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revision 79 by nigel, Sat Feb 24 21:40:52 2007 UTC revision 607 by ph10, Sun Jun 12 15:09:49 2011 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 26  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57    also some more of these special sequences that are concerned with the handling
58    of newlines; they are described below.
59    .P
60  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
61  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
62  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
63  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
64  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
65  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
66    alternative function, and how it differs from the normal function, are
67    discussed in the
68  .\" HREF  .\" HREF
69  \fBpcrematching\fP  \fBpcrematching\fP
70  .\"  .\"
71  page.  page.
72    .
73    .
74    .\" HTML <a name="newlines"></a>
75    .SH "NEWLINE CONVENTIONS"
76    .rs
77    .sp
78    PCRE supports five different conventions for indicating line breaks in
79    strings: a single CR (carriage return) character, a single LF (linefeed)
80    character, the two-character sequence CRLF, any of the three preceding, or any
81    Unicode newline sequence. The
82    .\" HREF
83    \fBpcreapi\fP
84    .\"
85    page has
86    .\" HTML <a href="pcreapi.html#newlines">
87    .\" </a>
88    further discussion
89    .\"
90    about newlines, and shows how to set the newline convention in the
91    \fIoptions\fP arguments for the compiling and matching functions.
92  .P  .P
93    It is also possible to specify a newline convention by starting a pattern
94    string with one of the following five sequences:
95    .sp
96      (*CR)        carriage return
97      (*LF)        linefeed
98      (*CRLF)      carriage return, followed by linefeed
99      (*ANYCRLF)   any of the three above
100      (*ANY)       all Unicode newline sequences
101    .sp
102    These override the default and the options given to \fBpcre_compile()\fP or
103    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
104    newline sequence, the pattern
105    .sp
106      (*CR)a.b
107    .sp
108    changes the convention to CR. That pattern matches "a\enb" because LF is no
109    longer a newline. Note that these special settings, which are not
110    Perl-compatible, are recognized only at the very start of a pattern, and that
111    they must be in upper case. If more than one of them is present, the last one
112    is used.
113    .P
114    The newline convention affects the interpretation of the dot metacharacter when
115    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
116    affect what the \eR escape sequence matches. By default, this is any Unicode
117    newline sequence, for Perl compatibility. However, this can be changed; see the
118    description of \eR in the section entitled
119    .\" HTML <a href="#newlineseq">
120    .\" </a>
121    "Newline sequences"
122    .\"
123    below. A change of \eR setting can be combined with a change of newline
124    convention.
125    .
126    .
127    .SH "CHARACTERS AND METACHARACTERS"
128    .rs
129    .sp
130  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
131  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
132  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 150  interpreted in some special way.
150  .P  .P
151  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
152  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
153  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
154  as follows:  are as follows:
155  .sp  .sp
156    \e      general escape character with several uses    \e      general escape character with several uses
157    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 182  a character class the only metacharacter
182  .sp  .sp
183  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
184  .  .
185    .
186  .SH BACKSLASH  .SH BACKSLASH
187  .rs  .rs
188  .sp  .sp
189  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
190  non-alphanumeric character, it takes away any special meaning that character may  character that is not a number or a letter, it takes away any special meaning
191  have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
192  outside character classes.  both inside and outside character classes.
193  .P  .P
194  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
195  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 106  otherwise be interpreted as a metacharac Line 197  otherwise be interpreted as a metacharac
197  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
198  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
199  .P  .P
200    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
201    backslash. All other characters (in particular, those whose codepoints are
202    greater than 127) are treated as literals.
203    .P
204  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
205  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
206  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
207  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
 pattern.  
208  .P  .P
209  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
210  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 126  Perl, $ and @ cause variable interpolati Line 220  Perl, $ and @ cause variable interpolati
220    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
221  .sp  .sp
222  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
223    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
224    by \eE later in the pattern, the literal interpretation continues to the end of
225    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
226    a character class, this causes an error, because the character class is not
227    terminated.
228  .  .
229  .  .
230  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 135  The \eQ...\eE sequence is recognized bot Line 234  The \eQ...\eE sequence is recognized bot
234  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
235  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
236  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
237  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
238  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
239  .sp  .sp
240    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
241    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
242    \ee        escape (hex 1B)    \ee        escape (hex 1B)
243    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
244    \en        newline (hex 0A)    \en        linefeed (hex 0A)
245    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
246    \et        tab (hex 09)    \et        tab (hex 09)
247    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
248    \exhh      character with hex code hh    \exhh      character with hex code hh
249    \ex{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
250  .sp  .sp
251  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
252  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
253  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
254  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
255    than 127, a compile-time error occurs. This locks out non-ASCII characters in
256    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
257    values are valid. A lower case letter is converted to upper case, and then the
258    0xc0 bits are flipped.)
259  .P  .P
260  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
261  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
262  appear between \ex{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
263  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
264  other than hexadecimal digits appear between \ex{ and }, or if there is no  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
265  terminating }, this form of escape is not recognized. Instead, the initial  point, which is 10FFFF.
266  \ex will be interpreted as a basic hexadecimal escape, with no following  .P
267  digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
268    there is no terminating }, this form of escape is not recognized. Instead, the
269    initial \ex will be interpreted as a basic hexadecimal escape, with no
270    following digits, giving a character whose value is zero.
271  .P  .P
272  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
273  syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
274  way they are handled. For example, \exdc is exactly the same as \ex{dc}.  example, \exdc is exactly the same as \ex{dc}.
275  .P  .P
276  After \e0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. If there are fewer than two
277  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
278  sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
279  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
280  pattern character that follows is itself an octal digit.  follows is itself an octal digit.
281  .P  .P
282  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
283  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
# Line 191  parenthesized subpatterns. Line 296  parenthesized subpatterns.
296  .P  .P
297  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
298  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
299  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
300  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
301  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
302    to \e777 are permitted. For example:
303  .sp  .sp
304    \e040   is another way of writing a space    \e040   is another way of writing a space
305  .\" JOIN  .\" JOIN
# Line 218  For example: Line 324  For example:
324  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
325  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
326  .P  .P
327  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
328  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
329  addition, inside a character class, the sequence \eb is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
330  backspace character (hex 08), and the sequence \eX is interpreted as the  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
331  character "X". Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
332  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
333  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
334    .
335    .
336    .SS "Absolute and relative back references"
337    .rs
338    .sp
339    The sequence \eg followed by an unsigned or a negative number, optionally
340    enclosed in braces, is an absolute or relative back reference. A named back
341    reference can be coded as \eg{name}. Back references are discussed
342    .\" HTML <a href="#backreferences">
343  .\" </a>  .\" </a>
344  (see below).  later,
345    .\"
346    following the discussion of
347    .\" HTML <a href="#subpattern">
348    .\" </a>
349    parenthesized subpatterns.
350  .\"  .\"
351  .  .
352  .  .
353    .SS "Absolute and relative subroutine calls"
354    .rs
355    .sp
356    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
357    a number enclosed either in angle brackets or single quotes, is an alternative
358    syntax for referencing a subpattern as a "subroutine". Details are discussed
359    .\" HTML <a href="#onigurumasubroutines">
360    .\" </a>
361    later.
362    .\"
363    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
364    synonymous. The former is a back reference; the latter is a
365    .\" HTML <a href="#subpatternsassubroutines">
366    .\" </a>
367    subroutine
368    .\"
369    call.
370    .
371    .
372    .\" HTML <a name="genericchartypes"></a>
373  .SS "Generic character types"  .SS "Generic character types"
374  .rs  .rs
375  .sp  .sp
376  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
377  .sp  .sp
378    \ed     any decimal digit    \ed     any decimal digit
379    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
380      \eh     any horizontal whitespace character
381      \eH     any character that is not a horizontal whitespace character
382    \es     any whitespace character    \es     any whitespace character
383    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
384      \ev     any vertical whitespace character
385      \eV     any character that is not a vertical whitespace character
386    \ew     any "word" character    \ew     any "word" character
387    \eW     any "non-word" character    \eW     any "non-word" character
388  .sp  .sp
389  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
390  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
391    .\" HTML <a href="#fullstopdot">
392    .\" </a>
393    the "." metacharacter
394    .\"
395    when PCRE_DOTALL is not set.
396  .P  .P
397  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
398    of characters into two disjoint sets. Any given character matches one, and only
399    one, of each pair. The sequences can appear both inside and outside character
400  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
401  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
402  there is no character to match.  there is no character to match.
403  .P  .P
404  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
405  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
406  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
407    included in a Perl script, \es may match the VT character. In PCRE, it never
408    does.
409  .P  .P
410  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
411  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
412  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
413  place (see  place (see
414  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 267  in the Line 419  in the
419  .\" HREF  .\" HREF
420  \fBpcreapi\fP  \fBpcreapi\fP
421  .\"  .\"
422  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
423  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
424  .P  accented letters, and these are then matched by \ew. The use of locales with
425  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Unicode is discouraged.
