very weird
1285: behaviour otherwise.
1286: </P>
1287: <P>
1288: <b>Note:</b> There are other PCRE-specific options that can be set by the
1.1.1.2 misho 1289: application when the compiling or matching functions are called. In some cases
1290: the pattern can contain special leading sequences such as (*CRLF) to override
1291: what the application has set or what has been defaulted. Details are given in
1292: the section entitled
1.1 misho 1293: <a href="#newlineseq">"Newline sequences"</a>
1.1.1.2 misho 1294: above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1295: can be used to set UTF and Unicode property modes; they are equivalent to
1296: setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1.1 misho 1297: <a name="subpattern"></a></P>
1298: <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1299: <P>
1300: Subpatterns are delimited by parentheses (round brackets), which can be nested.
1301: Turning part of a pattern into a subpattern does two things:
1302: <br>
1303: <br>
1304: 1. It localizes a set of alternatives. For example, the pattern
1305: <pre>
1306: cat(aract|erpillar|)
1307: </pre>
1308: matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1309: match "cataract", "erpillar" or an empty string.
1310: <br>
1311: <br>
1312: 2. It sets up the subpattern as a capturing subpattern. This means that, when
1313: the whole pattern matches, that portion of the subject string that matched the
1.1.1.2 misho 1314: subpattern is passed back to the caller via the <i>ovector</i> argument of the
1315: matching function. (This applies only to the traditional matching functions;
1316: the DFA matching functions do not support capturing.)
1317: </P>
1318: <P>
1319: Opening parentheses are counted from left to right (starting from 1) to obtain
1320: numbers for the capturing subpatterns. For example, if the string "the red
1321: king" is matched against the pattern
1.1 misho 1322: <pre>
1323: the ((red|white) (king|queen))
1324: </pre>
1325: the captured substrings are "red king", "red", and "king", and are numbered 1,
1326: 2, and 3, respectively.
1327: </P>
1328: <P>
1329: The fact that plain parentheses fulfil two functions is not always helpful.
1330: There are often times when a grouping subpattern is required without a
1331: capturing requirement. If an opening parenthesis is followed by a question mark
1332: and a colon, the subpattern does not do any capturing, and is not counted when
1333: computing the number of any subsequent capturing subpatterns. For example, if
1334: the string "the white queen" is matched against the pattern
1335: <pre>
1336: the ((?:red|white) (king|queen))
1337: </pre>
1338: the captured substrings are "white queen" and "queen", and are numbered 1 and
1339: 2. The maximum number of capturing subpatterns is 65535.
1340: </P>
1341: <P>
1342: As a convenient shorthand, if any option settings are required at the start of
1343: a non-capturing subpattern, the option letters may appear between the "?" and
1344: the ":". Thus the two patterns
1345: <pre>
1346: (?i:saturday|sunday)
1347: (?:(?i)saturday|sunday)
1348: </pre>
1349: match exactly the same set of strings. Because alternative branches are tried
1350: from left to right, and options are not reset until the end of the subpattern
1351: is reached, an option setting in one branch does affect subsequent branches, so
1352: the above patterns match "SUNDAY" as well as "Saturday".
1353: <a name="dupsubpatternnumber"></a></P>
1354: <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1355: <P>
1356: Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1357: the same numbers for its capturing parentheses. Such a subpattern starts with
1358: (?| and is itself a non-capturing subpattern. For example, consider this
1359: pattern:
1360: <pre>
1361: (?|(Sat)ur|(Sun))day
1362: </pre>
1363: Because the two alternatives are inside a (?| group, both sets of capturing
1364: parentheses are numbered one. Thus, when the pattern matches, you can look
1365: at captured substring number one, whichever alternative matched. This construct
1366: is useful when you want to capture part, but not all, of one of a number of
1367: alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1368: number is reset at the start of each branch. The numbers of any capturing
1369: parentheses that follow the subpattern start after the highest number used in
1370: any branch. The following example is taken from the Perl documentation. The
1371: numbers underneath show in which buffer the captured content will be stored.
1372: <pre>
1373: # before ---------------branch-reset----------- after
1374: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1375: # 1 2 2 3 2 3 4
1376: </pre>
1377: A back reference to a numbered subpattern uses the most recent value that is
1378: set for that number by any subpattern. The following pattern matches "abcabc"
1379: or "defdef":
1380: <pre>
1381: /(?|(abc)|(def))\1/
1382: </pre>
1383: In contrast, a subroutine call to a numbered subpattern always refers to the
1384: first one in the pattern with the given number. The following pattern matches
1385: "abcabc" or "defabc":
1386: <pre>
1387: /(?|(abc)|(def))(?1)/
1388: </pre>
1389: If a
1390: <a href="#conditions">condition test</a>
1391: for a subpattern's having matched refers to a non-unique number, the test is
1392: true if any of the subpatterns of that number have matched.
1393: </P>
1394: <P>
1395: An alternative approach to using this "branch reset" feature is to use
1396: duplicate named subpatterns, as described in the next section.
1397: </P>
1398: <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1399: <P>
1400: Identifying capturing parentheses by number is simple, but it can be very hard
1401: to keep track of the numbers in complicated regular expressions. Furthermore,
1402: if an expression is modified, the numbers may change. To help with this
1403: difficulty, PCRE supports the naming of subpatterns. This feature was not
1404: added to Perl until release 5.10. Python had the feature earlier, and PCRE
1405: introduced it at release 4.0, using the Python syntax. PCRE now supports both
1406: the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1407: have different names, but PCRE does not.
1408: </P>
1409: <P>
1410: In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1411: (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1412: parentheses from other parts of the pattern, such as
1413: <a href="#backreferences">back references,</a>
1414: <a href="#recursion">recursion,</a>
1415: and
1416: <a href="#conditions">conditions,</a>
1417: can be made by name as well as by number.
1418: </P>
1419: <P>
1420: Names consist of up to 32 alphanumeric characters and underscores. Named
1421: capturing parentheses are still allocated numbers as well as names, exactly as
1422: if the names were not present. The PCRE API provides function calls for
1423: extracting the name-to-number translation table from a compiled pattern. There
1424: is also a convenience function for extracting a captured substring by name.
1425: </P>
1426: <P>
1427: By default, a name must be unique within a pattern, but it is possible to relax
1428: this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1429: names are also always permitted for subpatterns with the same number, set up as
1430: described in the previous section.) Duplicate names can be useful for patterns
1431: where only one instance of the named parentheses can match. Suppose you want to
1432: match the name of a weekday, either as a 3-letter abbreviation or as the full
1433: name, and in both cases you want to extract the abbreviation. This pattern
1434: (ignoring the line breaks) does the job:
1435: <pre>
1436: (?<DN>Mon|Fri|Sun)(?:day)?|
1437: (?<DN>Tue)(?:sday)?|
1438: (?<DN>Wed)(?:nesday)?|
1439: (?<DN>Thu)(?:rsday)?|
1440: (?<DN>Sat)(?:urday)?
1441: </pre>
1442: There are five capturing substrings, but only one is ever set after a match.
1443: (An alternative way of solving this problem is to use a "branch reset"
1444: subpattern, as described in the previous section.)
1445: </P>
1446: <P>
1447: The convenience function for extracting the data by name returns the substring
1448: for the first (and in this example, the only) subpattern of that name that
1449: matched. This saves searching to find which numbered subpattern it was.
1450: </P>
1451: <P>
1452: If you make a back reference to a non-unique named subpattern from elsewhere in
1453: the pattern, the one that corresponds to the first occurrence of the name is
1454: used. In the absence of duplicate numbers (see the previous section) this is
1455: the one with the lowest number. If you use a named reference in a condition
1456: test (see the
1457: <a href="#conditions">section about conditions</a>
1458: below), either to check whether a subpattern has matched, or to check for
1459: recursion, all subpatterns with the same name are tested. If the condition is
1460: true for any one of them, the overall condition is true. This is the same
1461: behaviour as testing by number. For further details of the interfaces for
1462: handling named subpatterns, see the
1463: <a href="pcreapi.html"><b>pcreapi</b></a>
1464: documentation.
