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revision 555 by ph10, Tue Oct 26 08:26:20 2010 UTC revision 942 by ph10, Tue Feb 28 14:50:31 2012 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  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,  there is now also support for UTF-8 strings in the original library, and a
25  PCRE must be built to include UTF-8 support, and you must call  second library that supports 16-bit and UTF-16 character strings. To use these
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  features, PCRE must be built to include appropriate support. When using UTF
27  is also a special sequence that can be given at the start of a pattern:  strings you must either call the compiling function with the PCRE_UTF8 or
28    PCRE_UTF16 option, or the pattern must start with one of these special
29    sequences:
30  .sp  .sp
31    (*UTF8)    (*UTF8)
32      (*UTF16)
33  .sp  .sp
34  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8  Starting a pattern with such a sequence is equivalent to setting the relevant
35  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  option. This feature is not Perl-compatible. How setting a UTF mode affects
36  pattern matching is mentioned in several places below. There is also a summary  pattern matching is mentioned in several places below. There is also a summary
37  of UTF-8 features in the  of features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
38  .\" HREF  .\" HREF
39  \fBpcre\fP  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42  .P  .P
43  Another special sequence that may appear at the start of a pattern or in  Another special sequence that may appear at the start of a pattern or in
44  combination with (*UTF8) is:  combination with (*UTF8) or (*UTF16) is:
45  .sp  .sp
46    (*UCP)    (*UCP)
47  .sp  .sp
# Line 52  such as \ed and \ew to use Unicode prope Line 50  such as \ed and \ew to use Unicode prope
50  instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
51  table.  table.
52  .P  .P
53    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55    also some more of these special sequences that are concerned with the handling
56    of newlines; they are described below.
57    .P
58  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
59  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60  From release 6.0, PCRE offers a second matching function,  \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62  Perl-compatible. Some of the features discussed below are not available when  a different algorithm that is not Perl-compatible. Some of the features
63  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  discussed below are not available when DFA matching is used. The advantages and
64  alternative function, and how it differs from the normal function, are  disadvantages of the alternative functions, and how they differ from the normal
65  discussed in the  functions, are discussed in the
66  .\" HREF  .\" HREF
67  \fBpcrematching\fP  \fBpcrematching\fP
68  .\"  .\"
69  page.  page.
70  .  .
71  .  .
72    .\" HTML <a name="newlines"></a>
73  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
74  .rs  .rs
75  .sp  .sp
# Line 93  string with one of the following five se Line 97  string with one of the following five se
97    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
98    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
99  .sp  .sp
100  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function. For
101  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  example, on a Unix system where LF is the default newline sequence, the pattern
 newline sequence, the pattern  
102  .sp  .sp
103    (*CR)a.b    (*CR)a.b
104  .sp  .sp
# Line 129  corresponding characters in the subject. Line 132  corresponding characters in the subject.
132  .sp  .sp
133  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
134  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
135  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
136  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
137  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
138  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
139  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
140  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
141  UTF-8 support.  UTF support.
142  .P  .P
143  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
144  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 181  The following sections describe the use Line 184  The following sections describe the use
184  .rs  .rs
185  .sp  .sp
186  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
187  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
188  may 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
189  outside character classes.  both inside and outside character classes.
190  .P  .P
191  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.
192  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 191  otherwise be interpreted as a metacharac Line 194  otherwise be interpreted as a metacharac
194  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
195  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
196  .P  .P
197    In a UTF mode, only ASCII numbers and letters have any special meaning after a
198    backslash. All other characters (in particular, those whose codepoints are
199    greater than 127) are treated as literals.
200    .P
201  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
202  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
203  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 210  Perl, $ and @ cause variable interpolati Line 217  Perl, $ and @ cause variable interpolati
217    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
218  .sp  .sp
219  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
220  An isolated \eE that is not preceded by \eQ is ignored.  An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221    by \eE later in the pattern, the literal interpretation continues to the end of
222    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223    a character class, this causes an error, because the character class is not
224    terminated.
225  .  .
226  .  .
227  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 224  but when a pattern is being prepared by Line 235  but when a pattern is being prepared by
235  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
236  .sp  .sp
237    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
238    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
239    \ee        escape (hex 1B)    \ee        escape (hex 1B)
240    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
241    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
# Line 232  one of the following escape sequences th Line 243  one of the following escape sequences th
243    \et        tab (hex 09)    \et        tab (hex 09)
244    \eddd      character with octal code ddd, or back reference    \eddd      character with octal code ddd, or back reference
245    \exhh      character with hex code hh    \exhh      character with hex code hh
246    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247      \euhhhh    character with hex code hhhh (JavaScript mode only)
248  .sp  .sp
249  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
250  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.
251  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
252  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
253  .P  than 127, a compile-time error occurs. This locks out non-ASCII characters in
254  After \ex, from zero to two hexadecimal digits are read (letters can be in  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
255  upper or lower case). Any number of hexadecimal digits may appear between \ex{  lower case letter is converted to upper case, and then the 0xc0 bits are
256  and }, but the value of the character code must be less than 256 in non-UTF-8  flipped.)
257  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  .P
258  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  By default, after \ex, from zero to two hexadecimal digits are read (letters
259  point, which is 10FFFF.  can be in upper or lower case). Any number of hexadecimal digits may appear
260    between \ex{ and }, but the character code is constrained as follows:
261    .sp
262      8-bit non-UTF mode    less than 0x100
263      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
264      16-bit non-UTF mode   less than 0x10000
265      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
266    .sp
267    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
268    "surrogate" codepoints).
269  .P  .P
270  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
271  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
272  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
273  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
274  .P  .P
275    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
276    as just described only when it is followed by two hexadecimal digits.
277    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
278    code points greater than 256 is provided by \eu, which must be followed by
279    four hexadecimal digits; otherwise it matches a literal "u" character.
280    .P
281  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
282  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
283  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
284    \eu00dc in JavaScript mode).