426  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
427  character property support is available.  By default, in UTF-8 mode, characters with values greater than 128 never match
428    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
429    their original meanings from before UTF-8 support was available, mainly for
430    efficiency reasons. However, if PCRE is compiled with Unicode property support,
431    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
432    properties are used to determine character types, as follows:
433    .sp
434      \ed  any character that \ep{Nd} matches (decimal digit)
435      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
436      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
437    .sp
438    The upper case escapes match the inverse sets of characters. Note that \ed
439    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
440    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
441    \eB because they are defined in terms of \ew and \eW. Matching these sequences
442    is noticeably slower when PCRE_UCP is set.
443    .P
444    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
445    release 5.10. In contrast to the other sequences, which match only ASCII
446    characters by default, these always match certain high-valued codepoints in
447    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
448    are:
449    .sp
450      U+0009     Horizontal tab
451      U+0020     Space
452      U+00A0     Non-break space
453      U+1680     Ogham space mark
454      U+180E     Mongolian vowel separator
455      U+2000     En quad
456      U+2001     Em quad
457      U+2002     En space
458      U+2003     Em space
459      U+2004     Three-per-em space
460      U+2005     Four-per-em space
461      U+2006     Six-per-em space
462      U+2007     Figure space
463      U+2008     Punctuation space
464      U+2009     Thin space
465      U+200A     Hair space
466      U+202F     Narrow no-break space
467      U+205F     Medium mathematical space
468      U+3000     Ideographic space
469    .sp
470    The vertical space characters are:
471    .sp
472      U+000A     Linefeed
473      U+000B     Vertical tab
474      U+000C     Formfeed
475      U+000D     Carriage return
476      U+0085     Next line
477      U+2028     Line separator
478      U+2029     Paragraph separator
479    .
480    .
481    .\" HTML <a name="newlineseq"></a>
482    .SS "Newline sequences"
483    .rs
484    .sp
485    Outside a character class, by default, the escape sequence \eR matches any
486    Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
487    .sp
488      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
489    .sp
490    This is an example of an "atomic group", details of which are given
491    .\" HTML <a href="#atomicgroup">
492    .\" </a>
493    below.
494    .\"
495    This particular group matches either the two-character sequence CR followed by
496    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
497    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
498    line, U+0085). The two-character sequence is treated as a single unit that
499    cannot be split.
500    .P
501    In UTF-8 mode, two additional characters whose codepoints are greater than 255
502    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
503    Unicode character property support is not needed for these characters to be
504    recognized.
505    .P
506    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
507    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
508    either at compile time or when the pattern is matched. (BSR is an abbrevation
509    for "backslash R".) This can be made the default when PCRE is built; if this is
510    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
511    It is also possible to specify these settings by starting a pattern string with
512    one of the following sequences:
513    .sp
514      (*BSR_ANYCRLF)   CR, LF, or CRLF only
515      (*BSR_UNICODE)   any Unicode newline sequence
516    .sp
517    These override the default and the options given to \fBpcre_compile()\fP or
518    \fBpcre_compile2()\fP, but they can be overridden by options given to
519    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
520    which are not Perl-compatible, are recognized only at the very start of a
521    pattern, and that they must be in upper case. If more than one of them is
522    present, the last one is used. They can be combined with a change of newline
523    convention; for example, a pattern can start with:
524    .sp
525      (*ANY)(*BSR_ANYCRLF)
526    .sp
527    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
528    a character class, \eR is treated as an unrecognized escape sequence, and so
529    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
530  .  .
531  .  .
532  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 280  character property support is available. Line 534  character property support is available.
534  .rs  .rs
535  .sp  .sp
536  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
537  escape sequences to match generic character types are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
538  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
539  .sp  characters whose codepoints are less than 256, but they do work in this mode.
540   \ep{\fIxx\fP}   a character with the \fIxx\fP property  The extra escape sequences are:
541   \eP{\fIxx\fP}   a character without the \fIxx\fP property  .sp
542   \eX       an extended Unicode sequence    \ep{\fIxx\fP}   a character with the \fIxx\fP property
543  .sp    \eP{\fIxx\fP}   a character without the \fIxx\fP property
544  The property names represented by \fIxx\fP above are limited to the    \eX       an extended Unicode sequence
545  Unicode general category properties. Each character has exactly one such  .sp
546  property, specified by a two-letter abbreviation. For compatibility with Perl,  The property names represented by \fIxx\fP above are limited to the Unicode
547  negation can be specified by including a circumflex between the opening brace  script names, the general category properties, "Any", which matches any
548  and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.  character (including newline), and some special PCRE properties (described
549  .P  in the
550  If only one letter is specified with \ep or \eP, it includes all the properties  .\" HTML <a href="#extraprops">
551  that start with that letter. In this case, in the absence of negation, the  .\" </a>
552  curly brackets in the escape sequence are optional; these two examples have  next section).
553  the same effect:  .\"
554    Other Perl properties such as "InMusicalSymbols" are not currently supported by
555    PCRE. Note that \eP{Any} does not match any characters, so always causes a
556    match failure.
557    .P
558    Sets of Unicode characters are defined as belonging to certain scripts. A
559    character from one of these sets can be matched using a script name. For
560    example:
561    .sp
562      \ep{Greek}
563      \eP{Han}
564    .sp
565    Those that are not part of an identified script are lumped together as
566    "Common". The current list of scripts is:
567    .P
568    Arabic,
569    Armenian,
570    Avestan,
571    Balinese,
572    Bamum,
573    Bengali,
574    Bopomofo,
575    Braille,
576    Buginese,
577    Buhid,
578    Canadian_Aboriginal,
579    Carian,
580    Cham,
581    Cherokee,
582    Common,
583    Coptic,
584    Cuneiform,
585    Cypriot,
586    Cyrillic,
587    Deseret,
588    Devanagari,
589    Egyptian_Hieroglyphs,
590    Ethiopic,
591    Georgian,
592    Glagolitic,
593    Gothic,
594    Greek,
595    Gujarati,
596    Gurmukhi,
597    Han,
598    Hangul,
599    Hanunoo,
600    Hebrew,
601    Hiragana,
602    Imperial_Aramaic,
603    Inherited,
604    Inscriptional_Pahlavi,
605    Inscriptional_Parthian,
606    Javanese,
607    Kaithi,
608    Kannada,
609    Katakana,
610    Kayah_Li,
611    Kharoshthi,
612    Khmer,
613    Lao,
614    Latin,
615    Lepcha,
616    Limbu,
617    Linear_B,
618    Lisu,
619    Lycian,
620    Lydian,
621    Malayalam,
622    Meetei_Mayek,
623    Mongolian,
624    Myanmar,
625    New_Tai_Lue,
626    Nko,
627    Ogham,
628    Old_Italic,
629    Old_Persian,
630    Old_South_Arabian,
631    Old_Turkic,
632    Ol_Chiki,
633    Oriya,
634    Osmanya,
635    Phags_Pa,
636    Phoenician,
637    Rejang,
638    Runic,
639    Samaritan,
640    Saurashtra,
641    Shavian,
642    Sinhala,
643    Sundanese,
644    Syloti_Nagri,
645    Syriac,
646    Tagalog,
647    Tagbanwa,
648    Tai_Le,
649    Tai_Tham,
650    Tai_Viet,
651    Tamil,
652    Telugu,
653    Thaana,
654    Thai,
655    Tibetan,
656    Tifinagh,
657    Ugaritic,
658    Vai,
659    Yi.
660    .P
661    Each character has exactly one Unicode general category property, specified by
662    a two-letter abbreviation. For compatibility with Perl, negation can be
663    specified by including a circumflex between the opening brace and the property
664    name. For example, \ep{^Lu} is the same as \eP{Lu}.
665    .P
666    If only one letter is specified with \ep or \eP, it includes all the general
667    category properties that start with that letter. In this case, in the absence
668    of negation, the curly brackets in the escape sequence are optional; these two
669    examples have the same effect:
670  .sp  .sp
671    \ep{L}    \ep{L}
672    \epL    \epL
673  .sp  .sp
674  The following property codes are supported:  The following general category property codes are supported:
675  .sp  .sp
676    C     Other    C     Other
677    Cc    Control    Cc    Control
# Line 347  The following property codes are support Line 717  The following property codes are support
717    Zp    Paragraph separator    Zp    Paragraph separator
718    Zs    Space separator    Zs    Space separator
719  .sp  .sp
720  Extended properties such as "Greek" or "InMusicalSymbols" are not supported by  The special property L& is also supported: it matches a character that has
721  PCRE.  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
722    a modifier or "other".
723    .P
724    The Cs (Surrogate) property applies only to characters in the range U+D800 to
725    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
726    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
727    (see the discussion of PCRE_NO_UTF8_CHECK in the
728    .\" HREF
729    \fBpcreapi\fP
730    .\"
731    page). Perl does not support the Cs property.
732    .P
733    The long synonyms for property names that Perl supports (such as \ep{Letter})
734    are not supported by PCRE, nor is it permitted to prefix any of these
735    properties with "Is".