1465: </P>
1466: <P>
1467: <b>Warning:</b> You cannot use different names to distinguish between two
1468: subpatterns with the same number because PCRE uses only the numbers when
1469: matching. For this reason, an error is given at compile time if different names
1470: are given to subpatterns with the same number. However, you can give the same
1471: name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1472: </P>
1473: <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1474: <P>
1475: Repetition is specified by quantifiers, which can follow any of the following
1476: items:
1477: <pre>
1478: a literal data character
1479: the dot metacharacter
1480: the \C escape sequence
1.1.1.2 misho 1481: the \X escape sequence
1.1 misho 1482: the \R escape sequence
1483: an escape such as \d or \pL that matches a single character
1484: a character class
1485: a back reference (see next section)
1486: a parenthesized subpattern (including assertions)
1487: a subroutine call to a subpattern (recursive or otherwise)
1488: </pre>
1489: The general repetition quantifier specifies a minimum and maximum number of
1490: permitted matches, by giving the two numbers in curly brackets (braces),
1491: separated by a comma. The numbers must be less than 65536, and the first must
1492: be less than or equal to the second. For example:
1493: <pre>
1494: z{2,4}
1495: </pre>
1496: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1497: character. If the second number is omitted, but the comma is present, there is
1498: no upper limit; if the second number and the comma are both omitted, the
1499: quantifier specifies an exact number of required matches. Thus
1500: <pre>
1501: [aeiou]{3,}
1502: </pre>
1503: matches at least 3 successive vowels, but may match many more, while
1504: <pre>
1505: \d{8}
1506: </pre>
1507: matches exactly 8 digits. An opening curly bracket that appears in a position
1508: where a quantifier is not allowed, or one that does not match the syntax of a
1509: quantifier, is taken as a literal character. For example, {,6} is not a
1510: quantifier, but a literal string of four characters.
1511: </P>
1512: <P>
1.1.1.2 misho 1513: In UTF modes, quantifiers apply to characters rather than to individual data
1514: units. Thus, for example, \x{100}{2} matches two characters, each of
1515: which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1516: \X{3} matches three Unicode extended sequences, each of which may be several
1517: data units long (and they may be of different lengths).
1.1 misho 1518: </P>
1519: <P>
1520: The quantifier {0} is permitted, causing the expression to behave as if the
1521: previous item and the quantifier were not present. This may be useful for
1522: subpatterns that are referenced as
1523: <a href="#subpatternsassubroutines">subroutines</a>
1524: from elsewhere in the pattern (but see also the section entitled
1525: <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1526: below). Items other than subpatterns that have a {0} quantifier are omitted
1527: from the compiled pattern.
1528: </P>
1529: <P>
1530: For convenience, the three most common quantifiers have single-character
1531: abbreviations:
1532: <pre>
1533: * is equivalent to {0,}
1534: + is equivalent to {1,}
1535: ? is equivalent to {0,1}
1536: </pre>
1537: It is possible to construct infinite loops by following a subpattern that can
1538: match no characters with a quantifier that has no upper limit, for example:
1539: <pre>
1540: (a?)*
1541: </pre>
1542: Earlier versions of Perl and PCRE used to give an error at compile time for
1543: such patterns. However, because there are cases where this can be useful, such
1544: patterns are now accepted, but if any repetition of the subpattern does in fact
1545: match no characters, the loop is forcibly broken.
1546: </P>
1547: <P>
1548: By default, the quantifiers are "greedy", that is, they match as much as
1549: possible (up to the maximum number of permitted times), without causing the
1550: rest of the pattern to fail. The classic example of where this gives problems
1551: is in trying to match comments in C programs. These appear between /* and */
1552: and within the comment, individual * and / characters may appear. An attempt to
1553: match C comments by applying the pattern
1554: <pre>
1555: /\*.*\*/
1556: </pre>
1557: to the string
1558: <pre>
1559: /* first comment */ not comment /* second comment */
1560: </pre>
1561: fails, because it matches the entire string owing to the greediness of the .*
1562: item.
1563: </P>
1564: <P>
1565: However, if a quantifier is followed by a question mark, it ceases to be
1566: greedy, and instead matches the minimum number of times possible, so the
1567: pattern
1568: <pre>
1569: /\*.*?\*/
1570: </pre>
1571: does the right thing with the C comments. The meaning of the various
1572: quantifiers is not otherwise changed, just the preferred number of matches.
1573: Do not confuse this use of question mark with its use as a quantifier in its
1574: own right. Because it has two uses, it can sometimes appear doubled, as in
1575: <pre>
1576: \d??\d
1577: </pre>
1578: which matches one digit by preference, but can match two if that is the only
1579: way the rest of the pattern matches.
1580: </P>
1581: <P>
1582: If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1583: the quantifiers are not greedy by default, but individual ones can be made
1584: greedy by following them with a question mark. In other words, it inverts the
1585: default behaviour.
1586: </P>
1587: <P>
1588: When a parenthesized subpattern is quantified with a minimum repeat count that
1589: is greater than 1 or with a limited maximum, more memory is required for the
1590: compiled pattern, in proportion to the size of the minimum or maximum.
1591: </P>
1592: <P>
1593: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1594: to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1595: implicitly anchored, because whatever follows will be tried against every
1596: character position in the subject string, so there is no point in retrying the
1597: overall match at any position after the first. PCRE normally treats such a
1598: pattern as though it were preceded by \A.
1599: </P>
1600: <P>
1601: In cases where it is known that the subject string contains no newlines, it is
1602: worth setting PCRE_DOTALL in order to obtain this optimization, or
1603: alternatively using ^ to indicate anchoring explicitly.
1604: </P>
1605: <P>
1606: However, there is one situation where the optimization cannot be used. When .*
1607: is inside capturing parentheses that are the subject of a back reference
1608: elsewhere in the pattern, a match at the start may fail where a later one
1609: succeeds. Consider, for example:
1610: <pre>
1611: (.*)abc\1
1612: </pre>
1613: If the subject is "xyz123abc123" the match point is the fourth character. For
1614: this reason, such a pattern is not implicitly anchored.
1615: </P>
1616: <P>
1617: When a capturing subpattern is repeated, the value captured is the substring
1618: that matched the final iteration. For example, after
1619: <pre>
1620: (tweedle[dume]{3}\s*)+
1621: </pre>
1622: has matched "tweedledum tweedledee" the value of the captured substring is
1623: "tweedledee". However, if there are nested capturing subpatterns, the
1624: corresponding captured values may have been set in previous iterations. For
1625: example, after
1626: <pre>
1627: /(a|(b))+/
1628: </pre>
1629: matches "aba" the value of the second captured substring is "b".
1630: <a name="atomicgroup"></a></P>
1631: <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1632: <P>
1633: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1634: repetition, failure of what follows normally causes the repeated item to be
1635: re-evaluated to see if a different number of repeats allows the rest of the
1636: pattern to match. Sometimes it is useful to prevent this, either to change the
1637: nature of the match, or to cause it fail earlier than it otherwise might, when
1638: the author of the pattern knows there is no point in carrying on.
1639: </P>
1640: <P>
1641: Consider, for example, the pattern \d+foo when applied to the subject line
1642: <pre>
1643: 123456bar
1644: </pre>
1645: After matching all 6 digits and then failing to match "foo", the normal
1646: action of the matcher is to try again with only 5 digits matching the \d+
1647: item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1648: (a term taken from Jeffrey Friedl's book) provides the means for specifying
1649: that once a subpattern has matched, it is not to be re-evaluated in this way.
1650: </P>
1651: <P>
1652: If we use atomic grouping for the previous example, the matcher gives up
1653: immediately on failing to match "foo" the first time. The notation is a kind of
1654: special parenthesis, starting with (?> as in this example:
1655: <pre>
1656: (?>\d+)foo
1657: </pre>
1658: This kind of parenthesis "locks up" the part of the pattern it contains once
1659: it has matched, and a failure further into the pattern is prevented from
1660: backtracking into it. Backtracking past it to previous items, however, works as
1661: normal.
1662: </P>
1663: <P>
1664: An alternative description is that a subpattern of this type matches the string
1665: of characters that an identical standalone pattern would match, if anchored at
1666: the current point in the subject string.
1667: </P>
1668: <P>
1669: Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1670: the above example can be thought of as a maximizing repeat that must swallow
1671: everything it can. So, while both \d+ and \d+? are prepared to adjust the
1672: number of digits they match in order to make the rest of the pattern match,
1673: (?>\d+) can only match an entire sequence of digits.
