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