736    .P
737    No character that is in the Unicode table has the Cn (unassigned) property.
738    Instead, this property is assumed for any code point that is not in the
739    Unicode table.
740  .P  .P
741  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
742  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
# Line 366  atomic group Line 754  atomic group
754  (see below).  (see below).
755  .\"  .\"
756  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
757  preceding character.  preceding character. None of them have codepoints less than 256, so in
758    non-UTF-8 mode \eX matches any one character.
759  .P  .P
760  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
761  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
762  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
763  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
764    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
765    (*UCP).
766    .
767    .
768    .\" HTML <a name="extraprops"></a>
769    .SS PCRE's additional properties
770    .rs
771    .sp
772    As well as the standard Unicode properties described in the previous
773    section, PCRE supports four more that make it possible to convert traditional
774    escape sequences such as \ew and \es and POSIX character classes to use Unicode
775    properties. PCRE uses these non-standard, non-Perl properties internally when
776    PCRE_UCP is set. They are:
777    .sp
778      Xan   Any alphanumeric character
779      Xps   Any POSIX space character
780      Xsp   Any Perl space character
781      Xwd   Any Perl "word" character
782    .sp
783    Xan matches characters that have either the L (letter) or the N (number)
784    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
785    carriage return, and any other character that has the Z (separator) property.
786    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
787    same characters as Xan, plus underscore.
788    .
789    .
790    .\" HTML <a name="resetmatchstart"></a>
791    .SS "Resetting the match start"
792    .rs
793    .sp
794    The escape sequence \eK causes any previously matched characters not to be
795    included in the final matched sequence. For example, the pattern:
796    .sp
797      foo\eKbar
798    .sp
799    matches "foobar", but reports that it has matched "bar". This feature is
800    similar to a lookbehind assertion
801    .\" HTML <a href="#lookbehind">
802    .\" </a>
803    (described below).
804    .\"
805    However, in this case, the part of the subject before the real match does not
806    have to be of fixed length, as lookbehind assertions do. The use of \eK does
807    not interfere with the setting of
808    .\" HTML <a href="#subpattern">
809    .\" </a>
810    captured substrings.
811    .\"
812    For example, when the pattern
813    .sp
814      (foo)\eKbar
815    .sp
816    matches "foobar", the first substring is still set to "foo".
817    .P
818    Perl documents that the use of \eK within assertions is "not well defined". In
819    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
820    ignored in negative assertions.
821  .  .
822  .  .
823  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
824  .SS "Simple assertions"  .SS "Simple assertions"
825  .rs  .rs
826  .sp  .sp
827  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
828  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
829  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
830  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 386  subpatterns for more complicated asserti Line 832  subpatterns for more complicated asserti
832  .\" </a>  .\" </a>
833  below.  below.
834  .\"  .\"
835  The backslashed  The backslashed assertions are:
 assertions are:  
836  .sp  .sp
837    \eb     matches at a word boundary    \eb     matches at a word boundary
838    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
839    \eA     matches at start of subject    \eA     matches at the start of the subject
840    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
841    \ez     matches at end of subject            also matches before a newline at the end of the subject
842    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
843  .sp    \eG     matches at the first matching position in the subject
844  These assertions may not appear in character classes (but note that \eb has a  .sp
845  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
846    character. If any other of these assertions appears in a character class, by
847    default it matches the corresponding literal character (for example, \eB
848    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
849    escape sequence" error is generated instead.
850  .P  .P
851  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
852  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
853  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
854  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
855    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
856    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
857    of word" or "end of word" metasequence. However, whatever follows \eb normally
858    determines which it is. For example, the fragment \eba matches "a" at the start
859    of a word.
860  .P  .P
861  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
862  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 412  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 866  PCRE_NOTBOL or PCRE_NOTEOL options, whic
866  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
867  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
868  at a point other than the beginning of the subject, \eA can never match. The  at a point other than the beginning of the subject, \eA can never match. The
869  difference between \eZ and \ez is that \eZ matches before a newline that is the  difference between \eZ and \ez is that \eZ matches before a newline at the end
870  last character of the string as well as at the end of the string, whereas \ez  of the string as well as at the very end, whereas \ez matches only at the end.
 matches only at the end.  
871  .P  .P
872  The \eG assertion is true only when the current matching position is at the  The \eG assertion is true only when the current matching position is at the
873  start point of the match, as specified by the \fIstartoffset\fP argument of  start point of the match, as specified by the \fIstartoffset\fP argument of
# Line 458  to be anchored.) Line 911  to be anchored.)
911  .P  .P
912  A dollar character is an assertion that is true only if the current matching  A dollar character is an assertion that is true only if the current matching
913  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline
914  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
915  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
916  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
917  Dollar has no special meaning in a character class.  character class.
918  .P  .P
919  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
920  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
921  does not affect the \eZ assertion.  does not affect the \eZ assertion.
922  .P  .P
923  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
924  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
925  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
926  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
927  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
928  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
929  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
930  branches start with ^ are not anchored in multiline mode, and a match for  .P
931  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
932  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is  \en represents a newline) in multiline mode, but not otherwise. Consequently,
933  set.  patterns that are anchored in single line mode because all branches start with
934    ^ are not anchored in multiline mode, and a match for circumflex is possible
935    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
936    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
937  .P  .P
938  Note that the sequences \eA, \eZ, and \ez can be used to match the start and  Note that the sequences \eA, \eZ, and \ez can be used to match the start and
939  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
940  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
941  .  .
942  .  .
943  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
944    .SH "FULL STOP (PERIOD, DOT) AND \eN"
945  .rs  .rs
946  .sp  .sp
947  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
948  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
949  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
950  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
951  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
952  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
953  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
954    (including isolated CRs and LFs). When any Unicode line endings are being
955    recognized, dot does not match CR or LF or any of the other line ending
956    characters.
957    .P
958    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
959    option is set, a dot matches any one character, without exception. If the
960    two-character sequence CRLF is present in the subject string, it takes two dots
961    to match it.
962    .P
963    The handling of dot is entirely independent of the handling of circumflex and
964    dollar, the only relationship being that they both involve newlines. Dot has no
965    special meaning in a character class.
966    .P
967    The escape sequence \eN behaves like a dot, except that it is not affected by
968    the PCRE_DOTALL option. In other words, it matches any character except one
969    that signifies the end of a line.
970  .  .
971  .  .
972  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
973  .rs  .rs
974  .sp  .sp
975  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
976  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
977  provided in Perl in order to match individual bytes in UTF-8 mode. Because it  characters. The feature is provided in Perl in order to match individual bytes
978  breaks up UTF-8 characters into individual bytes, what remains in the string  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
979  may be a malformed UTF-8 string. For this reason, the \eC escape sequence is  rest of the string may start with a malformed UTF-8 character. For this reason,
980  best avoided.  the \eC escape sequence is best avoided.
981  .P  .P
982  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
983  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 520  the lookbehind. Line 993  the lookbehind.
993  .rs  .rs
994  .sp  .sp
995  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
996  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
997  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
998  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
999  escaped with a backslash.  a member of the class, it should be the first data character in the class
1000    (after an initial circumflex, if present) or escaped with a backslash.
1001  .P  .P
1002  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
1003  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
1004  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
1005  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
1006  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
1007  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 537  For example, the character class [aeiou] Line 1011  For example, the character class [aeiou]
1011  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
1012  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1013  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1014  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1015  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1016  string.  string.
1017  .P  .P
# Line 551  caseful version would. In UTF-8 mode, PC Line 1025  caseful version would. In UTF-8 mode, PC
1025  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1026  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1027  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1028  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1029  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
1030  UTF-8 support.  with UTF-8 support.
1031  .P  .P
1032  The newline character is never treated in any special way in character classes,  Characters that might indicate line breaks are never treated in any special way
1033  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  when matching character classes, whatever line-ending sequence is in use, and
1034  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1035    such as [^a] always matches one of these characters.
1036  .P  .P
1037  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
1038  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 581  example [\ex{100}-\ex{2ff}]. Line 1056  example [\ex{100}-\ex{2ff}].
1056  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
1057  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
1058  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1059  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1060  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1061  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1062  property support.  property support.
1063  .P  .P
1064  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1065  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1066  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1067  conveniently be used with the upper case character types to specify a more  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1068  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1069  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1070    .\" HTML <a href="#genericchartypes">
1071    .\" </a>
1072    "Generic character types"
1073    .\"
1074    above. The escape sequence \eb has a different meaning inside a character
1075    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1076    are not special inside a character class. Like any other unrecognized escape
1077    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1078    default, but cause an error if the PCRE_EXTRA option is set.
1079    .P
1080    A circumflex can conveniently be used with the upper case character types to
1081    specify a more restricted set of characters than the matching lower case type.
1082    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1083    whereas [\ew] includes underscore. A positive character class should be read as
1084    "something OR something OR ..." and a negative class as "NOT something AND NOT
1085    something AND NOT ...".