1674: </P>
1675: <P>
1676: Atomic groups in general can of course contain arbitrarily complicated
1677: subpatterns, and can be nested. However, when the subpattern for an atomic
1678: group is just a single repeated item, as in the example above, a simpler
1679: notation, called a "possessive quantifier" can be used. This consists of an
1680: additional + character following a quantifier. Using this notation, the
1681: previous example can be rewritten as
1682: <pre>
1683: \d++foo
1684: </pre>
1685: Note that a possessive quantifier can be used with an entire group, for
1686: example:
1687: <pre>
1688: (abc|xyz){2,3}+
1689: </pre>
1690: Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1691: option is ignored. They are a convenient notation for the simpler forms of
1692: atomic group. However, there is no difference in the meaning of a possessive
1693: quantifier and the equivalent atomic group, though there may be a performance
1694: difference; possessive quantifiers should be slightly faster.
1695: </P>
1696: <P>
1697: The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1698: Jeffrey Friedl originated the idea (and the name) in the first edition of his
1699: book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1700: package, and PCRE copied it from there. It ultimately found its way into Perl
1701: at release 5.10.
1702: </P>
1703: <P>
1704: PCRE has an optimization that automatically "possessifies" certain simple
1705: pattern constructs. For example, the sequence A+B is treated as A++B because
1706: there is no point in backtracking into a sequence of A's when B must follow.
1707: </P>
1708: <P>
1709: When a pattern contains an unlimited repeat inside a subpattern that can itself
1710: be repeated an unlimited number of times, the use of an atomic group is the
1711: only way to avoid some failing matches taking a very long time indeed. The
1712: pattern
1713: <pre>
1714: (\D+|<\d+>)*[!?]
1715: </pre>
1716: matches an unlimited number of substrings that either consist of non-digits, or
1717: digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1718: quickly. However, if it is applied to
1719: <pre>
1720: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1721: </pre>
1722: it takes a long time before reporting failure. This is because the string can
1723: be divided between the internal \D+ repeat and the external * repeat in a
1724: large number of ways, and all have to be tried. (The example uses [!?] rather
1725: than a single character at the end, because both PCRE and Perl have an
1726: optimization that allows for fast failure when a single character is used. They
1727: remember the last single character that is required for a match, and fail early
1728: if it is not present in the string.) If the pattern is changed so that it uses
1729: an atomic group, like this:
1730: <pre>
1731: ((?>\D+)|<\d+>)*[!?]
1732: </pre>
1733: sequences of non-digits cannot be broken, and failure happens quickly.
1734: <a name="backreferences"></a></P>
1735: <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1736: <P>
1737: Outside a character class, a backslash followed by a digit greater than 0 (and
1738: possibly further digits) is a back reference to a capturing subpattern earlier
1739: (that is, to its left) in the pattern, provided there have been that many
1740: previous capturing left parentheses.
1741: </P>
1742: <P>
1743: However, if the decimal number following the backslash is less than 10, it is
1744: always taken as a back reference, and causes an error only if there are not
1745: that many capturing left parentheses in the entire pattern. In other words, the
1746: parentheses that are referenced need not be to the left of the reference for
1747: numbers less than 10. A "forward back reference" of this type can make sense
1748: when a repetition is involved and the subpattern to the right has participated
1749: in an earlier iteration.
1750: </P>
1751: <P>
1752: It is not possible to have a numerical "forward back reference" to a subpattern
1753: whose number is 10 or more using this syntax because a sequence such as \50 is
1754: interpreted as a character defined in octal. See the subsection entitled
1755: "Non-printing characters"
1756: <a href="#digitsafterbackslash">above</a>
1757: for further details of the handling of digits following a backslash. There is
1758: no such problem when named parentheses are used. A back reference to any
1759: subpattern is possible using named parentheses (see below).
1760: </P>
1761: <P>
1762: Another way of avoiding the ambiguity inherent in the use of digits following a
1763: backslash is to use the \g escape sequence. This escape must be followed by an
1764: unsigned number or a negative number, optionally enclosed in braces. These
1765: examples are all identical:
1766: <pre>
1767: (ring), \1
1768: (ring), \g1
1769: (ring), \g{1}
1770: </pre>
1771: An unsigned number specifies an absolute reference without the ambiguity that
1772: is present in the older syntax. It is also useful when literal digits follow
1773: the reference. A negative number is a relative reference. Consider this
1774: example:
1775: <pre>
1776: (abc(def)ghi)\g{-1}
1777: </pre>
1778: The sequence \g{-1} is a reference to the most recently started capturing
1779: subpattern before \g, that is, is it equivalent to \2 in this example.
1780: Similarly, \g{-2} would be equivalent to \1. The use of relative references
1781: can be helpful in long patterns, and also in patterns that are created by
1782: joining together fragments that contain references within themselves.
1783: </P>
1784: <P>
1785: A back reference matches whatever actually matched the capturing subpattern in
1786: the current subject string, rather than anything matching the subpattern
1787: itself (see
1788: <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1789: below for a way of doing that). So the pattern
1790: <pre>
1791: (sens|respons)e and \1ibility
1792: </pre>
1793: matches "sense and sensibility" and "response and responsibility", but not
1794: "sense and responsibility". If caseful matching is in force at the time of the
1795: back reference, the case of letters is relevant. For example,
1796: <pre>
1797: ((?i)rah)\s+\1
1798: </pre>
1799: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1800: capturing subpattern is matched caselessly.
1801: </P>
1802: <P>
1803: There are several different ways of writing back references to named
1804: subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
1805: \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1806: back reference syntax, in which \g can be used for both numeric and named
1807: references, is also supported. We could rewrite the above example in any of
1808: the following ways:
1809: <pre>
1810: (?<p1>(?i)rah)\s+\k<p1>
1811: (?'p1'(?i)rah)\s+\k{p1}
1812: (?P<p1>(?i)rah)\s+(?P=p1)
1813: (?<p1>(?i)rah)\s+\g{p1}
1814: </pre>
1815: A subpattern that is referenced by name may appear in the pattern before or
1816: after the reference.
1817: </P>
1818: <P>
1819: There may be more than one back reference to the same subpattern. If a
1820: subpattern has not actually been used in a particular match, any back
1821: references to it always fail by default. For example, the pattern
1822: <pre>
1823: (a|(bc))\2
1824: </pre>
1825: always fails if it starts to match "a" rather than "bc". However, if the
1826: PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1827: unset value matches an empty string.
1828: </P>
1829: <P>
1830: Because there may be many capturing parentheses in a pattern, all digits
1831: following a backslash are taken as part of a potential back reference number.
1832: If the pattern continues with a digit character, some delimiter must be used to
1833: terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1.1.1.3 ! misho 1834: white space. Otherwise, the \g{ syntax or an empty comment (see
1.1 misho 1835: <a href="#comments">"Comments"</a>
1836: below) can be used.
1837: </P>
1838: <br><b>
1839: Recursive back references
1840: </b><br>
1841: <P>
1842: A back reference that occurs inside the parentheses to which it refers fails
1843: when the subpattern is first used, so, for example, (a\1) never matches.
1844: However, such references can be useful inside repeated subpatterns. For
1845: example, the pattern
1846: <pre>
1847: (a|b\1)+
1848: </pre>
1849: matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1850: the subpattern, the back reference matches the character string corresponding
1851: to the previous iteration. In order for this to work, the pattern must be such
1852: that the first iteration does not need to match the back reference. This can be
1853: done using alternation, as in the example above, or by a quantifier with a
1854: minimum of zero.
1855: </P>
1856: <P>
1857: Back references of this type cause the group that they reference to be treated
1858: as an
1859: <a href="#atomicgroup">atomic group.</a>
1860: Once the whole group has been matched, a subsequent matching failure cannot
1861: cause backtracking into the middle of the group.
1862: <a name="bigassertions"></a></P>
1863: <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1864: <P>
1865: An assertion is a test on the characters following or preceding the current
1866: matching point that does not actually consume any characters. The simple
1867: assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1868: <a href="#smallassertions">above.</a>
1869: </P>
1870: <P>
1871: More complicated assertions are coded as subpatterns. There are two kinds:
1872: those that look ahead of the current position in the subject string, and those
1873: that look behind it. An assertion subpattern is matched in the normal way,
1874: except that it does not cause the current matching position to be changed.
1875: </P>
1876: <P>
1877: Assertion subpatterns are not capturing subpatterns. If such an assertion
1878: contains capturing subpatterns within it, these are counted for the purposes of
1879: numbering the capturing subpatterns in the whole pattern. However, substring
1880: capturing is carried out only for positive assertions, because it does not make
1881: sense for negative assertions.
