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1  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"  .TH PCREPATTERN 3 "05 April 2013" "PCRE 8.33"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# 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 strings in the original library, and a  there is now also support for UTF-8 strings in the original library, an
25  second library that supports 16-bit and UTF-16 character strings. To use these  extra library that supports 16-bit and UTF-16 character strings, and a
26    third library that supports 32-bit and UTF-32 character strings. To use these
27  features, PCRE must be built to include appropriate support. When using UTF  features, PCRE must be built to include appropriate support. When using UTF
28  strings you must either call the compiling function with the PCRE_UTF8 or  strings you must either call the compiling function with the PCRE_UTF8,
29  PCRE_UTF16 option, or the pattern must start with one of these special  PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
30  sequences:  these special sequences:
31  .sp  .sp
32    (*UTF8)    (*UTF8)
33    (*UTF16)    (*UTF16)
34      (*UTF32)
35      (*UTF)
36  .sp  .sp
37    (*UTF) is a generic sequence that can be used with any of the libraries.
38  Starting a pattern with such a sequence is equivalent to setting the relevant  Starting a pattern with such a sequence is equivalent to setting the relevant
39  option. This feature is not Perl-compatible. How setting a UTF mode affects  option. This feature is not Perl-compatible. How setting a UTF mode affects
40  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
# Line 41  of features in the Line 45  of features in the
45  page.  page.
46  .P  .P
47  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
48  combination with (*UTF8) or (*UTF16) is:  combination with (*UTF8), (*UTF16), (*UTF32) or (*UTF) is:
49  .sp  .sp
50    (*UCP)    (*UCP)
51  .sp  .sp
# Line 57  of newlines; they are described below. Line 61  of newlines; they are described below.
61  .P  .P
62  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
63  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
64  \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching  \fBpcre[16|32]_exec()\fP (16- or 32-bit), is used. PCRE also has alternative
65  functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using  matching functions, \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP,
66  a different algorithm that is not Perl-compatible. Some of the features  which match using a different algorithm that is not Perl-compatible. Some of
67  discussed below are not available when DFA matching is used. The advantages and  the features discussed below are not available when DFA matching is used. The
68  disadvantages of the alternative functions, and how they differ from the normal  advantages and disadvantages of the alternative functions, and how they differ
69  functions, are discussed in the  from the normal functions, are discussed in the
70  .\" HREF  .\" HREF
71  \fBpcrematching\fP  \fBpcrematching\fP
72  .\"  .\"
73  page.  page.
74  .  .
75  .  .
76    .SH "EBCDIC CHARACTER CODES"
77    .rs
78    .sp
79    PCRE can be compiled to run in an environment that uses EBCDIC as its character
80    code rather than ASCII or Unicode (typically a mainframe system). In the
81    sections below, character code values are ASCII or Unicode; in an EBCDIC
82    environment these characters may have different code values, and there are no
83    code points greater than 255.
84    .
85    .
86  .\" HTML <a name="newlines"></a>  .\" HTML <a name="newlines"></a>
87  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
88  .rs  .rs
# Line 108  Perl-compatible, are recognized only at Line 122  Perl-compatible, are recognized only at
122  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
123  is used.  is used.
124  .P  .P
125  The newline convention affects the interpretation of the dot metacharacter when  The newline convention affects where the circumflex and dollar assertions are
126  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not  true. It also affects the interpretation of the dot metacharacter when
127  affect what the \eR escape sequence matches. By default, this is any Unicode  PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
128  newline sequence, for Perl compatibility. However, this can be changed; see the  what the \eR escape sequence matches. By default, this is any Unicode newline
129    sequence, for Perl compatibility. However, this can be changed; see the
130  description of \eR in the section entitled  description of \eR in the section entitled
131  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
132  .\" </a>  .\" </a>
# Line 246  one of the following escape sequences th Line 261  one of the following escape sequences th
261    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
262    \euhhhh    character with hex code hhhh (JavaScript mode only)    \euhhhh    character with hex code hhhh (JavaScript mode only)
263  .sp  .sp
264  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx on ASCII characters is as follows: if x is a lower
265  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  case letter, it is converted to upper case. Then bit 6 of the character (hex
266  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
267  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
268  than 127, a compile-time error occurs. This locks out non-ASCII characters in  data item (byte or 16-bit value) following \ec has a value greater than 127, a
269  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A  compile-time error occurs. This locks out non-ASCII characters in all modes.
270  lower case letter is converted to upper case, and then the 0xc0 bits are  .P
271  flipped.)  The \ec facility was designed for use with ASCII characters, but with the
272    extension to Unicode it is even less useful than it once was. It is, however,
273    recognized when PCRE is compiled in EBCDIC mode, where data items are always
274    bytes. In this mode, all values are valid after \ec. If the next character is a
275    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
276    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
277    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
278    characters also generate different values.