1086  .P  .P
1087  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1088  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 611  this notation. For example, Line 1102  this notation. For example,
1102    [01[:alpha:]%]    [01[:alpha:]%]
1103  .sp  .sp
1104  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1105  are  are:
1106  .sp  .sp
1107    alnum    letters and digits    alnum    letters and digits
1108    alpha    letters    alpha    letters
# Line 622  are Line 1113  are
1113    graph    printing characters, excluding space    graph    printing characters, excluding space
1114    lower    lower case letters    lower    lower case letters
1115    print    printing characters, including space    print    printing characters, including space
1116    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1117    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1118    upper    upper case letters    upper    upper case letters
1119    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 643  matches "1", "2", or any non-digit. PCRE Line 1134  matches "1", "2", or any non-digit. PCRE
1134  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1135  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1136  .P  .P
1137  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1138  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1139    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1140    character properties are used. This is achieved by replacing the POSIX classes
1141    by other sequences, as follows:
1142    .sp
1143      [:alnum:]  becomes  \ep{Xan}
1144      [:alpha:]  becomes  \ep{L}
1145      [:blank:]  becomes  \eh
1146      [:digit:]  becomes  \ep{Nd}
1147      [:lower:]  becomes  \ep{Ll}
1148      [:space:]  becomes  \ep{Xps}
1149      [:upper:]  becomes  \ep{Lu}
1150      [:word:]   becomes  \ep{Xwd}
1151    .sp
1152    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1153    classes are unchanged, and match only characters with code points less than
1154    128.
1155  .  .
1156  .  .
1157  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 656  the pattern Line 1163  the pattern
1163    gilbert|sullivan    gilbert|sullivan
1164  .sp  .sp
1165  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1166  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1167  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1168  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
 subpattern  
1169  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
1170  .\" </a>  .\" </a>
1171  (defined below),  (defined below),
# Line 672  alternative in the subpattern. Line 1178  alternative in the subpattern.
1178  .rs  .rs
1179  .sp  .sp
1180  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1181  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1182  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1183    The option letters are
1184  .sp  .sp
1185    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1186    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 687  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1194  PCRE_MULTILINE while unsetting PCRE_DOTA
1194  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1195  unset.  unset.
1196  .P  .P
1197  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1198  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1199  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1200  the global options (and it will therefore show up in data extracted by the  .P
1201  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1202    subpattern parentheses), the change applies to the remainder of the pattern
1203    that follows. If the change is placed right at the start of a pattern, PCRE
1204    extracts it into the global options (and it will therefore show up in data
1205    extracted by the \fBpcre_fullinfo()\fP function).
1206  .P  .P
1207  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1208  pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1209  .sp  .sp
1210    (a(?i)b)c    (a(?i)b)c
1211  .sp  .sp
# Line 710  branch is abandoned before the option se Line 1221  branch is abandoned before the option se
1221  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1222  behaviour otherwise.  behaviour otherwise.
1223  .P  .P
1224  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1225  same way as the Perl-compatible options by using the characters U and X  application when the compile or match functions are called. In some cases the
1226  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences such as (*CRLF) to override what
1227  earlier in the pattern than any of the additional features it turns on, even  the application has set or what has been defaulted. Details are given in the
1228  when it is at top level. It is best to put it at the start.  section entitled
1229    .\" HTML <a href="#newlineseq">
1230    .\" </a>
1231    "Newline sequences"
1232    .\"
1233    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1234    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1235    PCRE_UTF8 and the PCRE_UCP options, respectively.
1236  .  .
1237  .  .
1238  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 728  Turning part of a pattern into a subpatt Line 1246  Turning part of a pattern into a subpatt
1246  .sp  .sp
1247    cat(aract|erpillar|)    cat(aract|erpillar|)
1248  .sp  .sp
1249  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1250  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1251  .sp  .sp
1252  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1253  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1254  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1255  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1256  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1257  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1258  .sp  .sp
1259    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1260  .sp  .sp
# Line 754  the string "the white queen" is matched Line 1271  the string "the white queen" is matched
1271    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1272  .sp  .sp
1273  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1274  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1275  .P  .P
1276  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
1277  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 770  is reached, an option setting in one bra Line 1286  is reached, an option setting in one bra
1286  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1287  .  .
1288  .  .
1289    .\" HTML <a name="dupsubpatternnumber"></a>
1290    .SH "DUPLICATE SUBPATTERN NUMBERS"
1291    .rs
1292    .sp
1293    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1294    the same numbers for its capturing parentheses. Such a subpattern starts with
1295    (?| and is itself a non-capturing subpattern. For example, consider this
1296    pattern:
1297    .sp
1298      (?|(Sat)ur|(Sun))day
1299    .sp
1300    Because the two alternatives are inside a (?| group, both sets of capturing
1301    parentheses are numbered one. Thus, when the pattern matches, you can look
1302    at captured substring number one, whichever alternative matched. This construct
1303    is useful when you want to capture part, but not all, of one of a number of
1304    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1305    number is reset at the start of each branch. The numbers of any capturing
1306    parentheses that follow the subpattern start after the highest number used in
1307    any branch. The following example is taken from the Perl documentation. The
1308    numbers underneath show in which buffer the captured content will be stored.
1309    .sp
1310      # before  ---------------branch-reset----------- after
1311      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1312      # 1            2         2  3        2     3     4
1313    .sp
1314    A back reference to a numbered subpattern uses the most recent value that is
1315    set for that number by any subpattern. The following pattern matches "abcabc"
1316    or "defdef":
1317    .sp
1318      /(?|(abc)|(def))\e1/
1319    .sp
1320    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1321    refers to the first one in the pattern with the given number. The following
1322    pattern matches "abcabc" or "defabc":
1323    .sp
1324      /(?|(abc)|(def))(?1)/
1325    .sp
1326    If a
1327    .\" HTML <a href="#conditions">
1328    .\" </a>
1329    condition test
1330    .\"
1331    for a subpattern's having matched refers to a non-unique number, the test is
1332    true if any of the subpatterns of that number have matched.
1333    .P
1334    An alternative approach to using this "branch reset" feature is to use
1335    duplicate named subpatterns, as described in the next section.
1336    .
1337    .
1338  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1339  .rs  .rs
1340  .sp  .sp
1341  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1342  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1343  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1344  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1345  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1346  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1347  .P  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1348  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1349  PCRE API provides function calls for extracting the name-to-number translation  .P
1350  table from a compiled pattern. There is also a convenience function for  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1351  extracting a captured substring by name. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1352    parentheses from other parts of the pattern, such as
1353    .\" HTML <a href="#backreferences">
1354    .\" </a>
1355    back references,
1356    .\"
1357    .\" HTML <a href="#recursion">
1358    .\" </a>
1359    recursion,
1360    .\"
1361    and
1362    .\" HTML <a href="#conditions">
1363    .\" </a>
1364    conditions,
1365    .\"
1366    can be made by name as well as by number.
1367    .P
1368    Names consist of up to 32 alphanumeric characters and underscores. Named
1369    capturing parentheses are still allocated numbers as well as names, exactly as
1370    if the names were not present. The PCRE API provides function calls for
1371    extracting the name-to-number translation table from a compiled pattern. There
1372    is also a convenience function for extracting a captured substring by name.
1373    .P
1374    By default, a name must be unique within a pattern, but it is possible to relax
1375    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1376    names are also always permitted for subpatterns with the same number, set up as
1377    described in the previous section.) Duplicate names can be useful for patterns
1378    where only one instance of the named parentheses can match. Suppose you want to
1379    match the name of a weekday, either as a 3-letter abbreviation or as the full
1380    name, and in both cases you want to extract the abbreviation. This pattern
1381    (ignoring the line breaks) does the job:
1382    .sp
1383      (?<DN>Mon|Fri|Sun)(?:day)?|
1384      (?<DN>Tue)(?:sday)?|
1385      (?<DN>Wed)(?:nesday)?|
1386      (?<DN>Thu)(?:rsday)?|
1387      (?<DN>Sat)(?:urday)?
1388    .sp
1389    There are five capturing substrings, but only one is ever set after a match.
1390    (An alternative way of solving this problem is to use a "branch reset"
1391    subpattern, as described in the previous section.)
1392    .P
1393    The convenience function for extracting the data by name returns the substring
1394    for the first (and in this example, the only) subpattern of that name that
1395    matched. This saves searching to find which numbered subpattern it was.
1396    .P
1397    If you make a back reference to a non-unique named subpattern from elsewhere in
1398    the pattern, the one that corresponds to the first occurrence of the name is
1399    used. In the absence of duplicate numbers (see the previous section) this is
1400    the one with the lowest number. If you use a named reference in a condition
1401    test (see the
1402    .\"
1403    .\" HTML <a href="#conditions">
1404    .\" </a>
1405    section about conditions
1406    .\"
1407    below), either to check whether a subpattern has matched, or to check for
1408    recursion, all subpatterns with the same name are tested. If the condition is
1409    true for any one of them, the overall condition is true. This is the same
1410    behaviour as testing by number. For further details of the interfaces for
1411    handling named subpatterns, see the
1412  .\" HREF  .\" HREF
1413  \fBpcreapi\fP  \fBpcreapi\fP
1414  .\"  .\"
1415  documentation.  documentation.