1882: </P>
1883: <P>
1884: For compatibility with Perl, assertion subpatterns may be repeated; though
1885: it makes no sense to assert the same thing several times, the side effect of
1886: capturing parentheses may occasionally be useful. In practice, there only three
1887: cases:
1888: <br>
1889: <br>
1890: (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1891: However, it may contain internal capturing parenthesized groups that are called
1892: from elsewhere via the
1893: <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1894: <br>
1895: <br>
1896: (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1897: were {0,1}. At run time, the rest of the pattern match is tried with and
1898: without the assertion, the order depending on the greediness of the quantifier.
1899: <br>
1900: <br>
1901: (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1902: The assertion is obeyed just once when encountered during matching.
1903: </P>
1904: <br><b>
1905: Lookahead assertions
1906: </b><br>
1907: <P>
1908: Lookahead assertions start with (?= for positive assertions and (?! for
1909: negative assertions. For example,
1910: <pre>
1911: \w+(?=;)
1912: </pre>
1913: matches a word followed by a semicolon, but does not include the semicolon in
1914: the match, and
1915: <pre>
1916: foo(?!bar)
1917: </pre>
1918: matches any occurrence of "foo" that is not followed by "bar". Note that the
1919: apparently similar pattern
1920: <pre>
1921: (?!foo)bar
1922: </pre>
1923: does not find an occurrence of "bar" that is preceded by something other than
1924: "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1925: (?!foo) is always true when the next three characters are "bar". A
1926: lookbehind assertion is needed to achieve the other effect.
1927: </P>
1928: <P>
1929: If you want to force a matching failure at some point in a pattern, the most
1930: convenient way to do it is with (?!) because an empty string always matches, so
1931: an assertion that requires there not to be an empty string must always fail.
1932: The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1933: <a name="lookbehind"></a></P>
1934: <br><b>
1935: Lookbehind assertions
1936: </b><br>
1937: <P>
1938: Lookbehind assertions start with (?<= for positive assertions and (?<! for
1939: negative assertions. For example,
1940: <pre>
1941: (?<!foo)bar
1942: </pre>
1943: does find an occurrence of "bar" that is not preceded by "foo". The contents of
1944: a lookbehind assertion are restricted such that all the strings it matches must
1945: have a fixed length. However, if there are several top-level alternatives, they
1946: do not all have to have the same fixed length. Thus
1947: <pre>
1948: (?<=bullock|donkey)
1949: </pre>
1950: is permitted, but
1951: <pre>
1952: (?<!dogs?|cats?)
1953: </pre>
1954: causes an error at compile time. Branches that match different length strings
1955: are permitted only at the top level of a lookbehind assertion. This is an
1956: extension compared with Perl, which requires all branches to match the same
1957: length of string. An assertion such as
1958: <pre>
1959: (?<=ab(c|de))
1960: </pre>
1961: is not permitted, because its single top-level branch can match two different
1962: lengths, but it is acceptable to PCRE if rewritten to use two top-level
1963: branches:
1964: <pre>
1965: (?<=abc|abde)
1966: </pre>
1967: In some cases, the escape sequence \K
1968: <a href="#resetmatchstart">(see above)</a>
1969: can be used instead of a lookbehind assertion to get round the fixed-length
1970: restriction.
1971: </P>
1972: <P>
1973: The implementation of lookbehind assertions is, for each alternative, to
1974: temporarily move the current position back by the fixed length and then try to
1975: match. If there are insufficient characters before the current position, the
1976: assertion fails.
1977: </P>
1978: <P>
1.1.1.2 misho 1979: In a UTF mode, PCRE does not allow the \C escape (which matches a single data
1980: unit even in a UTF mode) to appear in lookbehind assertions, because it makes
1981: it impossible to calculate the length of the lookbehind. The \X and \R
1982: escapes, which can match different numbers of data units, are also not
1983: permitted.
1.1 misho 1984: </P>
1985: <P>
1986: <a href="#subpatternsassubroutines">"Subroutine"</a>
1987: calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1988: as the subpattern matches a fixed-length string.
1989: <a href="#recursion">Recursion,</a>
1990: however, is not supported.
1991: </P>
1992: <P>
1993: Possessive quantifiers can be used in conjunction with lookbehind assertions to
1994: specify efficient matching of fixed-length strings at the end of subject
1995: strings. Consider a simple pattern such as
1996: <pre>
1997: abcd$
1998: </pre>
1999: when applied to a long string that does not match. Because matching proceeds
2000: from left to right, PCRE will look for each "a" in the subject and then see if
2001: what follows matches the rest of the pattern. If the pattern is specified as
2002: <pre>
2003: ^.*abcd$
2004: </pre>
2005: the initial .* matches the entire string at first, but when this fails (because
2006: there is no following "a"), it backtracks to match all but the last character,
2007: then all but the last two characters, and so on. Once again the search for "a"
2008: covers the entire string, from right to left, so we are no better off. However,
2009: if the pattern is written as
2010: <pre>
2011: ^.*+(?<=abcd)
2012: </pre>
2013: there can be no backtracking for the .*+ item; it can match only the entire
2014: string. The subsequent lookbehind assertion does a single test on the last four
2015: characters. If it fails, the match fails immediately. For long strings, this
2016: approach makes a significant difference to the processing time.
2017: </P>
2018: <br><b>
2019: Using multiple assertions
2020: </b><br>
2021: <P>
2022: Several assertions (of any sort) may occur in succession. For example,
2023: <pre>
2024: (?<=\d{3})(?<!999)foo
2025: </pre>
2026: matches "foo" preceded by three digits that are not "999". Notice that each of
2027: the assertions is applied independently at the same point in the subject
2028: string. First there is a check that the previous three characters are all
2029: digits, and then there is a check that the same three characters are not "999".
2030: This pattern does <i>not</i> match "foo" preceded by six characters, the first
2031: of which are digits and the last three of which are not "999". For example, it
2032: doesn't match "123abcfoo". A pattern to do that is
2033: <pre>
2034: (?<=\d{3}...)(?<!999)foo
2035: </pre>
2036: This time the first assertion looks at the preceding six characters, checking
2037: that the first three are digits, and then the second assertion checks that the
2038: preceding three characters are not "999".
2039: </P>
2040: <P>
2041: Assertions can be nested in any combination. For example,
2042: <pre>
2043: (?<=(?<!foo)bar)baz
2044: </pre>
2045: matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2046: preceded by "foo", while
2047: <pre>
2048: (?<=\d{3}(?!999)...)foo
2049: </pre>
2050: is another pattern that matches "foo" preceded by three digits and any three
2051: characters that are not "999".
2052: <a name="conditions"></a></P>
2053: <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2054: <P>
2055: It is possible to cause the matching process to obey a subpattern
2056: conditionally or to choose between two alternative subpatterns, depending on
2057: the result of an assertion, or whether a specific capturing subpattern has
2058: already been matched. The two possible forms of conditional subpattern are:
2059: <pre>
2060: (?(condition)yes-pattern)
2061: (?(condition)yes-pattern|no-pattern)
2062: </pre>
2063: If the condition is satisfied, the yes-pattern is used; otherwise the
2064: no-pattern (if present) is used. If there are more than two alternatives in the
2065: subpattern, a compile-time error occurs. Each of the two alternatives may
2066: itself contain nested subpatterns of any form, including conditional
2067: subpatterns; the restriction to two alternatives applies only at the level of
2068: the condition. This pattern fragment is an example where the alternatives are
2069: complex:
2070: <pre>
2071: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2072:
2073: </PRE>
2074: </P>
2075: <P>
2076: There are four kinds of condition: references to subpatterns, references to
2077: recursion, a pseudo-condition called DEFINE, and assertions.
2078: </P>
2079: <br><b>
2080: Checking for a used subpattern by number
2081: </b><br>
2082: <P>
2083: If the text between the parentheses consists of a sequence of digits, the
2084: condition is true if a capturing subpattern of that number has previously
2085: matched. If there is more than one capturing subpattern with the same number
2086: (see the earlier
2087: <a href="#recursion">section about duplicate subpattern numbers),</a>
2088: the condition is true if any of them have matched. An alternative notation is
2089: to precede the digits with a plus or minus sign. In this case, the subpattern
2090: number is relative rather than absolute. The most recently opened parentheses
2091: can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2092: loops it can also make sense to refer to subsequent groups. The next
2093: parentheses to be opened can be referenced as (?(+1), and so on. (The value
2094: zero in any of these forms is not used; it provokes a compile-time error.)