285  .P  .P
286  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
287  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 279  parenthesized subpatterns. Line 307  parenthesized subpatterns.
307  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
308  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
309  digits following the backslash, and uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
310  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. The value of the character is
311  character specified in octal must be less than \e400. In UTF-8 mode, values up  constrained in the same way as characters specified in hexadecimal.
312  to \e777 are permitted. For example:  For example:
313  .sp  .sp
314    \e040   is another way of writing a space    \e040   is another way of writing a space
315  .\" JOIN  .\" JOIN
# Line 298  to \e777 are permitted. For example: Line 326  to \e777 are permitted. For example:
326              character with octal code 113              character with octal code 113
327  .\" JOIN  .\" JOIN
328    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
329              the byte consisting entirely of 1 bits              the value 255 (decimal)
330  .\" JOIN  .\" JOIN
331    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
332              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 307  Note that octal values of 100 or greater Line 335  Note that octal values of 100 or greater
335  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
336  .P  .P
337  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
338  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
339  sequence \eb is interpreted as the backspace character (hex 08). The sequences  interpreted as the backspace character (hex 08).
340  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other  .P
341  unrecognized escape sequences, they are treated as the literal characters "B",  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
342  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is  inside a character class. Like other unrecognized escape sequences, they are
343  set. Outside a character class, these sequences have different meanings.  treated as the literal characters "B", "R", and "X" by default, but cause an
344    error if the PCRE_EXTRA option is set. Outside a character class, these
345    sequences have different meanings.
346    .
347    .
348    .SS "Unsupported escape sequences"
349    .rs
350    .sp
351    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
352    handler and used to modify the case of following characters. By default, PCRE
353    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
354    option is set, \eU matches a "U" character, and \eu can be used to define a
355    character by code point, as described in the previous section.
356  .  .
357  .  .
358  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 374  This is the same as Line 414  This is the same as
414  .\" </a>  .\" </a>
415  the "." metacharacter  the "." metacharacter
416  .\"  .\"
417  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
418    PCRE does not support this.
419  .P  .P
420  Each pair of lower and upper case escape sequences partitions the complete set  Each pair of lower and upper case escape sequences partitions the complete set
421  of characters into two disjoint sets. Any given character matches one, and only  of characters into two disjoint sets. Any given character matches one, and only
# Line 406  or "french" in Windows, some character c Line 447  or "french" in Windows, some character c
447  accented letters, and these are then matched by \ew. The use of locales with  accented letters, and these are then matched by \ew. The use of locales with
448  Unicode is discouraged.  Unicode is discouraged.
449  .P  .P
450  By default, in UTF-8 mode, characters with values greater than 128 never match  By default, in a UTF mode, characters with values greater than 128 never match
451  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
452  their original meanings from before UTF-8 support was available, mainly for  their original meanings from before UTF support was available, mainly for
453  efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
454  and the PCRE_UCP option is set, the behaviour is changed so that Unicode  and the PCRE_UCP option is set, the behaviour is changed so that Unicode
455  properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
# Line 423  any Unicode letter, and underscore. Note Line 464  any Unicode letter, and underscore. Note
464  \eB because they are defined in terms of \ew and \eW. Matching these sequences  \eB because they are defined in terms of \ew and \eW. Matching these sequences
465  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
466  .P  .P
467  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
468  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
469  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is  characters by default, these always match certain high-valued codepoints,
470  set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
471  .sp  .sp
472    U+0009     Horizontal tab    U+0009     Horizontal tab
473    U+0020     Space    U+0020     Space
# Line 457  The vertical space characters are: Line 498  The vertical space characters are:
498    U+0085     Next line    U+0085     Next line
499    U+2028     Line separator    U+2028     Line separator
500    U+2029     Paragraph separator    U+2029     Paragraph separator
501    .sp
502    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
503    relevant.
504  .  .
505  .  .
506  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 464  The vertical space characters are: Line 508  The vertical space characters are:
508  .rs  .rs
509  .sp  .sp
510  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
511  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
512  equivalent to the following:  following:
513  .sp  .sp
514    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
515  .sp  .sp
# Line 480  U+000B), FF (formfeed, U+000C), CR (carr Line 524  U+000B), FF (formfeed, U+000C), CR (carr
524  line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
525  cannot be split.  cannot be split.
526  .P  .P
527  In UTF-8 mode, two additional characters whose codepoints are greater than 255  In other modes, two additional characters whose codepoints are greater than 255
528  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
529  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
530  recognized.  recognized.
# Line 496  one of the following sequences: Line 540  one of the following sequences:
540    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
541    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
542  .sp  .sp
543  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function, but
544  \fBpcre_compile2()\fP, but they can be overridden by options given to  they can themselves be overridden by options given to a matching function. Note
545  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  that these special settings, which are not Perl-compatible, are recognized only
546  which are not Perl-compatible, are recognized only at the very start of a  at the very start of a pattern, and that they must be in upper case. If more
547  pattern, and that they must be in upper case. If more than one of them is  than one of them is present, the last one is used. They can be combined with a
548  present, the last one is used. They can be combined with a change of newline  change of newline convention; for example, a pattern can start with:
 convention; for example, a pattern can start with:  
549  .sp  .sp
550    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
551  .sp  .sp
552  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
553  a character class, \eR is treated as an unrecognized escape sequence, and so  sequences. Inside a character class, \eR is treated as an unrecognized escape
554  matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.  sequence, and so matches the letter "R" by default, but causes an error if
555    PCRE_EXTRA is set.
556  .  .
557  .  .