279  .P  .P
280  By default, after \ex, from zero to two hexadecimal digits are read (letters  By default, after \ex, from zero to two hexadecimal digits are read (letters
281  can be in upper or lower case). Any number of hexadecimal digits may appear  can be in upper or lower case). Any number of hexadecimal digits may appear
# Line 263  between \ex{ and }, but the character co Line 285  between \ex{ and }, but the character co
285    8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint    8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
286    16-bit non-UTF mode   less than 0x10000    16-bit non-UTF mode   less than 0x10000
287    16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint    16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
288      32-bit non-UTF mode   less than 0x80000000
289      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
290  .sp  .sp
291  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
292  "surrogate" codepoints).  "surrogate" codepoints), and 0xffef.
293  .P  .P
294  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
295  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
# Line 277  as just described only when it is follow Line 301  as just described only when it is follow
301  Otherwise, it matches a literal "x" character. In JavaScript mode, support for  Otherwise, it matches a literal "x" character. In JavaScript mode, support for
302  code points greater than 256 is provided by \eu, which must be followed by  code points greater than 256 is provided by \eu, which must be followed by
303  four hexadecimal digits; otherwise it matches a literal "u" character.  four hexadecimal digits; otherwise it matches a literal "u" character.
304    Character codes specified by \eu in JavaScript mode are constrained in the same
305    was as those specified by \ex in non-JavaScript mode.
306  .P  .P
307  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
308  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
# Line 311  subsequent digits stand for themselves. Line 337  subsequent digits stand for themselves.
337  constrained in the same way as characters specified in hexadecimal.  constrained in the same way as characters specified in hexadecimal.
338  For example:  For example:
339  .sp  .sp
340    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
341  .\" JOIN  .\" JOIN
342    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
343              previous capturing subpatterns              previous capturing subpatterns
# Line 469  release 5.10. In contrast to the other s Line 495  release 5.10. In contrast to the other s
495  characters by default, these always match certain high-valued codepoints,  characters by default, these always match certain high-valued codepoints,
496  whether or not PCRE_UCP is set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
497  .sp  .sp
498    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
499    U+0020     Space    U+0020     Space
500    U+00A0     Non-break space    U+00A0     Non-break space
501    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 491  whether or not PCRE_UCP is set. The hori Line 517  whether or not PCRE_UCP is set. The hori
517  .sp  .sp
518  The vertical space characters are:  The vertical space characters are:
519  .sp  .sp
520    U+000A     Linefeed    U+000A     Linefeed (LF)
521    U+000B     Vertical tab    U+000B     Vertical tab (VT)
522    U+000C     Form feed    U+000C     Form feed (FF)
523    U+000D     Carriage return    U+000D     Carriage return (CR)
524    U+0085     Next line    U+0085     Next line (NEL)
525    U+2028     Line separator    U+2028     Line separator
526    U+2029     Paragraph separator    U+2029     Paragraph separator
527  .sp  .sp
# Line 549  change of newline convention; for exampl Line 575  change of newline convention; for exampl
575  .sp  .sp
576    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
577  .sp  .sp
578  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
579  sequences. Inside a character class, \eR is treated as an unrecognized escape  (*UCP) special sequences. Inside a character class, \eR is treated as an
580  sequence, and so matches the letter "R" by default, but causes an error if  unrecognized escape sequence, and so matches the letter "R" by default, but
581  PCRE_EXTRA is set.  causes an error if PCRE_EXTRA is set.
582  .  .
583  .  .
584  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 567  The extra escape sequences are: Line 593  The extra escape sequences are:
593  .sp  .sp
594    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
595    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
596    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
597  .sp  .sp
598  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
599  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
# Line 760  a modifier or "other". Line 786  a modifier or "other".
786  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
787  U+DFFF. Such characters are not valid in Unicode strings and so  U+DFFF. Such characters are not valid in Unicode strings and so
788  cannot be tested by PCRE, unless UTF validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
789  (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
790    PCRE_NO_UTF32_CHECK in the
791  .\" HREF  .\" HREF
792  \fBpcreapi\fP  \fBpcreapi\fP
793  .\"  .\"
# Line 775  Instead, this property is assumed for an Line 802  Instead, this property is assumed for an
802  Unicode table.  Unicode table.
803  .P  .P
804  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
805  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
806    the behaviour of current versions of Perl.
807  .P  .P
808  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
809  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
810  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
811    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
812    PCRE_UCP option or by starting the pattern with (*UCP).
813    .
814    .