1416    .P
1417    \fBWarning:\fP You cannot use different names to distinguish between two
1418    subpatterns with the same number because PCRE uses only the numbers when
1419    matching. For this reason, an error is given at compile time if different names
1420    are given to subpatterns with the same number. However, you can give the same
1421    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1422  .  .
1423  .  .
1424  .SH REPETITION  .SH REPETITION
# Line 797  Repetition is specified by quantifiers, Line 1428  Repetition is specified by quantifiers,
1428  items:  items:
1429  .sp  .sp
1430    a literal data character    a literal data character
1431    the . metacharacter    the dot metacharacter
1432    the \eC escape sequence    the \eC escape sequence
1433    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1434    an escape such as \ed that matches a single character    the \eR escape sequence
1435      an escape such as \ed or \epL that matches a single character
1436    a character class    a character class
1437    a back reference (see next section)    a back reference (see next section)
1438    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1439      a recursive or "subroutine" call to a subpattern
1440  .sp  .sp
1441  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1442  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 835  support is available, \eX{3} matches thr Line 1468  support is available, \eX{3} matches thr
1468  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1469  .P  .P
1470  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1471  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1472    subpatterns that are referenced as
1473    .\" HTML <a href="#subpatternsassubroutines">
1474    .\" </a>
1475    subroutines
1476    .\"
1477    from elsewhere in the pattern (but see also the section entitled
1478    .\" HTML <a href="#subdefine">
1479    .\" </a>
1480    "Defining subpatterns for use by reference only"
1481    .\"
1482    below). Items other than subpatterns that have a {0} quantifier are omitted
1483    from the compiled pattern.
1484  .P  .P
1485  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1486  quantifiers have single-character abbreviations:  abbreviations:
1487  .sp  .sp
1488    *    is equivalent to {0,}    *    is equivalent to {0,}
1489    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 886  own right. Because it has two uses, it c Line 1531  own right. Because it has two uses, it c
1531  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1532  way the rest of the pattern matches.  way the rest of the pattern matches.
1533  .P  .P
1534  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1535  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1536  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1537  default behaviour.  default behaviour.
# Line 896  is greater than 1 or with a limited maxi Line 1541  is greater than 1 or with a limited maxi
1541  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1542  .P  .P
1543  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1544  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1545  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1546  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1547  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 907  worth setting PCRE_DOTALL in order to ob Line 1552  worth setting PCRE_DOTALL in order to ob
1552  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1553  .P  .P
1554  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1555  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1556  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1557  succeed. Consider, for example:  succeeds. Consider, for example:
1558  .sp  .sp
1559    (.*)abc\e1    (.*)abc\e1
1560  .sp  .sp
# Line 935  matches "aba" the value of the second ca Line 1580  matches "aba" the value of the second ca
1580  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1581  .rs  .rs
1582  .sp  .sp
1583  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1584  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1585  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1586  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1587  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1588  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1589  .P  .P
1590  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1591  .sp  .sp
# Line 952  item, and then with 4, and so on, before Line 1597  item, and then with 4, and so on, before
1597  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1598  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1599  .P  .P
1600  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1601  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1602  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1603  .sp  .sp
# Line 982  previous example can be rewritten as Line 1627  previous example can be rewritten as
1627  .sp  .sp
1628    \ed++foo    \ed++foo
1629  .sp  .sp
1630    Note that a possessive quantifier can be used with an entire group, for
1631    example:
1632    .sp
1633      (abc|xyz){2,3}+
1634    .sp
1635  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1636  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1637  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1638  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1639  .P  difference; possessive quantifiers should be slightly faster.
1640  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1641  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1642    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1643    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1644    package, and PCRE copied it from there. It ultimately found its way into Perl
1645    at release 5.10.
1646    .P
1647    PCRE has an optimization that automatically "possessifies" certain simple
1648    pattern constructs. For example, the sequence A+B is treated as A++B because
1649    there is no point in backtracking into a sequence of A's when B must follow.
1650  .P  .P
1651  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1652  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1030  However, if the decimal number following Line 1688  However, if the decimal number following
1688  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1689  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1690  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1691  numbers less than 10. See the subsection entitled "Non-printing characters"  numbers less than 10. A "forward back reference" of this type can make sense
1692    when a repetition is involved and the subpattern to the right has participated
1693    in an earlier iteration.
1694    .P
1695    It is not possible to have a numerical "forward back reference" to a subpattern
1696    whose number is 10 or more using this syntax because a sequence such as \e50 is
1697    interpreted as a character defined in octal. See the subsection entitled
1698    "Non-printing characters"
1699  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1700  .\" </a>  .\" </a>
1701  above  above
1702  .\"  .\"
1703  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1704    no such problem when named parentheses are used. A back reference to any
1705    subpattern is possible using named parentheses (see below).
1706    .P
1707    Another way of avoiding the ambiguity inherent in the use of digits following a
1708    backslash is to use the \eg escape sequence. This escape must be followed by an
1709    unsigned number or a negative number, optionally enclosed in braces. These
1710    examples are all identical:
1711    .sp
1712      (ring), \e1
1713      (ring), \eg1
1714      (ring), \eg{1}
1715    .sp
1716    An unsigned number specifies an absolute reference without the ambiguity that
1717    is present in the older syntax. It is also useful when literal digits follow
1718    the reference. A negative number is a relative reference. Consider this
1719    example:
1720    .sp
1721      (abc(def)ghi)\eg{-1}
1722    .sp
1723    The sequence \eg{-1} is a reference to the most recently started capturing
1724    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1725    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1726    can be helpful in long patterns, and also in patterns that are created by
1727    joining together fragments that contain references within themselves.
1728  .P  .P
1729  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1730  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1057  back reference, the case of letters is r Line 1746  back reference, the case of letters is r
1746  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1747  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1748  .P  .P
1749  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1750  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1751  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1752    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1753    references, is also supported. We could rewrite the above example in any of
1754    the following ways:
1755    .sp
1756      (?<p1>(?i)rah)\es+\ek<p1>
1757      (?'p1'(?i)rah)\es+\ek{p1}
1758      (?P<p1>(?i)rah)\es+(?P=p1)
1759      (?<p1>(?i)rah)\es+\eg{p1}
1760  .sp  .sp
1761    A subpattern that is referenced by name may appear in the pattern before or
1762    after the reference.
1763    .P
1764  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1765  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1766  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1767  .sp  .sp
1768    (a|(bc))\e2    (a|(bc))\e2
1769  .sp  .sp
1770  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1771  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1772  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1773  with a digit character, some delimiter must be used to terminate the back  .P
1774  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1775  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1776    If the pattern continues with a digit character, some delimiter must be used to
1777    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1778    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1779  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1780  .\" </a>  .\" </a>
1781  "Comments"  "Comments"
1782  .\"  .\"
1783  below) can be used.  below) can be used.
1784  .P  .
1785    .SS "Recursive back references"
1786    .rs
1787    .sp
1788  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1789  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1790  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1093  to the previous iteration. In order for Line 1798  to the previous iteration. In order for
1798  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1799  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1800  minimum of zero.  minimum of zero.
1801    .P
1802    Back references of this type cause the group that they reference to be treated
1803    as an
1804    .\" HTML <a href="#atomicgroup">
1805    .\" </a>
1806    atomic group.
1807    .\"
1808    Once the whole group has been matched, a subsequent matching failure cannot
1809    cause backtracking into the middle of the group.
1810  .  .
1811  .  .
1812  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1123  because it does not make sense for negat Line 1837  because it does not make sense for negat
1837  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1838  .rs  .rs
1839  .sp  .sp
1840  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1841  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1842  .sp  .sp
1843    \ew+(?=;)    \ew+(?=;)
1844  .sp  .sp
# Line 1146  lookbehind assertion is needed to achiev Line 1860  lookbehind assertion is needed to achiev
1860  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1861  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1862  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1863    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1864  .  .
1865  .  .
1866  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1159  negative assertions. For example, Line 1874  negative assertions. For example,
1874  .sp  .sp
1875  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1876  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1877  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1878  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1879  .sp  .sp
1880    (?<=bullock|donkey)    (?<=bullock|donkey)
1881  .sp  .sp
# Line 1170  is permitted, but Line 1885  is permitted, but
1885  .sp  .sp
1886  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1887  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1888  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1889  match the same length of string. An assertion such as  length of string. An assertion such as
1890  .sp  .sp
1891    (?<=ab(c|de))    (?<=ab(c|de))
1892  .sp  .sp
1893  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1894  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1895    branches:
1896  .sp  .sp
1897    (?<=abc|abde)    (?<=abc|abde)
1898  .sp  .sp
1899    In some cases, the escape sequence \eK
1900    .\" HTML <a href="#resetmatchstart">
1901    .\" </a>
1902    (see above)
1903    .\"
1904    can be used instead of a lookbehind assertion to get round the fixed-length
1905    restriction.