558  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 517  matches the letter "R" by default, but c Line 561  matches the letter "R" by default, but c
561  .sp  .sp
562  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
563  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
564  When not in UTF-8 mode, these sequences are of course limited to testing  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
565  characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
566  The extra escape sequences are:  The extra escape sequences are:
567  .sp  .sp
# Line 552  Armenian, Line 596  Armenian,
596  Avestan,  Avestan,
597  Balinese,  Balinese,
598  Bamum,  Bamum,
599    Batak,
600  Bengali,  Bengali,
601  Bopomofo,  Bopomofo,
602    Brahmi,
603  Braille,  Braille,
604  Buginese,  Buginese,
605  Buhid,  Buhid,
606  Canadian_Aboriginal,  Canadian_Aboriginal,
607  Carian,  Carian,
608    Chakma,
609  Cham,  Cham,
610  Cherokee,  Cherokee,
611  Common,  Common,
# Line 601  Lisu, Line 648  Lisu,
648  Lycian,  Lycian,
649  Lydian,  Lydian,
650  Malayalam,  Malayalam,
651    Mandaic,
652  Meetei_Mayek,  Meetei_Mayek,
653    Meroitic_Cursive,
654    Meroitic_Hieroglyphs,
655    Miao,
656  Mongolian,  Mongolian,
657  Myanmar,  Myanmar,
658  New_Tai_Lue,  New_Tai_Lue,
# Line 620  Rejang, Line 671  Rejang,
671  Runic,  Runic,
672  Samaritan,  Samaritan,
673  Saurashtra,  Saurashtra,
674    Sharada,
675  Shavian,  Shavian,
676  Sinhala,  Sinhala,
677    Sora_Sompeng,
678  Sundanese,  Sundanese,
679  Syloti_Nagri,  Syloti_Nagri,
680  Syriac,  Syriac,
# Line 630  Tagbanwa, Line 683  Tagbanwa,
683  Tai_Le,  Tai_Le,
684  Tai_Tham,  Tai_Tham,
685  Tai_Viet,  Tai_Viet,
686    Takri,
687  Tamil,  Tamil,
688  Telugu,  Telugu,
689  Thaana,  Thaana,
# Line 704  the Lu, Ll, or Lt property, in other wor Line 758  the Lu, Ll, or Lt property, in other wor
758  a modifier or "other".  a modifier or "other".
759  .P  .P
760  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
761  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
762  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
763  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
764  .\" HREF  .\" HREF
765  \fBpcreapi\fP  \fBpcreapi\fP
766  .\"  .\"
# Line 737  atomic group Line 791  atomic group
791  .\"  .\"
792  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
793  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
794  non-UTF-8 mode \eX matches any one character.  8-bit non-UTF-8 mode \eX matches any one character.
795    .P
796    Note that recent versions of Perl have changed \eX to match what Unicode calls
797    an "extended grapheme cluster", which has a more complicated definition.
798  .P  .P
799  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
800  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
801  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
802  properties in PCRE by default, though you can make them do so by setting the  properties in PCRE by default, though you can make them do so by setting the
803  PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with  PCRE_UCP option or by starting the pattern with (*UCP).
 (*UCP).  
804  .  .
805  .  .
806  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
# Line 773  same characters as Xan, plus underscore. Line 829  same characters as Xan, plus underscore.
829  .SS "Resetting the match start"  .SS "Resetting the match start"
830  .rs  .rs
831  .sp  .sp
832  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
833  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
834  .sp  .sp
835    foo\eKbar    foo\eKbar
836  .sp  .sp
# Line 834  escape sequence" error is generated inst Line 889  escape sequence" error is generated inst
889  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
890  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
891  \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
892  first or last character matches \ew, respectively. In UTF-8 mode, the meanings  first or last character matches \ew, respectively. In a UTF mode, the meanings
893  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
894  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
895  of word" or "end of word" metasequence. However, whatever follows \eb normally  of word" or "end of word" metasequence. However, whatever follows \eb normally
# Line 929  end of the subject in both modes, and if Line 984  end of the subject in both modes, and if
984  .sp  .sp
985  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
986  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
987  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
988  .P  .P
989  When a line ending is defined as a single character, dot never matches that  When a line ending is defined as a single character, dot never matches that
990  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 947  The handling of dot is entirely independ Line 1002  The handling of dot is entirely independ
1002  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
1003  special meaning in a character class.  special meaning in a character class.
1004  .P  .P
1005  The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not  The escape sequence \eN behaves like a dot, except that it is not affected by
1006  set. In other words, it matches any one character except one that signifies the  the PCRE_DOTALL option. In other words, it matches any character except one
1007  end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
1008  .  name; PCRE does not support this.
1009  .  .
1010  .SH "MATCHING A SINGLE BYTE"  .
1011  .rs  .SH "MATCHING A SINGLE DATA UNIT"
1012  .sp  .rs
1013  Outside a character class, the escape sequence \eC matches any one byte, both  .sp
1014  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  Outside a character class, the escape sequence \eC matches any one data unit,
1015  characters. The feature is provided in Perl in order to match individual bytes  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1016  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1017  what remains in the string may be a malformed UTF-8 string. For this reason,  matches line-ending characters. The feature is provided in Perl in order to
1018  the \eC escape sequence is best avoided.  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1019    used. Because \eC breaks up characters into individual data units, matching one
1020    unit with \eC in a UTF mode means that the rest of the string may start with a
1021    malformed UTF character. This has undefined results, because PCRE assumes that
1022    it is dealing with valid UTF strings (and by default it checks this at the
1023    start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1024  .P  .P
1025  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1026  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1027  .\" </a>  .\" </a>
1028  (described below),  (described below)
1029  .\"  .\"
1030  because in UTF-8 mode this would make it impossible to calculate the length of  in a UTF mode, because this would make it impossible to calculate the length of
1031  the lookbehind.  the lookbehind.
1032    .P
1033    In general, the \eC escape sequence is best avoided. However, one
1034    way of using it that avoids the problem of malformed UTF characters is to use a
1035    lookahead to check the length of the next character, as in this pattern, which
1036    could be used with a UTF-8 string (ignore white space and line breaks):
1037    .sp
1038      (?| (?=[\ex00-\ex7f])(\eC) |
1039          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1040          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1041          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1042    .sp
1043    A group that starts with (?| resets the capturing parentheses numbers in each
1044    alternative (see
1045    .\" HTML <a href="#dupsubpatternnumber">
1046    .\" </a>
1047    "Duplicate Subpattern Numbers"
1048    .\"
1049    below). The assertions at the start of each branch check the next UTF-8
1050    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1051    character's individual bytes are then captured by the appropriate number of
1052    groups.