815    .SS Extended grapheme clusters
816    .rs
817  .sp  .sp
818  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
819  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
820  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
821  .\" </a>  .\" </a>
822  (see below).  (see below).
823  .\"  .\"
824  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
825  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
826  8-bit non-UTF-8 mode \eX matches any one character.  .sp
827  .P    (?>\ePM\epM*)
828  Note that recent versions of Perl have changed \eX to match what Unicode calls  .sp
829  an "extended grapheme cluster", which has a more complicated definition.  That is, it matched a character without the "mark" property, followed by zero
830  .P  or more characters with the "mark" property. Characters with the "mark"
831  Matching characters by Unicode property is not fast, because PCRE has to search  property are typically non-spacing accents that affect the preceding character.
832  a structure that contains data for over fifteen thousand characters. That is  .P
833  why the traditional escape sequences such as \ed and \ew do not use Unicode  This simple definition was extended in Unicode to include more complicated
834  properties in PCRE by default, though you can make them do so by setting the  kinds of composite character by giving each character a grapheme breaking
835  PCRE_UCP option or by starting the pattern with (*UCP).  property, and creating rules that use these properties to define the boundaries
836    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
837    one of these clusters.
838    .P
839    \eX always matches at least one character. Then it decides whether to add
840    additional characters according to the following rules for ending a cluster:
841    .P
842    1. End at the end of the subject string.
843    .P
844    2. Do not end between CR and LF; otherwise end after any control character.
845    .P
846    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
847    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
848    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
849    character; an LVT or T character may be follwed only by a T character.
850    .P
851    4. Do not end before extending characters or spacing marks. Characters with
852    the "mark" property always have the "extend" grapheme breaking property.
853    .P
854    5. Do not end after prepend characters.
855    .P
856    6. Otherwise, end the cluster.
857  .  .
858  .  .
859  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
860  .SS PCRE's additional properties  .SS PCRE's additional properties
861  .rs  .rs
862  .sp  .sp
863  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described above, PCRE supports four
864  section, PCRE supports four more that make it possible to convert traditional  more that make it possible to convert traditional escape sequences such as \ew
865  escape sequences such as \ew and \es and POSIX character classes to use Unicode  and \es and POSIX character classes to use Unicode properties. PCRE uses these
866  properties. PCRE uses these non-standard, non-Perl properties internally when  non-standard, non-Perl properties internally when PCRE_UCP is set. However,
867  PCRE_UCP is set. They are:  they may also be used explicitly. These properties are:
868  .sp  .sp
869    Xan   Any alphanumeric character    Xan   Any alphanumeric character
870    Xps   Any POSIX space character    Xps   Any POSIX space character
# Line 823  property. Xps matches the characters tab Line 876  property. Xps matches the characters tab
876  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
877  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
878  same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
879    .P
880    There is another non-standard property, Xuc, which matches any character that
881    can be represented by a Universal Character Name in C++ and other programming
882    languages. These are the characters $, @, ` (grave accent), and all characters
883    with Unicode code points greater than or equal to U+00A0, except for the
884    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
885    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
886    where H is a hexadecimal digit. Note that the Xuc property does not match these
887    sequences but the characters that they represent.)
888  .  .
889  .  .
890  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 928  regular expression. Line 990  regular expression.
990  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
991  .rs  .rs
992  .sp  .sp
993    The circumflex and dollar metacharacters are zero-width assertions. That is,
994    they test for a particular condition being true without consuming any
995    characters from the subject string.
996    .P
997  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
998  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is at
999  at the start of the subject string. If the \fIstartoffset\fP argument of  the start of the subject string. If the \fIstartoffset\fP argument of
1000  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1001  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1002  meaning  meaning
# Line 947  constrained to match only at the start o Line 1013  constrained to match only at the start o
1013  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1014  to be anchored.)  to be anchored.)
1015  .P  .P
1016  A dollar character is an assertion that is true only if the current matching  The dollar character is an assertion that is true only if the current matching
1017  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline at
1018  at the end of the string (by default). Dollar need not be the last character of  the end of the string (by default). Note, however, that it does not actually
1019  the pattern if a number of alternatives are involved, but it should be the last  match the newline. Dollar need not be the last character of the pattern if a
1020  item in any branch in which it appears. Dollar has no special meaning in a  number of alternatives are involved, but it should be the last item in any