1906    .P
1907  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1908  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1909  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1910  match is deemed to fail.  assertion fails.
1911  .P  .P
1912  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1913  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1914  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1915  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1916  .P  .P
1917  Atomic groups can be used in conjunction with lookbehind assertions to specify  .\" HTML <a href="#subpatternsassubroutines">
1918  efficient matching at the end of the subject string. Consider a simple pattern  .\" </a>
1919  such as  "Subroutine"
1920    .\"
1921    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1922    as the subpattern matches a fixed-length string.
1923    .\" HTML <a href="#recursion">
1924    .\" </a>
1925    Recursion,
1926    .\"
1927    however, is not supported.
1928    .P
1929    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1930    specify efficient matching of fixed-length strings at the end of subject
1931    strings. Consider a simple pattern such as
1932  .sp  .sp
1933    abcd$    abcd$
1934  .sp  .sp
# Line 1208  then all but the last two characters, an Line 1944  then all but the last two characters, an
1944  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1945  if the pattern is written as  if the pattern is written as
1946  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1947    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1948  .sp  .sp
1949  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1950  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1951  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1952  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1254  is another pattern that matches "foo" pr Line 1986  is another pattern that matches "foo" pr
1986  characters that are not "999".  characters that are not "999".
1987  .  .
1988  .  .
1989    .\" HTML <a name="conditions"></a>
1990  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1991  .rs  .rs
1992  .sp  .sp
1993  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1994  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1995  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1996  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1997  .sp  .sp
1998    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1999    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2000  .sp  .sp
2001  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2002  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
2003  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2004    itself contain nested subpatterns of any form, including conditional
2005    subpatterns; the restriction to two alternatives applies only at the level of
2006    the condition. This pattern fragment is an example where the alternatives are
2007    complex:
2008    .sp
2009      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2010    .sp
2011  .P  .P
2012  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
2013  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
2014  subpattern of that number has previously matched. The number must be greater  .
2015  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
2016  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
2017  it into three parts for ease of discussion:  .sp
2018    If the text between the parentheses consists of a sequence of digits, the
2019    condition is true if a capturing subpattern of that number has previously
2020    matched. If there is more than one capturing subpattern with the same number
2021    (see the earlier
2022    .\"
2023    .\" HTML <a href="#recursion">
2024    .\" </a>
2025    section about duplicate subpattern numbers),
2026    .\"
2027    the condition is true if any of them have matched. An alternative notation is
2028    to precede the digits with a plus or minus sign. In this case, the subpattern
2029    number is relative rather than absolute. The most recently opened parentheses
2030    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2031    loops it can also make sense to refer to subsequent groups. The next
2032    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2033    zero in any of these forms is not used; it provokes a compile-time error.)
2034    .P
2035    Consider the following pattern, which contains non-significant white space to
2036    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2037    three parts for ease of discussion:
2038  .sp  .sp
2039    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
2040  .sp  .sp
2041  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2042  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2043  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2044  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2045  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2046  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2047  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2048  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2049  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2050  .P  .P
2051  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
2052  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2053  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2054      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2055    .sp
2056    This makes the fragment independent of the parentheses in the larger pattern.
2057    .
2058    .SS "Checking for a used subpattern by name"
2059    .rs
2060    .sp
2061    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2062    subpattern by name. For compatibility with earlier versions of PCRE, which had
2063    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2064    there is a possible ambiguity with this syntax, because subpattern names may
2065    consist entirely of digits. PCRE looks first for a named subpattern; if it
2066    cannot find one and the name consists entirely of digits, PCRE looks for a
2067    subpattern of that number, which must be greater than zero. Using subpattern
2068    names that consist entirely of digits is not recommended.
2069    .P
2070    Rewriting the above example to use a named subpattern gives this:
2071    .sp
2072      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2073    .sp
2074    If the name used in a condition of this kind is a duplicate, the test is
2075    applied to all subpatterns of the same name, and is true if any one of them has
2076    matched.
2077    .
2078    .SS "Checking for pattern recursion"
2079    .rs
2080    .sp
2081    If the condition is the string (R), and there is no subpattern with the name R,
2082    the condition is true if a recursive call to the whole pattern or any
2083    subpattern has been made. If digits or a name preceded by ampersand follow the
2084    letter R, for example:
2085    .sp
2086      (?(R3)...) or (?(R&name)...)
2087    .sp
2088    the condition is true if the most recent recursion is into a subpattern whose
2089    number or name is given. This condition does not check the entire recursion
2090    stack. If the name used in a condition of this kind is a duplicate, the test is
2091    applied to all subpatterns of the same name, and is true if any one of them is
2092    the most recent recursion.
2093  .P  .P
2094  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
2095    .\" HTML <a href="#recursion">
2096    .\" </a>
2097    The syntax for recursive patterns
2098    .\"
2099    is described below.
2100    .
2101    .\" HTML <a name="subdefine"></a>
2102    .SS "Defining subpatterns for use by reference only"
2103    .rs
2104    .sp
2105    If the condition is the string (DEFINE), and there is no subpattern with the
2106    name DEFINE, the condition is always false. In this case, there may be only one
2107    alternative in the subpattern. It is always skipped if control reaches this
2108    point in the pattern; the idea of DEFINE is that it can be used to define
2109    "subroutines" that can be referenced from elsewhere. (The use of
2110    .\" HTML <a href="#subpatternsassubroutines">
2111    .\" </a>
2112    "subroutines"
2113    .\"
2114    is described below.) For example, a pattern to match an IPv4 address such as
2115    "192.168.23.245" could be written like this (ignore whitespace and line
2116    breaks):
2117    .sp
2118      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2119      \eb (?&byte) (\e.(?&byte)){3} \eb
2120    .sp
2121    The first part of the pattern is a DEFINE group inside which a another group
2122    named "byte" is defined. This matches an individual component of an IPv4
2123    address (a number less than 256). When matching takes place, this part of the
2124    pattern is skipped because DEFINE acts like a false condition. The rest of the
2125    pattern uses references to the named group to match the four dot-separated
2126    components of an IPv4 address, insisting on a word boundary at each end.
2127    .
2128    .SS "Assertion conditions"
2129    .rs
2130    .sp
2131    If the condition is not in any of the above formats, it must be an assertion.
2132  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2133  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2134  alternatives on the second line:  alternatives on the second line:
# Line 1312  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2148  dd-aaa-dd or dd-dd-dd, where aaa are let
2148  .SH COMMENTS  .SH COMMENTS
2149  .rs  .rs
2150  .sp  .sp
2151  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2152  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2153  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2154    subpattern name or number. The characters that make up a comment play no part
2155    in the pattern matching.
2156  .P  .P
2157  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2158  character class introduces a comment that continues up to the next newline  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2159  character in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2160    this case continues to immediately after the next newline character or
2161    character sequence in the pattern. Which characters are interpreted as newlines
2162    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2163    sequence at the start of the pattern, as described in the section entitled
2164    .\" HTML <a href="#newlines">
2165    .\" </a>
2166    "Newline conventions"
2167    .\"
2168    above. Note that the end of this type of comment is a literal newline sequence
2169    in the pattern; escape sequences that happen to represent a newline do not
2170    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2171    default newline convention is in force:
2172    .sp
2173      abc #comment \en still comment
2174    .sp
2175    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2176    a newline in the pattern. The sequence \en is still literal at this stage, so
2177    it does not terminate the comment. Only an actual character with the code value
2178    0x0a (the default newline) does so.
2179  .  .
2180  .  .
2181    .\" HTML <a name="recursion"></a>
2182  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
2183  .rs  .rs
2184  .sp  .sp
2185  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2186  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2187  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
2188  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2189  that allows regular expressions to recurse (amongst other things). It does this  .P
2190  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
2191  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
2192  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2193    pattern using code interpolation to solve the parentheses problem can be
2194    created like this:
2195  .sp  .sp
2196    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2197  .sp  .sp
2198  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
2199  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2200  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2201  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2202  .P  supports special syntax for recursion of the entire pattern, and also for
2203  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2204  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2205  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2206  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2207  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2208    provided that it occurs inside that subpattern. (If not, it is a
2209    .\" HTML <a href="#subpatternsassubroutines">
2210    .\" </a>
2211    "subroutine"
2212    .\"
2213    call, which is described in the next section.) The special item (?R) or (?0) is
2214    a recursive call of the entire regular expression.
2215  .P  .P
2216  For example, this PCRE pattern solves the nested parentheses problem (assume  This PCRE pattern solves the nested parentheses problem (assume the
2217  the PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2218  .sp  .sp
2219    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2220  .sp  .sp
2221  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2222  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2223  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2224  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2225    to avoid backtracking into sequences of non-parentheses.