1053  .  .
1054  .  .
1055  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 982  bracket causes a compile-time error. If Line 1063  bracket causes a compile-time error. If
1063  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
1064  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1065  .P  .P
1066  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 a UTF mode, the
1067  character may be more than one byte long. A matched character must be in the  character may be more than one data unit long. A matched character must be in
1068  set of characters defined by the class, unless the first character in the class  the set of characters defined by the class, unless the first character in the
1069  definition is a circumflex, in which case the subject character must not be in  class definition is a circumflex, in which case the subject character must not
1070  the set defined by the class. If a circumflex is actually required as a member  be in the set defined by the class. If a circumflex is actually required as a
1071  of the class, ensure it is not the first character, or escape it with a  member of the class, ensure it is not the first character, or escape it with a
1072  backslash.  backslash.
1073  .P  .P
1074  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
# Line 998  circumflex is not an assertion; it still Line 1079  circumflex is not an assertion; it still
1079  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
1080  string.  string.
1081  .P  .P
1082  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1083  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  included in a class as a literal string of data units, or by using the \ex{
1084    escaping mechanism.
1085  .P  .P
1086  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
1087  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
1088  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1089  caseful version would. In UTF-8 mode, PCRE always understands the concept of  caseful version would. In a UTF mode, PCRE always understands the concept of
1090  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
1091  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1092  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1093  If you want to use caseless matching in UTF8-mode for characters 128 and above,  If you want to use caseless matching in a UTF mode for characters 128 and
1094  you must ensure that PCRE is compiled with Unicode property support as well as  above, you must ensure that PCRE is compiled with Unicode property support as
1095  with UTF-8 support.  well as with UTF support.
1096  .P  .P
1097  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1098  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 1032  followed by two other characters. The oc Line 1114  followed by two other characters. The oc
1114  "]" can also be used to end a range.  "]" can also be used to end a range.
1115  .P  .P
1116  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
1117  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1118  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\ex{100}-\ex{2ff}].  
1119  .P  .P
1120  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
1121  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
1122  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1123  tables for a French locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1124  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1125  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
1126  property support.  property support.
1127  .P  .P
1128  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1129  \eW may also appear in a character class, and add the characters that they  \eV, \ew, and \eW may appear in a character class, and add the characters that
1130  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1131  circumflex can conveniently be used with the upper case character types to  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1132    and their upper case partners, just as it does when they appear outside a
1133    character class, as described in the section entitled
1134    .\" HTML <a href="#genericchartypes">
1135    .\" </a>
1136    "Generic character types"
1137    .\"
1138    above. The escape sequence \eb has a different meaning inside a character
1139    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1140    are not special inside a character class. Like any other unrecognized escape
1141    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1142    default, but cause an error if the PCRE_EXTRA option is set.
1143    .P
1144    A circumflex can conveniently be used with the upper case character types to
1145  specify a more restricted set of characters than the matching lower case type.  specify a more restricted set of characters than the matching lower case type.
1146  For example, the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore,
1147    whereas [\ew] includes underscore. A positive character class should be read as
1148    "something OR something OR ..." and a negative class as "NOT something AND NOT
1149    something AND NOT ...".
1150  .P  .P
1151  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1152  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 1101  matches "1", "2", or any non-digit. PCRE Line 1198  matches "1", "2", or any non-digit. PCRE
1198  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
1199  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1200  .P  .P
1201  By default, in UTF-8 mode, characters with values greater than 128 do not match  By default, in UTF modes, characters with values greater than 128 do not match
1202  any of the POSIX character classes. However, if the PCRE_UCP option is passed  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1203  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1204  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
# Line 1172  extracts it into the global options (and Line 1269  extracts it into the global options (and
1269  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1270  .P  .P
1271  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1272  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1273  .sp  .sp
1274    (a(?i)b)c    (a(?i)b)c
1275  .sp  .sp
# Line 1189  option settings happen at compile time. Line 1286  option settings happen at compile time.
1286  behaviour otherwise.  behaviour otherwise.
1287  .P  .P
1288  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1289  application when the compile or match functions are called. In some cases the  application when the compiling or matching functions are called. In some cases
1290  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1291  the application has set or what has been defaulted. Details are given in the  what the application has set or what has been defaulted. Details are given in
1292  section entitled  the section entitled
1293  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1294  .\" </a>  .\" </a>
1295  "Newline sequences"  "Newline sequences"
1296  .\"  .\"
1297  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1298  to set UTF-8 and Unicode property modes; they are equivalent to setting the  can be used to set UTF and Unicode property modes; they are equivalent to
1299  PCRE_UTF8 and the PCRE_UCP options, respectively.  setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1300  .  .
1301  .  .