2095: </P>
2096: <P>
2097: Consider the following pattern, which contains non-significant white space to
2098: make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2099: three parts for ease of discussion:
2100: <pre>
2101: ( \( )? [^()]+ (?(1) \) )
2102: </pre>
2103: The first part matches an optional opening parenthesis, and if that
2104: character is present, sets it as the first captured substring. The second part
2105: matches one or more characters that are not parentheses. The third part is a
2106: conditional subpattern that tests whether or not the first set of parentheses
2107: matched. If they did, that is, if subject started with an opening parenthesis,
2108: the condition is true, and so the yes-pattern is executed and a closing
2109: parenthesis is required. Otherwise, since no-pattern is not present, the
2110: subpattern matches nothing. In other words, this pattern matches a sequence of
2111: non-parentheses, optionally enclosed in parentheses.
2112: </P>
2113: <P>
2114: If you were embedding this pattern in a larger one, you could use a relative
2115: reference:
2116: <pre>
2117: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2118: </pre>
2119: This makes the fragment independent of the parentheses in the larger pattern.
2120: </P>
2121: <br><b>
2122: Checking for a used subpattern by name
2123: </b><br>
2124: <P>
2125: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2126: subpattern by name. For compatibility with earlier versions of PCRE, which had
2127: this facility before Perl, the syntax (?(name)...) is also recognized. However,
2128: there is a possible ambiguity with this syntax, because subpattern names may
2129: consist entirely of digits. PCRE looks first for a named subpattern; if it
2130: cannot find one and the name consists entirely of digits, PCRE looks for a
2131: subpattern of that number, which must be greater than zero. Using subpattern
2132: names that consist entirely of digits is not recommended.
2133: </P>
2134: <P>
2135: Rewriting the above example to use a named subpattern gives this:
2136: <pre>
2137: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
2138: </pre>
2139: If the name used in a condition of this kind is a duplicate, the test is
2140: applied to all subpatterns of the same name, and is true if any one of them has
2141: matched.
2142: </P>
2143: <br><b>
2144: Checking for pattern recursion
2145: </b><br>
2146: <P>
2147: If the condition is the string (R), and there is no subpattern with the name R,
2148: the condition is true if a recursive call to the whole pattern or any
2149: subpattern has been made. If digits or a name preceded by ampersand follow the
2150: letter R, for example:
2151: <pre>
2152: (?(R3)...) or (?(R&name)...)
2153: </pre>
2154: the condition is true if the most recent recursion is into a subpattern whose
2155: number or name is given. This condition does not check the entire recursion
2156: stack. If the name used in a condition of this kind is a duplicate, the test is
2157: applied to all subpatterns of the same name, and is true if any one of them is
2158: the most recent recursion.
2159: </P>
2160: <P>
2161: At "top level", all these recursion test conditions are false.
2162: <a href="#recursion">The syntax for recursive patterns</a>
2163: is described below.
2164: <a name="subdefine"></a></P>
2165: <br><b>
2166: Defining subpatterns for use by reference only
2167: </b><br>
2168: <P>
2169: If the condition is the string (DEFINE), and there is no subpattern with the
2170: name DEFINE, the condition is always false. In this case, there may be only one
2171: alternative in the subpattern. It is always skipped if control reaches this
2172: point in the pattern; the idea of DEFINE is that it can be used to define
2173: subroutines that can be referenced from elsewhere. (The use of
2174: <a href="#subpatternsassubroutines">subroutines</a>
2175: is described below.) For example, a pattern to match an IPv4 address such as
1.1.1.3 ! misho 2176: "192.168.23.245" could be written like this (ignore white space and line
1.1 misho 2177: breaks):
2178: <pre>
2179: (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2180: \b (?&byte) (\.(?&byte)){3} \b
2181: </pre>
2182: The first part of the pattern is a DEFINE group inside which a another group
2183: named "byte" is defined. This matches an individual component of an IPv4
2184: address (a number less than 256). When matching takes place, this part of the
2185: pattern is skipped because DEFINE acts like a false condition. The rest of the
2186: pattern uses references to the named group to match the four dot-separated
2187: components of an IPv4 address, insisting on a word boundary at each end.
2188: </P>
2189: <br><b>
2190: Assertion conditions
2191: </b><br>
2192: <P>
2193: If the condition is not in any of the above formats, it must be an assertion.
2194: This may be a positive or negative lookahead or lookbehind assertion. Consider
2195: this pattern, again containing non-significant white space, and with the two
2196: alternatives on the second line:
2197: <pre>
2198: (?(?=[^a-z]*[a-z])
2199: \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2200: </pre>
2201: The condition is a positive lookahead assertion that matches an optional
2202: sequence of non-letters followed by a letter. In other words, it tests for the
2203: presence of at least one letter in the subject. If a letter is found, the
2204: subject is matched against the first alternative; otherwise it is matched
2205: against the second. This pattern matches strings in one of the two forms
2206: dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2207: <a name="comments"></a></P>
2208: <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2209: <P>
2210: There are two ways of including comments in patterns that are processed by
2211: PCRE. In both cases, the start of the comment must not be in a character class,
2212: nor in the middle of any other sequence of related characters such as (?: or a
2213: subpattern name or number. The characters that make up a comment play no part
2214: in the pattern matching.
2215: </P>
2216: <P>
2217: The sequence (?# marks the start of a comment that continues up to the next
2218: closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2219: option is set, an unescaped # character also introduces a comment, which in
2220: this case continues to immediately after the next newline character or
2221: character sequence in the pattern. Which characters are interpreted as newlines
1.1.1.2 misho 2222: is controlled by the options passed to a compiling function or by a special
1.1 misho 2223: sequence at the start of the pattern, as described in the section entitled
2224: <a href="#newlines">"Newline conventions"</a>
2225: above. Note that the end of this type of comment is a literal newline sequence
2226: in the pattern; escape sequences that happen to represent a newline do not
2227: count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2228: default newline convention is in force:
2229: <pre>
2230: abc #comment \n still comment
2231: </pre>
2232: On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2233: a newline in the pattern. The sequence \n is still literal at this stage, so
2234: it does not terminate the comment. Only an actual character with the code value
2235: 0x0a (the default newline) does so.
2236: <a name="recursion"></a></P>
2237: <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2238: <P>
2239: Consider the problem of matching a string in parentheses, allowing for
2240: unlimited nested parentheses. Without the use of recursion, the best that can
2241: be done is to use a pattern that matches up to some fixed depth of nesting. It
2242: is not possible to handle an arbitrary nesting depth.
2243: </P>
2244: <P>
2245: For some time, Perl has provided a facility that allows regular expressions to
2246: recurse (amongst other things). It does this by interpolating Perl code in the
2247: expression at run time, and the code can refer to the expression itself. A Perl
2248: pattern using code interpolation to solve the parentheses problem can be
2249: created like this:
2250: <pre>
2251: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
2252: </pre>
2253: The (?p{...}) item interpolates Perl code at run time, and in this case refers
2254: recursively to the pattern in which it appears.
2255: </P>
2256: <P>
2257: Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2258: supports special syntax for recursion of the entire pattern, and also for
2259: individual subpattern recursion. After its introduction in PCRE and Python,
2260: this kind of recursion was subsequently introduced into Perl at release 5.10.
2261: </P>
2262: <P>
2263: A special item that consists of (? followed by a number greater than zero and a
2264: closing parenthesis is a recursive subroutine call of the subpattern of the
2265: given number, provided that it occurs inside that subpattern. (If not, it is a
2266: <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2267: call, which is described in the next section.) The special item (?R) or (?0) is
2268: a recursive call of the entire regular expression.
2269: </P>
2270: <P>
2271: This PCRE pattern solves the nested parentheses problem (assume the
2272: PCRE_EXTENDED option is set so that white space is ignored):
2273: <pre>
2274: \( ( [^()]++ | (?R) )* \)
2275: </pre>
2276: First it matches an opening parenthesis. Then it matches any number of
2277: substrings which can either be a sequence of non-parentheses, or a recursive
2278: match of the pattern itself (that is, a correctly parenthesized substring).