2226  .P  .P
2227  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
2228  pattern, so instead you could use this:  pattern, so instead you could use this:
2229  .sp  .sp
2230    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2231  .sp  .sp
2232  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2233  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2234  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2235  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2236  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. Instead of (?1) in the
2237  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2238  .sp  parentheses preceding the recursion. In other words, a negative number counts
2239    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  capturing parentheses leftwards from the point at which it is encountered.
2240  .sp  .P
2241  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2242  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2243  when applying the pattern to strings that do not match. For example, when this  reference is not inside the parentheses that are referenced. They are always
2244  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2245    .\" </a>
2246    "subroutine"
2247    .\"
2248    calls, as described in the next section.
2249    .P
2250    An alternative approach is to use named parentheses instead. The Perl syntax
2251    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2252    could rewrite the above example as follows:
2253    .sp
2254      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2255    .sp
2256    If there is more than one subpattern with the same name, the earliest one is
2257    used.
2258    .P
2259    This particular example pattern that we have been looking at contains nested
2260    unlimited repeats, and so the use of a possessive quantifier for matching
2261    strings of non-parentheses is important when applying the pattern to strings
2262    that do not match. For example, when this pattern is applied to
2263  .sp  .sp
2264    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2265  .sp  .sp
2266  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2267  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2268  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2269  before failure can be reported.  before failure can be reported.
2270  .P  .P
2271  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2272  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2273  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2274  .\" HREF  .\" HREF
2275  \fBpcrecallout\fP  \fBpcrecallout\fP
2276  .\"  .\"
# Line 1393  documentation). If the pattern above is Line 2278  documentation). If the pattern above is
2278  .sp  .sp
2279    (ab(cd)ef)    (ab(cd)ef)
2280  .sp  .sp
2281  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2282  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2283  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2284    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2285       ^                        ^  .P
2286       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2287  .sp  obtain extra memory to store data during a recursion, which it does by using
2288  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2289  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2290  .P  .P
2291  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2292  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1418  different alternatives for the recursive Line 2300  different alternatives for the recursive
2300  is the actual recursive call.  is the actual recursive call.
2301  .  .
2302  .  .
2303    .\" HTML <a name="recursiondifference"></a>
2304    .SS "Recursion difference from Perl"
2305    .rs
2306    .sp
2307    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2308    treated as an atomic group. That is, once it has matched some of the subject
2309    string, it is never re-entered, even if it contains untried alternatives and
2310    there is a subsequent matching failure. This can be illustrated by the
2311    following pattern, which purports to match a palindromic string that contains
2312    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2313    .sp
2314      ^(.|(.)(?1)\e2)$
2315    .sp
2316    The idea is that it either matches a single character, or two identical
2317    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2318    it does not if the pattern is longer than three characters. Consider the
2319    subject string "abcba":
2320    .P
2321    At the top level, the first character is matched, but as it is not at the end
2322    of the string, the first alternative fails; the second alternative is taken
2323    and the recursion kicks in. The recursive call to subpattern 1 successfully
2324    matches the next character ("b"). (Note that the beginning and end of line
2325    tests are not part of the recursion).
2326    .P
2327    Back at the top level, the next character ("c") is compared with what
2328    subpattern 2 matched, which was "a". This fails. Because the recursion is
2329    treated as an atomic group, there are now no backtracking points, and so the
2330    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2331    try the second alternative.) However, if the pattern is written with the
2332    alternatives in the other order, things are different:
2333    .sp
2334      ^((.)(?1)\e2|.)$
2335    .sp
2336    This time, the recursing alternative is tried first, and continues to recurse
2337    until it runs out of characters, at which point the recursion fails. But this
2338    time we do have another alternative to try at the higher level. That is the big
2339    difference: in the previous case the remaining alternative is at a deeper
2340    recursion level, which PCRE cannot use.
2341    .P
2342    To change the pattern so that it matches all palindromic strings, not just
2343    those with an odd number of characters, it is tempting to change the pattern to
2344    this:
2345    .sp
2346      ^((.)(?1)\e2|.?)$
2347    .sp
2348    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2349    deeper recursion has matched a single character, it cannot be entered again in
2350    order to match an empty string. The solution is to separate the two cases, and
2351    write out the odd and even cases as alternatives at the higher level:
2352    .sp
2353      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2354    .sp
2355    If you want to match typical palindromic phrases, the pattern has to ignore all
2356    non-word characters, which can be done like this:
2357    .sp
2358      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2359    .sp
2360    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2361    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2362    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2363    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2364    more) to match typical phrases, and Perl takes so long that you think it has
2365    gone into a loop.
2366    .P
2367    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2368    string does not start with a palindrome that is shorter than the entire string.
2369    For example, although "abcba" is correctly matched, if the subject is "ababa",
2370    PCRE finds the palindrome "aba" at the start, then fails at top level because
2371    the end of the string does not follow. Once again, it cannot jump back into the
2372    recursion to try other alternatives, so the entire match fails.
2373    .
2374    .
2375  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2376  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2377  .rs  .rs
2378  .sp  .sp
2379  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
2380  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2381  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2382  pattern  before or after the reference. A numbered reference can be absolute or
2383    relative, as in these examples:
2384    .sp
2385      (...(absolute)...)...(?2)...
2386      (...(relative)...)...(?-1)...
2387      (...(?+1)...(relative)...
2388    .sp
2389    An earlier example pointed out that the pattern
2390  .sp  .sp
2391    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2392  .sp  .sp
# Line 1435  matches "sense and sensibility" and "res Line 2396  matches "sense and sensibility" and "res
2396    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2397  .sp  .sp
2398  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2399  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2400  refer.  .P
2401    Like recursive subpatterns, a subroutine call is always treated as an atomic
2402    group. That is, once it has matched some of the subject string, it is never
2403    re-entered, even if it contains untried alternatives and there is a subsequent
2404    matching failure. Any capturing parentheses that are set during the subroutine
2405    call revert to their previous values afterwards.
2406    .P
2407    When a subpattern is used as a subroutine, processing options such as
2408    case-independence are fixed when the subpattern is defined. They cannot be
2409    changed for different calls. For example, consider this pattern:
2410    .sp
2411      (abc)(?i:(?-1))
2412    .sp
2413    It matches "abcabc". It does not match "abcABC" because the change of
2414    processing option does not affect the called subpattern.
2415    .
2416    .
2417    .\" HTML <a name="onigurumasubroutines"></a>
2418    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2419    .rs
2420    .sp
2421    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2422    a number enclosed either in angle brackets or single quotes, is an alternative
2423    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2424    are two of the examples used above, rewritten using this syntax:
2425    .sp
2426      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2427      (sens|respons)e and \eg'1'ibility
2428    .sp
2429    PCRE supports an extension to Oniguruma: if a number is preceded by a
2430    plus or a minus sign it is taken as a relative reference. For example:
2431    .sp
2432      (abc)(?i:\eg<-1>)
2433    .sp
2434    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2435    synonymous. The former is a back reference; the latter is a subroutine call.
2436  .  .
2437  .  .
2438  .SH CALLOUTS  .SH CALLOUTS
# Line 1457  function is to be called. If you want to Line 2453  function is to be called. If you want to
2453  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2454  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2455  .sp  .sp
2456    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2457  .sp  .sp
2458  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2459  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1473  description of the interface to the call Line 2469  description of the interface to the call
2469  \fBpcrecallout\fP  \fBpcrecallout\fP
2470  .\"  .\"
2471  documentation.  documentation.
2472    .
2473    .
2474    .\" HTML <a name="backtrackcontrol"></a>
2475    .SH "BACKTRACKING CONTROL"
2476    .rs
2477    .sp
2478    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2479    are described in the Perl documentation as "experimental and subject to change
2480    or removal in a future version of Perl". It goes on to say: "Their usage in
2481    production code should be noted to avoid problems during upgrades." The same
2482    remarks apply to the PCRE features described in this section.
2483    .P
2484    Since these verbs are specifically related to backtracking, most of them can be
2485    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2486    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2487    failing negative assertion, they cause an error if encountered by
2488    \fBpcre_dfa_exec()\fP.
2489  .P  .P
2490  .in 0  If any of these verbs are used in an assertion or subroutine subpattern
2491  Last updated: 28 February 2005  (including recursive subpatterns), their effect is confined to that subpattern;
2492  .br  it does not extend to the surrounding pattern. Note that such subpatterns are
2493  Copyright (c) 1997-2005 University of Cambridge.  processed as anchored at the point where they are tested.
2494    .P
2495    The new verbs make use of what was previously invalid syntax: an opening
2496    parenthesis followed by an asterisk. They are generally of the form
2497    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2498    depending on whether or not an argument is present. An name is a sequence of
2499    letters, digits, and underscores. If the name is empty, that is, if the closing
2500    parenthesis immediately follows the colon, the effect is as if the colon were
2501    not there. Any number of these verbs may occur in a pattern.