1302  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1213  Turning part of a pattern into a subpatt Line 1310  Turning part of a pattern into a subpatt
1310  .sp  .sp
1311    cat(aract|erpillar|)    cat(aract|erpillar|)
1312  .sp  .sp
1313  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1314  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1315  .sp  .sp
1316  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
1317  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
1318  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 the
1319  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1320  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1321  .P  .P
1322  For example, if the string "the red king" is matched against the pattern  Opening parentheses are counted from left to right (starting from 1) to obtain
1323    numbers for the capturing subpatterns. For example, if the string "the red
1324    king" is matched against the pattern
1325  .sp  .sp
1326    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1327  .sp  .sp
# Line 1271  at captured substring number one, whiche Line 1370  at captured substring number one, whiche
1370  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1371  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1372  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1373  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1374  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1375  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1376  .sp  .sp
1377    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1378    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# Line 1286  or "defdef": Line 1384  or "defdef":
1384  .sp  .sp
1385    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1386  .sp  .sp
1387  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1388  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1389  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1390  .sp  .sp
1391    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1392  .sp  .sp
# Line 1399  items: Line 1497  items:
1497    a literal data character    a literal data character
1498    the dot metacharacter    the dot metacharacter
1499    the \eC escape sequence    the \eC escape sequence
1500    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1501    the \eR escape sequence    the \eR escape sequence
1502    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1503    a character class    a character class
1504    a back reference (see next section)    a back reference (see next section)
1505    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1506    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1507  .sp  .sp
1508  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1509  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1430  where a quantifier is not allowed, or on Line 1528  where a quantifier is not allowed, or on
1528  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1529  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1530  .P  .P
1531  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1532  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1533  which is represented by a two-byte sequence. Similarly, when Unicode property  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1534  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended sequences, each of which may be several
1535  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1536  .P  .P
1537  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
1538  previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
# Line 1443  subpatterns that are referenced as Line 1541  subpatterns that are referenced as
1541  .\" </a>  .\" </a>
1542  subroutines  subroutines
1543  .\"  .\"
1544  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1545  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1546    .\" </a>
1547    "Defining subpatterns for use by reference only"
1548    .\"
1549    below). Items other than subpatterns that have a {0} quantifier are omitted
1550    from the compiled pattern.
1551  .P  .P
1552  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1553  abbreviations:  abbreviations:
# Line 1669  no such problem when named parentheses a Line 1772  no such problem when named parentheses a
1772  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1773  .P  .P
1774  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1775  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1776  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1777  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1778  .sp  .sp
1779    (ring), \e1    (ring), \e1
1780    (ring), \eg1    (ring), \eg1
# Line 1685  example: Line 1788  example:
1788    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1789  .sp  .sp
1790  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1791  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1792  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1793  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1794  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1795  .P  .P
1796  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1797  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1790  those that look ahead of the current pos Line 1893  those that look ahead of the current pos
1893  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1894  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1895  .P  .P
1896  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1897  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1898  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1899  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1900  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1901  because it does not make sense for negative assertions.  .P
1902    For compatibility with Perl, assertion subpatterns may be repeated; though
1903    it makes no sense to assert the same thing several times, the side effect of
1904    capturing parentheses may occasionally be useful. In practice, there only three
1905    cases:
1906    .sp
1907    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1908    However, it may contain internal capturing parenthesized groups that are called
1909    from elsewhere via the
1910    .\" HTML <a href="#subpatternsassubroutines">
1911    .\" </a>
1912    subroutine mechanism.
1913    .\"
1914    .sp
1915    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1916    were {0,1}. At run time, the rest of the pattern match is tried with and
1917    without the assertion, the order depending on the greediness of the quantifier.
1918    .sp
1919    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1920    The assertion is obeyed just once when encountered during matching.
1921  .  .
1922  .  .
1923  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1824  lookbehind assertion is needed to achiev Line 1946  lookbehind assertion is needed to achiev
1946  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
1947  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
1948  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.
1949  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1950  .  .
1951  .  .
1952  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1850  is permitted, but Line 1971  is permitted, but
1971  .sp  .sp
1972  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1973  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
1974  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1975  match the same length of string. An assertion such as  length of string. An assertion such as
1976  .sp  .sp
1977    (?<=ab(c|de))    (?<=ab(c|de))
1978  .sp  .sp
# Line 1861  branches: Line 1982  branches:
1982  .sp  .sp
1983    (?<=abc|abde)    (?<=abc|abde)
1984  .sp  .sp
1985  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1986  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1987  .\" </a>  .\" </a>
1988  (see above)  (see above)
# Line 1874  temporarily move the current position ba Line 1995  temporarily move the current position ba
1995  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1996  assertion fails.  assertion fails.
1997  .P  .P
1998  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
1999  to appear in lookbehind assertions, because it makes it impossible to calculate  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2000  the length of the lookbehind. The \eX and \eR escapes, which can match  it impossible to calculate the length of the lookbehind. The \eX and \eR
2001  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2002    permitted.
2003  .P  .P
2004  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2005  .\" </a>  .\" </a>
# Line 1965  already been matched. The two possible f Line 2087  already been matched. The two possible f
2087  .sp  .sp
2088  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2089  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
2090  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2091    itself contain nested subpatterns of any form, including conditional
2092    subpatterns; the restriction to two alternatives applies only at the level of
2093    the condition. This pattern fragment is an example where the alternatives are
2094    complex:
2095    .sp
2096      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2097    .sp
2098  .P  .P
2099  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2100  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1982  matched. If there is more than one captu Line 2111  matched. If there is more than one captu
2111  .\" </a>  .\" </a>
2112  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2113  .\"  .\"
2114  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2115  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2116  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2117  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2118  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2119  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2120    zero in any of these forms is not used; it provokes a compile-time error.)
2121  .P  .P
2122  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2123  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1998  three parts for ease of discussion: Line 2128  three parts for ease of discussion:
2128  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2129  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
2130  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
2131  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2132  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,
2133  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
2134  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2135  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 2055  The syntax for recursive patterns Line 2185  The syntax for recursive patterns
2185  .\"  .\"
2186  is described below.  is described below.
2187  .  .
2188    .\" HTML <a name="subdefine"></a>
2189  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2190  .rs  .rs
2191  .sp  .sp
# Line 2062  If the condition is the string (DEFINE), Line 2193  If the condition is the string (DEFINE),
2193  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2194  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2195  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2196  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2197  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2198  .\" </a>  .\" </a>
2199  "subroutines"  subroutines
2200  .\"  .\"
2201  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2202  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2203    breaks):
2204  .sp  .sp
2205    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2206    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2103  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2235  dd-aaa-dd or dd-dd-dd, where aaa are let
2235  .SH COMMENTS  .SH COMMENTS
2236  .rs  .rs
2237  .sp  .sp
2238  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
2239  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,
2240  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
2241    subpattern name or number. The characters that make up a comment play no part
2242    in the pattern matching.