2279: Finally there is a closing parenthesis. Note the use of a possessive quantifier
2280: to avoid backtracking into sequences of non-parentheses.
2281: </P>
2282: <P>
2283: If this were part of a larger pattern, you would not want to recurse the entire
2284: pattern, so instead you could use this:
2285: <pre>
2286: ( \( ( [^()]++ | (?1) )* \) )
2287: </pre>
2288: We have put the pattern into parentheses, and caused the recursion to refer to
2289: them instead of the whole pattern.
2290: </P>
2291: <P>
2292: In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2293: is made easier by the use of relative references. Instead of (?1) in the
2294: pattern above you can write (?-2) to refer to the second most recently opened
2295: parentheses preceding the recursion. In other words, a negative number counts
2296: capturing parentheses leftwards from the point at which it is encountered.
2297: </P>
2298: <P>
2299: It is also possible to refer to subsequently opened parentheses, by writing
2300: references such as (?+2). However, these cannot be recursive because the
2301: reference is not inside the parentheses that are referenced. They are always
2302: <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2303: calls, as described in the next section.
2304: </P>
2305: <P>
2306: An alternative approach is to use named parentheses instead. The Perl syntax
2307: for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2308: could rewrite the above example as follows:
2309: <pre>
2310: (?<pn> \( ( [^()]++ | (?&pn) )* \) )
2311: </pre>
2312: If there is more than one subpattern with the same name, the earliest one is
2313: used.
2314: </P>
2315: <P>
2316: This particular example pattern that we have been looking at contains nested
2317: unlimited repeats, and so the use of a possessive quantifier for matching
2318: strings of non-parentheses is important when applying the pattern to strings
2319: that do not match. For example, when this pattern is applied to
2320: <pre>
2321: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2322: </pre>
2323: it yields "no match" quickly. However, if a possessive quantifier is not used,
2324: the match runs for a very long time indeed because there are so many different
2325: ways the + and * repeats can carve up the subject, and all have to be tested
2326: before failure can be reported.
2327: </P>
2328: <P>
2329: At the end of a match, the values of capturing parentheses are those from
2330: the outermost level. If you want to obtain intermediate values, a callout
2331: function can be used (see below and the
2332: <a href="pcrecallout.html"><b>pcrecallout</b></a>
2333: documentation). If the pattern above is matched against
2334: <pre>
2335: (ab(cd)ef)
2336: </pre>
2337: the value for the inner capturing parentheses (numbered 2) is "ef", which is
2338: the last value taken on at the top level. If a capturing subpattern is not
2339: matched at the top level, its final captured value is unset, even if it was
2340: (temporarily) set at a deeper level during the matching process.
2341: </P>
2342: <P>
2343: If there are more than 15 capturing parentheses in a pattern, PCRE has to
2344: obtain extra memory to store data during a recursion, which it does by using
2345: <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2346: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2347: </P>
2348: <P>
2349: Do not confuse the (?R) item with the condition (R), which tests for recursion.
2350: Consider this pattern, which matches text in angle brackets, allowing for
2351: arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2352: recursing), whereas any characters are permitted at the outer level.
2353: <pre>
2354: < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
2355: </pre>
2356: In this pattern, (?(R) is the start of a conditional subpattern, with two
2357: different alternatives for the recursive and non-recursive cases. The (?R) item
2358: is the actual recursive call.
2359: <a name="recursiondifference"></a></P>
2360: <br><b>
2361: Differences in recursion processing between PCRE and Perl
2362: </b><br>
2363: <P>
2364: Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2365: (like Python, but unlike Perl), a recursive subpattern call is always treated
2366: as an atomic group. That is, once it has matched some of the subject string, it
2367: is never re-entered, even if it contains untried alternatives and there is a
2368: subsequent matching failure. This can be illustrated by the following pattern,
2369: which purports to match a palindromic string that contains an odd number of
2370: characters (for example, "a", "aba", "abcba", "abcdcba"):
2371: <pre>
2372: ^(.|(.)(?1)\2)$
2373: </pre>
2374: The idea is that it either matches a single character, or two identical
2375: characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2376: it does not if the pattern is longer than three characters. Consider the
2377: subject string "abcba":
2378: </P>
2379: <P>
2380: At the top level, the first character is matched, but as it is not at the end
2381: of the string, the first alternative fails; the second alternative is taken
2382: and the recursion kicks in. The recursive call to subpattern 1 successfully
2383: matches the next character ("b"). (Note that the beginning and end of line
2384: tests are not part of the recursion).
2385: </P>
2386: <P>
2387: Back at the top level, the next character ("c") is compared with what
2388: subpattern 2 matched, which was "a". This fails. Because the recursion is
2389: treated as an atomic group, there are now no backtracking points, and so the
2390: entire match fails. (Perl is able, at this point, to re-enter the recursion and
2391: try the second alternative.) However, if the pattern is written with the
2392: alternatives in the other order, things are different:
2393: <pre>
2394: ^((.)(?1)\2|.)$
2395: </pre>
2396: This time, the recursing alternative is tried first, and continues to recurse
2397: until it runs out of characters, at which point the recursion fails. But this
2398: time we do have another alternative to try at the higher level. That is the big
2399: difference: in the previous case the remaining alternative is at a deeper
2400: recursion level, which PCRE cannot use.
2401: </P>
2402: <P>
2403: To change the pattern so that it matches all palindromic strings, not just
2404: those with an odd number of characters, it is tempting to change the pattern to
2405: this:
2406: <pre>
2407: ^((.)(?1)\2|.?)$
2408: </pre>
2409: Again, this works in Perl, but not in PCRE, and for the same reason. When a
2410: deeper recursion has matched a single character, it cannot be entered again in
2411: order to match an empty string. The solution is to separate the two cases, and
2412: write out the odd and even cases as alternatives at the higher level:
2413: <pre>
2414: ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2415: </pre>
2416: If you want to match typical palindromic phrases, the pattern has to ignore all
2417: non-word characters, which can be done like this:
2418: <pre>
2419: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2420: </pre>
2421: If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2422: man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2423: the use of the possessive quantifier *+ to avoid backtracking into sequences of
2424: non-word characters. Without this, PCRE takes a great deal longer (ten times or
2425: more) to match typical phrases, and Perl takes so long that you think it has
2426: gone into a loop.
2427: </P>
2428: <P>
2429: <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2430: string does not start with a palindrome that is shorter than the entire string.
2431: For example, although "abcba" is correctly matched, if the subject is "ababa",
2432: PCRE finds the palindrome "aba" at the start, then fails at top level because
2433: the end of the string does not follow. Once again, it cannot jump back into the
2434: recursion to try other alternatives, so the entire match fails.
2435: </P>
2436: <P>
2437: The second way in which PCRE and Perl differ in their recursion processing is
2438: in the handling of captured values. In Perl, when a subpattern is called
2439: recursively or as a subpattern (see the next section), it has no access to any
2440: values that were captured outside the recursion, whereas in PCRE these values
2441: can be referenced. Consider this pattern:
2442: <pre>
2443: ^(.)(\1|a(?2))
2444: </pre>
2445: In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2446: then in the second group, when the back reference \1 fails to match "b", the
2447: second alternative matches "a" and then recurses. In the recursion, \1 does
2448: now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2449: match because inside the recursive call \1 cannot access the externally set
2450: value.
2451: <a name="subpatternsassubroutines"></a></P>
2452: <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2453: <P>
2454: If the syntax for a recursive subpattern call (either by number or by
2455: name) is used outside the parentheses to which it refers, it operates like a
2456: subroutine in a programming language. The called subpattern may be defined
2457: before or after the reference. A numbered reference can be absolute or
2458: relative, as in these examples:
2459: <pre>
2460: (...(absolute)...)...(?2)...
2461: (...(relative)...)...(?-1)...
2462: (...(?+1)...(relative)...
2463: </pre>
2464: An earlier example pointed out that the pattern
2465: <pre>
2466: (sens|respons)e and \1ibility
2467: </pre>
2468: matches "sense and sensibility" and "response and responsibility", but not
2469: "sense and responsibility". If instead the pattern
2470: <pre>
2471: (sens|respons)e and (?1)ibility
2472: </pre>
2473: is used, it does match "sense and responsibility" as well as the other two
2474: strings. Another example is given in the discussion of DEFINE above.