2502    .P
2503    PCRE contains some optimizations that are used to speed up matching by running
2504    some checks at the start of each match attempt. For example, it may know the
2505    minimum length of matching subject, or that a particular character must be
2506    present. When one of these optimizations suppresses the running of a match, any
2507    included backtracking verbs will not, of course, be processed. You can suppress
2508    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2509    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2510    pattern with (*NO_START_OPT).
2511    .
2512    .
2513    .SS "Verbs that act immediately"
2514    .rs
2515    .sp
2516    The following verbs act as soon as they are encountered. They may not be
2517    followed by a name.
2518    .sp
2519       (*ACCEPT)
2520    .sp
2521    This verb causes the match to end successfully, skipping the remainder of the
2522    pattern. When inside a recursion, only the innermost pattern is ended
2523    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2524    captured. (This feature was added to PCRE at release 8.00.) For example:
2525    .sp
2526      A((?:A|B(*ACCEPT)|C)D)
2527    .sp
2528    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2529    the outer parentheses.
2530    .sp
2531      (*FAIL) or (*F)
2532    .sp
2533    This verb causes the match to fail, forcing backtracking to occur. It is
2534    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2535    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2536    Perl features that are not present in PCRE. The nearest equivalent is the
2537    callout feature, as for example in this pattern:
2538    .sp
2539      a+(?C)(*FAIL)
2540    .sp
2541    A match with the string "aaaa" always fails, but the callout is taken before
2542    each backtrack happens (in this example, 10 times).
2543    .
2544    .
2545    .SS "Recording which path was taken"
2546    .rs
2547    .sp
2548    There is one verb whose main purpose is to track how a match was arrived at,
2549    though it also has a secondary use in conjunction with advancing the match
2550    starting point (see (*SKIP) below).
2551    .sp
2552      (*MARK:NAME) or (*:NAME)
2553    .sp
2554    A name is always required with this verb. There may be as many instances of
2555    (*MARK) as you like in a pattern, and their names do not have to be unique.
2556    .P
2557    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2558    to the caller via the \fIpcre_extra\fP data structure, as described in the
2559    .\" HTML <a href="pcreapi.html#extradata">
2560    .\" </a>
2561    section on \fIpcre_extra\fP
2562    .\"
2563    in the
2564    .\" HREF
2565    \fBpcreapi\fP
2566    .\"
2567    documentation. No data is returned for a partial match. Here is an example of
2568    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2569    outputting of (*MARK) data:
2570    .sp
2571      /X(*MARK:A)Y|X(*MARK:B)Z/K
2572      XY
2573       0: XY
2574      MK: A
2575      XZ
2576       0: XZ
2577      MK: B
2578    .sp
2579    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2580    indicates which of the two alternatives matched. This is a more efficient way
2581    of obtaining this information than putting each alternative in its own
2582    capturing parentheses.
2583    .P
2584    A name may also be returned after a failed match if the final path through the
2585    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2586    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2587    starting point for matching is advanced, the final check is often with an empty
2588    string, causing a failure before (*MARK) is reached. For example:
2589    .sp
2590      /X(*MARK:A)Y|X(*MARK:B)Z/K
2591      XP
2592      No match
2593    .sp
2594    There are three potential starting points for this match (starting with X,
2595    starting with P, and with an empty string). If the pattern is anchored, the
2596    result is different:
2597    .sp
2598      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2599      XP
2600      No match, mark = B
2601    .sp
2602    PCRE's start-of-match optimizations can also interfere with this. For example,
2603    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2604    subject length for a match, a shorter subject will not be scanned at all.
2605    .P
2606    Note that similar anomalies (though different in detail) exist in Perl, no
2607    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2608    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2609    .
2610    .
2611    .SS "Verbs that act after backtracking"
2612    .rs
2613    .sp
2614    The following verbs do nothing when they are encountered. Matching continues
2615    with what follows, but if there is no subsequent match, causing a backtrack to
2616    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2617    the verb. However, when one of these verbs appears inside an atomic group, its
2618    effect is confined to that group, because once the group has been matched,
2619    there is never any backtracking into it. In this situation, backtracking can
2620    "jump back" to the left of the entire atomic group. (Remember also, as stated
2621    above, that this localization also applies in subroutine calls and assertions.)
2622    .P
2623    These verbs differ in exactly what kind of failure occurs when backtracking
2624    reaches them.
2625    .sp
2626      (*COMMIT)
2627    .sp
2628    This verb, which may not be followed by a name, causes the whole match to fail
2629    outright if the rest of the pattern does not match. Even if the pattern is
2630    unanchored, no further attempts to find a match by advancing the starting point
2631    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2632    finding a match at the current starting point, or not at all. For example:
2633    .sp
2634      a+(*COMMIT)b
2635    .sp
2636    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2637    dynamic anchor, or "I've started, so I must finish." The name of the most
2638    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2639    match failure.
2640    .P
2641    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2642    unless PCRE's start-of-match optimizations are turned off, as shown in this
2643    \fBpcretest\fP example:
2644    .sp
2645      /(*COMMIT)abc/
2646      xyzabc
2647       0: abc
2648      xyzabc\eY
2649      No match
2650    .sp
2651    PCRE knows that any match must start with "a", so the optimization skips along
2652    the subject to "a" before running the first match attempt, which succeeds. When
2653    the optimization is disabled by the \eY escape in the second subject, the match
2654    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2655    starting points.
2656    .sp
2657      (*PRUNE) or (*PRUNE:NAME)
2658    .sp
2659    This verb causes the match to fail at the current starting position in the
2660    subject if the rest of the pattern does not match. If the pattern is
2661    unanchored, the normal "bumpalong" advance to the next starting character then
2662    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2663    reached, or when matching to the right of (*PRUNE), but if there is no match to
2664    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2665    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2666    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2667    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2668    match fails completely; the name is passed back if this is the final attempt.
2669    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2670    pattern (*PRUNE) has the same effect as (*COMMIT).
2671    .sp
2672      (*SKIP)
2673    .sp
2674    This verb, when given without a name, is like (*PRUNE), except that if the
2675    pattern is unanchored, the "bumpalong" advance is not to the next character,
2676    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2677    signifies that whatever text was matched leading up to it cannot be part of a
2678    successful match. Consider:
2679    .sp
2680      a+(*SKIP)b
2681    .sp
2682    If the subject is "aaaac...", after the first match attempt fails (starting at
2683    the first character in the string), the starting point skips on to start the
2684    next attempt at "c". Note that a possessive quantifer does not have the same
2685    effect as this example; although it would suppress backtracking during the
2686    first match attempt, the second attempt would start at the second character
2687    instead of skipping on to "c".
2688    .sp
2689      (*SKIP:NAME)
2690    .sp
2691    When (*SKIP) has an associated name, its behaviour is modified. If the
2692    following pattern fails to match, the previous path through the pattern is
2693    searched for the most recent (*MARK) that has the same name. If one is found,
2694    the "bumpalong" advance is to the subject position that corresponds to that
2695    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2696    matching name is found, normal "bumpalong" of one character happens (the
2697    (*SKIP) is ignored).
2698    .sp
2699      (*THEN) or (*THEN:NAME)
2700    .sp
2701    This verb causes a skip to the next alternation in the innermost enclosing
2702    group if the rest of the pattern does not match. That is, it cancels pending
2703    backtracking, but only within the current alternation. Its name comes from the
2704    observation that it can be used for a pattern-based if-then-else block:
2705    .sp
2706      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2707    .sp
2708    If the COND1 pattern matches, FOO is tried (and possibly further items after
2709    the end of the group if FOO succeeds); on failure the matcher skips to the
2710    second alternative and tries COND2, without backtracking into COND1. The
2711    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2712    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2713    like (*PRUNE).
2714    .
2715    .P
2716    The above verbs provide four different "strengths" of control when subsequent
2717    matching fails. (*THEN) is the weakest, carrying on the match at the next
2718    alternation. (*PRUNE) comes next, failing the match at the current starting
2719    position, but allowing an advance to the next character (for an unanchored
2720    pattern). (*SKIP) is similar, except that the advance may be more than one
2721    character. (*COMMIT) is the strongest, causing the entire match to fail.
2722    .P
2723    If more than one is present in a pattern, the "stongest" one wins. For example,
2724    consider this pattern, where A, B, etc. are complex pattern fragments:
2725    .sp
2726      (A(*COMMIT)B(*THEN)C|D)
2727    .sp
2728    Once A has matched, PCRE is committed to this match, at the current starting
2729    position. If subsequently B matches, but C does not, the normal (*THEN) action
2730    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2731    overrides.
2732    .
2733    .
2734    .SH "SEE ALSO"
2735    .rs
2736    .sp
2737    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2738    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2739    .
2740    .
2741    .SH AUTHOR
2742    .rs
2743    .sp
2744    .nf
2745    Philip Hazel
2746    University Computing Service
2747    Cambridge CB2 3QH, England.
2748    .fi
2749    .
2750    .
2751    .SH REVISION
2752    .rs
2753    .sp
2754    .nf
2755    Last updated: 12 June 2011
2756    Copyright (c) 1997-2011 University of Cambridge.
2757    .fi

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