2243  .P  .P
2244  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
2245  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2246  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2247    this case continues to immediately after the next newline character or
2248    character sequence in the pattern. Which characters are interpreted as newlines
2249    is controlled by the options passed to a compiling function or by a special
2250    sequence at the start of the pattern, as described in the section entitled
2251    .\" HTML <a href="#newlines">
2252    .\" </a>
2253    "Newline conventions"
2254    .\"
2255    above. Note that the end of this type of comment is a literal newline sequence
2256    in the pattern; escape sequences that happen to represent a newline do not
2257    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2258    default newline convention is in force:
2259    .sp
2260      abc #comment \en still comment
2261    .sp
2262    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2263    a newline in the pattern. The sequence \en is still literal at this stage, so
2264    it does not terminate the comment. Only an actual character with the code value
2265    0x0a (the default newline) does so.
2266  .  .
2267  .  .
2268  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2138  individual subpattern recursion. After i Line 2291  individual subpattern recursion. After i
2291  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2292  .P  .P
2293  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2294  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2295  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2296  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2297  .\" </a>  .\" </a>
2298  "subroutine"  non-recursive subroutine
2299  .\"  .\"
2300  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2301  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2167  We have put the pattern into parentheses Line 2320  We have put the pattern into parentheses
2320  them instead of the whole pattern.  them instead of the whole pattern.
2321  .P  .P
2322  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2323  is made easier by the use of relative references (a Perl 5.10 feature).  is made easier by the use of relative references. Instead of (?1) in the
2324  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2325  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2326  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2327  .P  .P
2328  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2329  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2330  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2331  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2332  .\" </a>  .\" </a>
2333  "subroutine"  non-recursive subroutine
2334  .\"  .\"
2335  calls, as described in the next section.  calls, as described in the next section.
2336  .P  .P
# Line 2215  documentation). If the pattern above is Line 2367  documentation). If the pattern above is
2367  .sp  .sp
2368  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2369  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2370  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2371  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2372  .P  .P
2373  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2374  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2236  is the actual recursive call. Line 2388  is the actual recursive call.
2388  .  .
2389  .  .
2390  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2391  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2392  .rs  .rs
2393  .sp  .sp
2394  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2395  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2396  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2397  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2398  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2399  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2400    characters (for example, "a", "aba", "abcba", "abcdcba"):
2401  .sp  .sp
2402    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2403  .sp  .sp
# Line 2274  time we do have another alternative to t Line 2427  time we do have another alternative to t
2427  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2428  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2429  .P  .P
2430  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2431  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2432    this:
2433  .sp  .sp
2434    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2435  .sp  .sp
# Line 2304  For example, although "abcba" is correct Line 2458  For example, although "abcba" is correct
2458  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2459  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2460  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2461    .P
2462    The second way in which PCRE and Perl differ in their recursion processing is
2463    in the handling of captured values. In Perl, when a subpattern is called
2464    recursively or as a subpattern (see the next section), it has no access to any
2465    values that were captured outside the recursion, whereas in PCRE these values
2466    can be referenced. Consider this pattern:
2467    .sp
2468      ^(.)(\e1|a(?2))
2469    .sp
2470    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2471    then in the second group, when the back reference \e1 fails to match "b", the
2472    second alternative matches "a" and then recurses. In the recursion, \e1 does
2473    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2474    match because inside the recursive call \e1 cannot access the externally set
2475    value.
2476  .  .
2477  .  .
2478  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2479  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2480  .rs  .rs
2481  .sp  .sp
2482  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2483  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
2484  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2485  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2486  relative, as in these examples:  relative, as in these examples:
2487  .sp  .sp
# Line 2332  matches "sense and sensibility" and "res Line 2501  matches "sense and sensibility" and "res
2501  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
2502  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2503  .P  .P
2504  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2505  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2506  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2507  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2508  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2509  .P  .P
2510  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2511  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2512  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2513  .sp  .sp
2514    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2515  .sp  .sp
# Line 2379  same pair of parentheses when there is a Line 2548  same pair of parentheses when there is a
2548  .P  .P
2549  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2550  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
2551  function by putting its entry point in the global variable \fIpcre_callout\fP.  function by putting its entry point in the global variable \fIpcre_callout\fP
2552  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2553    variable contains NULL, which disables all calling out.
2554  .P  .P
2555  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2556  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
# Line 2389  For example, this pattern has two callou Line 2559  For example, this pattern has two callou
2559  .sp  .sp
2560    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2561  .sp  .sp
2562  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2563  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2564  255.  255.
2565  .P  .P
2566  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  During matching, when PCRE reaches a callout point, the external function is
2567  set), the external function is called. It is provided with the number of the  called. It is provided with the number of the callout, the position in the
2568  callout, the position in the pattern, and, optionally, one item of data  pattern, and, optionally, one item of data originally supplied by the caller of
2569  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2570  may cause matching to proceed, to backtrack, or to fail altogether. A complete  backtrack, or to fail altogether. A complete description of the interface to
2571  description of the interface to the callout function is given in the  the callout function is given in the
2572  .\" HREF  .\" HREF
2573  \fBpcrecallout\fP  \fBpcrecallout\fP
2574  .\"  .\"
# Line 2416  production code should be noted to avoid Line 2586  production code should be noted to avoid
2586  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2587  .P  .P
2588  Since these verbs are specifically related to backtracking, most of them can be  Since these verbs are specifically related to backtracking, most of them can be
2589  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses  used only when the pattern is to be matched using one of the traditional
2590  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2591  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2592  \fBpcre_dfa_exec()\fP.  if encountered by a DFA matching function.
2593  .P  .P
2594  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or in a subpattern that is
2595  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2596  it does not extend to the surrounding pattern. Note that such subpatterns are  to that subpattern; it does not extend to the surrounding pattern, with one
2597  processed as anchored at the point where they are tested.  exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2598    a successful positive assertion \fIis\fP passed back when a match succeeds
2599    (compare capturing parentheses in assertions). Note that such subpatterns are
2600    processed as anchored at the point where they are tested. Note also that Perl's
2601    treatment of subroutines is different in some cases.