2475: </P>
2476: <P>
2477: All subroutine calls, whether recursive or not, are always treated as atomic
2478: groups. That is, once a subroutine has matched some of the subject string, it
2479: is never re-entered, even if it contains untried alternatives and there is a
2480: subsequent matching failure. Any capturing parentheses that are set during the
2481: subroutine call revert to their previous values afterwards.
2482: </P>
2483: <P>
2484: Processing options such as case-independence are fixed when a subpattern is
2485: defined, so if it is used as a subroutine, such options cannot be changed for
2486: different calls. For example, consider this pattern:
2487: <pre>
2488: (abc)(?i:(?-1))
2489: </pre>
2490: It matches "abcabc". It does not match "abcABC" because the change of
2491: processing option does not affect the called subpattern.
2492: <a name="onigurumasubroutines"></a></P>
2493: <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2494: <P>
2495: For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2496: a number enclosed either in angle brackets or single quotes, is an alternative
2497: syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2498: are two of the examples used above, rewritten using this syntax:
2499: <pre>
2500: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
2501: (sens|respons)e and \g'1'ibility
2502: </pre>
2503: PCRE supports an extension to Oniguruma: if a number is preceded by a
2504: plus or a minus sign it is taken as a relative reference. For example:
2505: <pre>
2506: (abc)(?i:\g<-1>)
2507: </pre>
2508: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
2509: synonymous. The former is a back reference; the latter is a subroutine call.
2510: </P>
2511: <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2512: <P>
2513: Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2514: code to be obeyed in the middle of matching a regular expression. This makes it
2515: possible, amongst other things, to extract different substrings that match the
2516: same pair of parentheses when there is a repetition.
2517: </P>
2518: <P>
2519: PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2520: code. The feature is called "callout". The caller of PCRE provides an external
1.1.1.2 misho 2521: function by putting its entry point in the global variable <i>pcre_callout</i>
2522: (8-bit library) or <i>pcre16_callout</i> (16-bit library). By default, this
2523: variable contains NULL, which disables all calling out.
1.1 misho 2524: </P>
2525: <P>
2526: Within a regular expression, (?C) indicates the points at which the external
2527: function is to be called. If you want to identify different callout points, you
2528: can put a number less than 256 after the letter C. The default value is zero.
2529: For example, this pattern has two callout points:
2530: <pre>
2531: (?C1)abc(?C2)def
2532: </pre>
1.1.1.2 misho 2533: If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
1.1 misho 2534: automatically installed before each item in the pattern. They are all numbered
2535: 255.
2536: </P>
2537: <P>
1.1.1.2 misho 2538: During matching, when PCRE reaches a callout point, the external function is
2539: called. It is provided with the number of the callout, the position in the
2540: pattern, and, optionally, one item of data originally supplied by the caller of
2541: the matching function. The callout function may cause matching to proceed, to
2542: backtrack, or to fail altogether. A complete description of the interface to
2543: the callout function is given in the
1.1 misho 2544: <a href="pcrecallout.html"><b>pcrecallout</b></a>
2545: documentation.
2546: <a name="backtrackcontrol"></a></P>
2547: <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2548: <P>
2549: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2550: are described in the Perl documentation as "experimental and subject to change
2551: or removal in a future version of Perl". It goes on to say: "Their usage in
2552: production code should be noted to avoid problems during upgrades." The same
2553: remarks apply to the PCRE features described in this section.
2554: </P>
2555: <P>
2556: Since these verbs are specifically related to backtracking, most of them can be
1.1.1.2 misho 2557: used only when the pattern is to be matched using one of the traditional
2558: matching functions, which use a backtracking algorithm. With the exception of
2559: (*FAIL), which behaves like a failing negative assertion, they cause an error
2560: if encountered by a DFA matching function.
1.1 misho 2561: </P>
2562: <P>
2563: If any of these verbs are used in an assertion or in a subpattern that is
2564: called as a subroutine (whether or not recursively), their effect is confined
2565: to that subpattern; it does not extend to the surrounding pattern, with one
2566: exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2567: a successful positive assertion <i>is</i> passed back when a match succeeds
2568: (compare capturing parentheses in assertions). Note that such subpatterns are
2569: processed as anchored at the point where they are tested. Note also that Perl's
1.1.1.3 ! misho 2570: treatment of subroutines and assertions is different in some cases.
1.1 misho 2571: </P>
2572: <P>
2573: The new verbs make use of what was previously invalid syntax: an opening
2574: parenthesis followed by an asterisk. They are generally of the form
2575: (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2576: depending on whether or not an argument is present. A name is any sequence of
1.1.1.3 ! misho 2577: characters that does not include a closing parenthesis. The maximum length of
! 2578: name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
! 2579: is empty, that is, if the closing parenthesis immediately follows the colon,
! 2580: the effect is as if the colon were not there. Any number of these verbs may
! 2581: occur in a pattern.
! 2582: <a name="nooptimize"></a></P>
! 2583: <br><b>
! 2584: Optimizations that affect backtracking verbs
! 2585: </b><br>
1.1 misho 2586: <P>
2587: PCRE contains some optimizations that are used to speed up matching by running
2588: some checks at the start of each match attempt. For example, it may know the
2589: minimum length of matching subject, or that a particular character must be
2590: present. When one of these optimizations suppresses the running of a match, any
2591: included backtracking verbs will not, of course, be processed. You can suppress
2592: the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2593: when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
1.1.1.3 ! misho 2594: pattern with (*NO_START_OPT). There is more discussion of this option in the
! 2595: section entitled
! 2596: <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
! 2597: in the
! 2598: <a href="pcreapi.html"><b>pcreapi</b></a>
! 2599: documentation.
1.1 misho 2600: </P>
2601: <P>
2602: Experiments with Perl suggest that it too has similar optimizations, sometimes
2603: leading to anomalous results.
2604: </P>
2605: <br><b>
2606: Verbs that act immediately
2607: </b><br>
2608: <P>
2609: The following verbs act as soon as they are encountered. They may not be
2610: followed by a name.
2611: <pre>
2612: (*ACCEPT)
2613: </pre>
2614: This verb causes the match to end successfully, skipping the remainder of the
2615: pattern. However, when it is inside a subpattern that is called as a
2616: subroutine, only that subpattern is ended successfully. Matching then continues
2617: at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2618: far is captured. For example:
2619: <pre>
2620: A((?:A|B(*ACCEPT)|C)D)
2621: </pre>
2622: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2623: the outer parentheses.
2624: <pre>
2625: (*FAIL) or (*F)
2626: </pre>
2627: This verb causes a matching failure, forcing backtracking to occur. It is
2628: equivalent to (?!) but easier to read. The Perl documentation notes that it is
2629: probably useful only when combined with (?{}) or (??{}). Those are, of course,
2630: Perl features that are not present in PCRE. The nearest equivalent is the
2631: callout feature, as for example in this pattern:
2632: <pre>
2633: a+(?C)(*FAIL)
2634: </pre>
2635: A match with the string "aaaa" always fails, but the callout is taken before
2636: each backtrack happens (in this example, 10 times).
2637: </P>
2638: <br><b>
2639: Recording which path was taken
2640: </b><br>
2641: <P>
2642: There is one verb whose main purpose is to track how a match was arrived at,
2643: though it also has a secondary use in conjunction with advancing the match
2644: starting point (see (*SKIP) below).
2645: <pre>
2646: (*MARK:NAME) or (*:NAME)
2647: </pre>
2648: A name is always required with this verb. There may be as many instances of
2649: (*MARK) as you like in a pattern, and their names do not have to be unique.
2650: </P>
2651: <P>
2652: When a match succeeds, the name of the last-encountered (*MARK) on the matching
1.1.1.2 misho 2653: path is passed back to the caller as described in the section entitled
2654: <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
1.1 misho 2655: in the
2656: <a href="pcreapi.html"><b>pcreapi</b></a>
2657: documentation. Here is an example of <b>pcretest</b> output, where the /K
2658: modifier requests the retrieval and outputting of (*MARK) data:
2659: <pre>
2660: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2661: data> XY
2662: 0: XY
2663: MK: A
2664: XZ
2665: 0: XZ
2666: MK: B
2667: </pre>
2668: The (*MARK) name is tagged with "MK:" in this output, and in this example it
2669: indicates which of the two alternatives matched. This is a more efficient way
2670: of obtaining this information than putting each alternative in its own
2671: capturing parentheses.