2602  .P  .P
2603  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2604  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2605  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2606  depending on whether or not an argument is present. An name is a sequence of  depending on whether or not an argument is present. A name is any sequence of
2607  letters, digits, and underscores. If the name is empty, that is, if the closing  characters that does not include a closing parenthesis. If the name is empty,
2608  parenthesis immediately follows the colon, the effect is as if the colon were  that is, if the closing parenthesis immediately follows the colon, the effect
2609  not there. Any number of these verbs may occur in a pattern.  is as if the colon were not there. Any number of these verbs may occur in a
2610  .P  pattern.
2611    .
2612    .
2613    .\" HTML <a name="nooptimize"></a>
2614    .SS "Optimizations that affect backtracking verbs"
2615    .rs
2616    .sp
2617  PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
2618  some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
2619  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2620  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations suppresses the running of a match, any
2621  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2622  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2623  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2624    pattern with (*NO_START_OPT). There is more discussion of this option in the
2625    section entitled
2626    .\" HTML <a href="pcreapi.html#execoptions">
2627    .\" </a>
2628    "Option bits for \fBpcre_exec()\fP"
2629    .\"
2630    in the
2631    .\" HREF
2632    \fBpcreapi\fP
2633    .\"
2634    documentation.
2635    .P
2636    Experiments with Perl suggest that it too has similar optimizations, sometimes
2637    leading to anomalous results.
2638  .  .
2639  .  .
2640  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2452  followed by a name. Line 2646  followed by a name.
2646     (*ACCEPT)     (*ACCEPT)
2647  .sp  .sp
2648  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2649  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2650  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2651  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2652    far is captured. For example:
2653  .sp  .sp
2654    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2655  .sp  .sp
# Line 2463  the outer parentheses. Line 2658  the outer parentheses.
2658  .sp  .sp
2659    (*FAIL) or (*F)    (*FAIL) or (*F)
2660  .sp  .sp
2661  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2662  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2663  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2664  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2487  starting point (see (*SKIP) below). Line 2682  starting point (see (*SKIP) below).
2682  A name is always required with this verb. There may be as many instances of  A name is always required with this verb. There may be as many instances of
2683  (*MARK) as you like in a pattern, and their names do not have to be unique.  (*MARK) as you like in a pattern, and their names do not have to be unique.
2684  .P  .P
2685  When a match succeeds, the name of the last-encountered (*MARK) is passed back  When a match succeeds, the name of the last-encountered (*MARK) on the matching
2686  to the caller via the \fIpcre_extra\fP data structure, as described in the  path is passed back to the caller as described in the section entitled
2687  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2688  .\" </a>  .\" </a>
2689  section on \fIpcre_extra\fP  "Extra data for \fBpcre_exec()\fP"
2690  .\"  .\"
2691  in the  in the
2692  .\" HREF  .\" HREF
2693  \fBpcreapi\fP  \fBpcreapi\fP
2694  .\"  .\"
2695  documentation. No data is returned for a partial match. Here is an example of  documentation. Here is an example of \fBpcretest\fP output, where the /K
2696  \fBpcretest\fP output, where the /K modifier requests the retrieval and  modifier requests the retrieval and outputting of (*MARK) data:
 outputting of (*MARK) data:  
2697  .sp  .sp
2698    /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2699    XY    data> XY
2700     0: XY     0: XY
2701    MK: A    MK: A
2702    XZ    XZ
# Line 2514  indicates which of the two alternatives Line 2708  indicates which of the two alternatives
2708  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2709  capturing parentheses.  capturing parentheses.
2710  .P  .P
2711  A name may also be returned after a failed match if the final path through the  If (*MARK) is encountered in a positive assertion, its name is recorded and
2712  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  passed back if it is the last-encountered. This does not happen for negative
2713  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  assertions.
2714  starting point for matching is advanced, the final check is often with an empty  .P
2715  string, causing a failure before (*MARK) is reached. For example:  After a partial match or a failed match, the name of the last encountered
2716  .sp  (*MARK) in the entire match process is returned. For example:
   /X(*MARK:A)Y|X(*MARK:B)Z/K  
   XP  
   No match  
 .sp  
 There are three potential starting points for this match (starting with X,  
 starting with P, and with an empty string). If the pattern is anchored, the  
 result is different:  
2717  .sp  .sp
2718    /^X(*MARK:A)Y|^X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2719    XP    data> XP
2720    No match, mark = B    No match, mark = B
2721  .sp  .sp
2722  PCRE's start-of-match optimizations can also interfere with this. For example,  Note that in this unanchored example the mark is retained from the match
2723  if, as a result of a call to \fBpcre_study()\fP, it knows the minimum  attempt that started at the letter "X" in the subject. Subsequent match
2724  subject length for a match, a shorter subject will not be scanned at all.  attempts starting at "P" and then with an empty string do not get as far as the
2725  .P  (*MARK) item, but nevertheless do not reset it.
2726  Note that similar anomalies (though different in detail) exist in Perl, no  .P
2727  doubt for the same reasons. The use of (*MARK) data after a failed match of an  If you are interested in (*MARK) values after failed matches, you should
2728  unanchored pattern is not recommended, unless (*COMMIT) is involved.  probably set the PCRE_NO_START_OPTIMIZE option
2729    .\" HTML <a href="#nooptimize">
2730    .\" </a>
2731    (see above)
2732    .\"
2733    to ensure that the match is always attempted.
2734  .  .
2735  .  .
2736  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
# Line 2575  Note that (*COMMIT) at the start of a pa Line 2767  Note that (*COMMIT) at the start of a pa
2767  unless PCRE's start-of-match optimizations are turned off, as shown in this  unless PCRE's start-of-match optimizations are turned off, as shown in this
2768  \fBpcretest\fP example:  \fBpcretest\fP example:
2769  .sp  .sp
2770    /(*COMMIT)abc/      re> /(*COMMIT)abc/
2771    xyzabc    data> xyzabc
2772     0: abc     0: abc
2773    xyzabc\eY    xyzabc\eY
2774    No match    No match
# Line 2597  reached, or when matching to the right o Line 2789  reached, or when matching to the right o
2789  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2790  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2791  but there are some uses of (*PRUNE) that cannot be expressed in any other way.  but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2792  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2793  match fails completely; the name is passed back if this is the final attempt.  anchored pattern (*PRUNE) has the same effect as (*COMMIT).
 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored  
 pattern (*PRUNE) has the same effect as (*COMMIT).  
2794  .sp  .sp
2795    (*SKIP)    (*SKIP)
2796  .sp  .sp
# Line 2626  following pattern fails to match, the pr Line 2816  following pattern fails to match, the pr
2816  searched for the most recent (*MARK) that has the same name. If one is found,  searched for the most recent (*MARK) that has the same name. If one is found,
2817  the "bumpalong" advance is to the subject position that corresponds to that  the "bumpalong" advance is to the subject position that corresponds to that
2818  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2819  matching name is found, normal "bumpalong" of one character happens (the  matching name is found, the (*SKIP) is ignored.
 (*SKIP) is ignored).  
2820  .sp  .sp
2821    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2822  .sp  .sp
2823  This verb causes a skip to the next alternation in the innermost enclosing  This verb causes a skip to the next innermost alternative if the rest of the
2824  group if the rest of the pattern does not match. That is, it cancels pending  pattern does not match. That is, it cancels pending backtracking, but only
2825  backtracking, but only within the current alternation. Its name comes from the  within the current alternative. Its name comes from the observation that it can
2826  observation that it can be used for a pattern-based if-then-else block:  be used for a pattern-based if-then-else block:
2827  .sp  .sp
2828    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2829  .sp  .sp
2830  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2831  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2832  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. The
2833  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2834  overall match fails. If (*THEN) is not directly inside an alternation, it acts  If (*THEN) is not inside an alternation, it acts like (*PRUNE).
 like (*PRUNE).  
 .  
 .P  
 The above verbs provide four different "strengths" of control when subsequent  
 matching fails. (*THEN) is the weakest, carrying on the match at the next  
 alternation. (*PRUNE) comes next, failing the match at the current starting  
 position, but allowing an advance to the next character (for an unanchored  
 pattern). (*SKIP) is similar, except that the advance may be more than one  
 character. (*COMMIT) is the strongest, causing the entire match to fail.  
2835  .P  .P
2836  If more than one is present in a pattern, the "stongest" one wins. For example,  Note that a subpattern that does not contain a | character is just a part of
2837  consider this pattern, where A, B, etc. are complex pattern fragments:  the enclosing alternative; it is not a nested alternation with only one
2838    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2839    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2840    pattern fragments that do not contain any | characters at this level:
2841    .sp
2842      A (B(*THEN)C) | D
2843    .sp
2844    If A and B are matched, but there is a failure in C, matching does not
2845    backtrack into A; instead it moves to the next alternative, that is, D.
2846    However, if the subpattern containing (*THEN) is given an alternative, it
2847    behaves differently:
2848    .sp
2849      A (B(*THEN)C | (*FAIL)) | D
2850    .sp
2851    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2852    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2853    because there are no more alternatives to try. In this case, matching does now
2854    backtrack into A.
2855    .P
2856    Note also that a conditional subpattern is not considered as having two
2857    alternatives, because only one is ever used. In other words, the | character in
2858    a conditional subpattern has a different meaning. Ignoring white space,
2859    consider:
2860    .sp
2861      ^.*? (?(?=a) a | b(*THEN)c )
2862    .sp
2863    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2864    it initially matches zero characters. The condition (?=a) then fails, the
2865    character "b" is matched, but "c" is not. At this point, matching does not
2866    backtrack to .*? as might perhaps be expected from the presence of the |
2867    character. The conditional subpattern is part of the single alternative that
2868    comprises the whole pattern, and so the match fails. (If there was a backtrack
2869    into .*?, allowing it to match "b", the match would succeed.)
2870    .P
2871    The verbs just described provide four different "strengths" of control when
2872    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2873    next alternative. (*PRUNE) comes next, failing the match at the current
2874    starting position, but allowing an advance to the next character (for an
2875    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2876    than one character. (*COMMIT) is the strongest, causing the entire match to
2877    fail.
2878    .P
2879    If more than one such verb is present in a pattern, the "strongest" one wins.
2880    For example, consider this pattern, where A, B, etc. are complex pattern
2881    fragments:
2882  .sp  .sp
2883    (A(*COMMIT)B(*THEN)C|D)    (A(*COMMIT)B(*THEN)C|D)
2884  .sp  .sp
2885  Once A has matched, PCRE is committed to this match, at the current starting  Once A has matched, PCRE is committed to this match, at the current starting
2886  position. If subsequently B matches, but C does not, the normal (*THEN) action  position. If subsequently B matches, but C does not, the normal (*THEN) action
2887  of trying the next alternation (that is, D) does not happen because (*COMMIT)  of trying the next alternative (that is, D) does not happen because (*COMMIT)
2888  overrides.  overrides.
2889  .  .
2890  .  .
# Line 2668  overrides. Line 2892  overrides.
2892  .rs  .rs
2893  .sp  .sp
2894  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2895  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2896  .  .
2897  .  .
2898  .SH AUTHOR  .SH AUTHOR
# Line 2685  Cambridge CB2 3QH, England. Line 2909  Cambridge CB2 3QH, England.
2909  .rs  .rs
2910  .sp  .sp
2911  .nf  .nf
2912  Last updated: 26 October 2010  Last updated: 24 February 2012
2913  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2914  .fi  .fi

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