2672: </P>
2673: <P>
2674: If (*MARK) is encountered in a positive assertion, its name is recorded and
2675: passed back if it is the last-encountered. This does not happen for negative
2676: assertions.
2677: </P>
2678: <P>
2679: After a partial match or a failed match, the name of the last encountered
2680: (*MARK) in the entire match process is returned. For example:
2681: <pre>
2682: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2683: data> XP
2684: No match, mark = B
2685: </pre>
2686: Note that in this unanchored example the mark is retained from the match
1.1.1.3 ! misho 2687: attempt that started at the letter "X" in the subject. Subsequent match
! 2688: attempts starting at "P" and then with an empty string do not get as far as the
! 2689: (*MARK) item, but nevertheless do not reset it.
! 2690: </P>
! 2691: <P>
! 2692: If you are interested in (*MARK) values after failed matches, you should
! 2693: probably set the PCRE_NO_START_OPTIMIZE option
! 2694: <a href="#nooptimize">(see above)</a>
! 2695: to ensure that the match is always attempted.
1.1 misho 2696: </P>
2697: <br><b>
2698: Verbs that act after backtracking
2699: </b><br>
2700: <P>
2701: The following verbs do nothing when they are encountered. Matching continues
2702: with what follows, but if there is no subsequent match, causing a backtrack to
2703: the verb, a failure is forced. That is, backtracking cannot pass to the left of
2704: the verb. However, when one of these verbs appears inside an atomic group, its
2705: effect is confined to that group, because once the group has been matched,
2706: there is never any backtracking into it. In this situation, backtracking can
2707: "jump back" to the left of the entire atomic group. (Remember also, as stated
2708: above, that this localization also applies in subroutine calls and assertions.)
2709: </P>
2710: <P>
2711: These verbs differ in exactly what kind of failure occurs when backtracking
2712: reaches them.
2713: <pre>
2714: (*COMMIT)
2715: </pre>
2716: This verb, which may not be followed by a name, causes the whole match to fail
2717: outright if the rest of the pattern does not match. Even if the pattern is
2718: unanchored, no further attempts to find a match by advancing the starting point
2719: take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2720: finding a match at the current starting point, or not at all. For example:
2721: <pre>
2722: a+(*COMMIT)b
2723: </pre>
2724: This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2725: dynamic anchor, or "I've started, so I must finish." The name of the most
2726: recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2727: match failure.
2728: </P>
2729: <P>
2730: Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2731: unless PCRE's start-of-match optimizations are turned off, as shown in this
2732: <b>pcretest</b> example:
2733: <pre>
2734: re> /(*COMMIT)abc/
2735: data> xyzabc
2736: 0: abc
2737: xyzabc\Y
2738: No match
2739: </pre>
2740: PCRE knows that any match must start with "a", so the optimization skips along
2741: the subject to "a" before running the first match attempt, which succeeds. When
2742: the optimization is disabled by the \Y escape in the second subject, the match
2743: starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2744: starting points.
2745: <pre>
2746: (*PRUNE) or (*PRUNE:NAME)
2747: </pre>
2748: This verb causes the match to fail at the current starting position in the
2749: subject if the rest of the pattern does not match. If the pattern is
2750: unanchored, the normal "bumpalong" advance to the next starting character then
2751: happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2752: reached, or when matching to the right of (*PRUNE), but if there is no match to
2753: the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2754: (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2755: but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2756: The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2757: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2758: <pre>
2759: (*SKIP)
2760: </pre>
2761: This verb, when given without a name, is like (*PRUNE), except that if the
2762: pattern is unanchored, the "bumpalong" advance is not to the next character,
2763: but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2764: signifies that whatever text was matched leading up to it cannot be part of a
2765: successful match. Consider:
2766: <pre>
2767: a+(*SKIP)b
2768: </pre>
2769: If the subject is "aaaac...", after the first match attempt fails (starting at
2770: the first character in the string), the starting point skips on to start the
2771: next attempt at "c". Note that a possessive quantifer does not have the same
2772: effect as this example; although it would suppress backtracking during the
2773: first match attempt, the second attempt would start at the second character
2774: instead of skipping on to "c".
2775: <pre>
2776: (*SKIP:NAME)
2777: </pre>
2778: When (*SKIP) has an associated name, its behaviour is modified. If the
2779: following pattern fails to match, the previous path through the pattern is
2780: searched for the most recent (*MARK) that has the same name. If one is found,
2781: the "bumpalong" advance is to the subject position that corresponds to that
2782: (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2783: matching name is found, the (*SKIP) is ignored.
2784: <pre>
2785: (*THEN) or (*THEN:NAME)
2786: </pre>
2787: This verb causes a skip to the next innermost alternative if the rest of the
2788: pattern does not match. That is, it cancels pending backtracking, but only
2789: within the current alternative. Its name comes from the observation that it can
2790: be used for a pattern-based if-then-else block:
2791: <pre>
2792: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2793: </pre>
2794: If the COND1 pattern matches, FOO is tried (and possibly further items after
2795: the end of the group if FOO succeeds); on failure, the matcher skips to the
2796: second alternative and tries COND2, without backtracking into COND1. The
2797: behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2798: If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2799: </P>
2800: <P>
2801: Note that a subpattern that does not contain a | character is just a part of
2802: the enclosing alternative; it is not a nested alternation with only one
2803: alternative. The effect of (*THEN) extends beyond such a subpattern to the
2804: enclosing alternative. Consider this pattern, where A, B, etc. are complex
2805: pattern fragments that do not contain any | characters at this level:
2806: <pre>
2807: A (B(*THEN)C) | D
2808: </pre>
2809: If A and B are matched, but there is a failure in C, matching does not
2810: backtrack into A; instead it moves to the next alternative, that is, D.
2811: However, if the subpattern containing (*THEN) is given an alternative, it
2812: behaves differently:
2813: <pre>
2814: A (B(*THEN)C | (*FAIL)) | D
2815: </pre>
2816: The effect of (*THEN) is now confined to the inner subpattern. After a failure
2817: in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2818: because there are no more alternatives to try. In this case, matching does now
2819: backtrack into A.
2820: </P>
2821: <P>
2822: Note also that a conditional subpattern is not considered as having two
2823: alternatives, because only one is ever used. In other words, the | character in
2824: a conditional subpattern has a different meaning. Ignoring white space,
2825: consider:
2826: <pre>
2827: ^.*? (?(?=a) a | b(*THEN)c )
2828: </pre>
2829: If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2830: it initially matches zero characters. The condition (?=a) then fails, the
2831: character "b" is matched, but "c" is not. At this point, matching does not
2832: backtrack to .*? as might perhaps be expected from the presence of the |
2833: character. The conditional subpattern is part of the single alternative that
2834: comprises the whole pattern, and so the match fails. (If there was a backtrack
2835: into .*?, allowing it to match "b", the match would succeed.)
2836: </P>
2837: <P>
2838: The verbs just described provide four different "strengths" of control when
2839: subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2840: next alternative. (*PRUNE) comes next, failing the match at the current
2841: starting position, but allowing an advance to the next character (for an
2842: unanchored pattern). (*SKIP) is similar, except that the advance may be more
2843: than one character. (*COMMIT) is the strongest, causing the entire match to
2844: fail.
2845: </P>
2846: <P>
2847: If more than one such verb is present in a pattern, the "strongest" one wins.
2848: For example, consider this pattern, where A, B, etc. are complex pattern
2849: fragments:
2850: <pre>
2851: (A(*COMMIT)B(*THEN)C|D)
2852: </pre>
2853: Once A has matched, PCRE is committed to this match, at the current starting
2854: position. If subsequently B matches, but C does not, the normal (*THEN) action
2855: of trying the next alternative (that is, D) does not happen because (*COMMIT)
2856: overrides.
2857: </P>
2858: <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2859: <P>
2860: <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
1.1.1.2 misho 2861: <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>.
1.1 misho 2862: </P>
2863: <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2864: <P>
2865: Philip Hazel
2866: <br>
2867: University Computing Service
2868: <br>
2869: Cambridge CB2 3QH, England.
2870: <br>
2871: </P>
2872: <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2873: <P>
1.1.1.3 ! misho 2874: Last updated: 17 June 2012
1.1 misho 2875: <br>
1.1.1.2 misho 2876: Copyright © 1997-2012 University of Cambridge.
1.1 misho 2877: <br>
2878: <p>
2879: Return to the <a href="index.html">PCRE index page</a>.
2880: </p>
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