1021  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1022  .P  .P
1023  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
1024  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
# Line 1013  name; PCRE does not support this. Line 1079  name; PCRE does not support this.
1079  .sp  .sp
1080  Outside a character class, the escape sequence \eC matches any one data unit,  Outside a character class, the escape sequence \eC matches any one data unit,
1081  whether or not a UTF mode is set. In the 8-bit library, one data unit is one  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1082  byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always  byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1083    a 32-bit unit. Unlike a dot, \eC always
1084  matches line-ending characters. The feature is provided in Perl in order to  matches line-ending characters. The feature is provided in Perl in order to
1085  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1086  used. Because \eC breaks up characters into individual data units, matching one  used. Because \eC breaks up characters into individual data units, matching one
1087  unit with \eC in a UTF mode means that the rest of the string may start with a  unit with \eC in a UTF mode means that the rest of the string may start with a
1088  malformed UTF character. This has undefined results, because PCRE assumes that  malformed UTF character. This has undefined results, because PCRE assumes that
1089  it is dealing with valid UTF strings (and by default it checks this at the  it is dealing with valid UTF strings (and by default it checks this at the
1090  start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option  start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1091  is used).  PCRE_NO_UTF32_CHECK option is used).
1092  .P  .P
1093  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1094  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 1080  circumflex is not an assertion; it still Line 1147  circumflex is not an assertion; it still
1147  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
1148  string.  string.
1149  .P  .P
1150  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1151  included in a class as a literal string of data units, or by using the \ex{  can be included in a class as a literal string of data units, or by using the
1152  escaping mechanism.  \ex{ escaping mechanism.
1153  .P  .P
1154  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
1155  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
# Line 1295  the section entitled Line 1362  the section entitled
1362  .\" </a>  .\" </a>
1363  "Newline sequences"  "Newline sequences"
1364  .\"  .\"
1365  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1366  can be used to set UTF and Unicode property modes; they are equivalent to  sequences that can be used to set UTF and Unicode property modes; they are
1367  setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.  equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1368    options, respectively. The (*UTF) sequence is a generic version that can be
1369    used with any of the libraries. However, the application can set the
1370    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1371  .  .
1372  .  .
1373  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1532  quantifier, but a literal string of four Line 1602  quantifier, but a literal string of four
1602  In UTF modes, quantifiers apply to characters rather than to individual data  In UTF modes, quantifiers apply to characters rather than to individual data
1603  units. Thus, for example, \ex{100}{2} matches two characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1604  which is represented by a two-byte sequence in a UTF-8 string. Similarly,  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1605  \eX{3} matches three Unicode extended sequences, each of which may be several  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1606  data units long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1607  .P  .P
1608  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
1609  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 1619  In cases where it is known that the subj Line 1689  In cases where it is known that the subj
1689  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1690  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1691  .P  .P
1692  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1693  is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
1694  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1695  succeeds. Consider, for example:  succeeds. Consider, for example:
# Line 1629  succeeds. Consider, for example: Line 1699  succeeds. Consider, for example:
1699  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1700  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1701  .P  .P
1702    Another case where implicit anchoring is not applied is when the leading .* is
1703    inside an atomic group. Once again, a match at the start may fail where a later
1704    one succeeds. Consider this pattern:
1705    .sp
1706      (?>.*?a)b
1707    .sp
1708    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1709    (*PRUNE) and (*SKIP) also disable this optimization.
1710    .P
1711  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1712  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1713  .sp  .sp
# Line 1897  except that it does not cause the curren Line 1976  except that it does not cause the curren
1976  Assertion subpatterns are not capturing subpatterns. If such an assertion  Assertion subpatterns are not capturing subpatterns. If such an assertion
1977  contains capturing subpatterns within it, these are counted for the purposes of  contains capturing subpatterns within it, these are counted for the purposes of
1978  numbering the capturing subpatterns in the whole pattern. However, substring  numbering the capturing subpatterns in the whole pattern. However, substring
1979  capturing is carried out only for positive assertions, because it does not make  capturing is carried out only for positive assertions. (Perl sometimes, but not
1980  sense for negative assertions.  always, does do capturing in negative assertions.)
1981  .P  .P
1982  For compatibility with Perl, assertion subpatterns may be repeated; though  For compatibility with Perl, assertion subpatterns may be repeated; though
1983  it makes no sense to assert the same thing several times, the side effect of  it makes no sense to assert the same thing several times, the side effect of
# Line 2550  same pair of parentheses when there is a Line 2629  same pair of parentheses when there is a
2629  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
2630  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
2631  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
2632  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this  (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2633  variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2634  .P  .P
2635  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2636  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 2562  For example, this pattern has two callou Line 2641  For example, this pattern has two callou
2641  .sp  .sp
2642  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2643  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2644  255.  255. If there is a conditional group in the pattern whose condition is an
2645    assertion, an additional callout is inserted just before the condition. An
2646    explicit callout may also be set at this position, as in this example:
2647    .sp
2648      (?(?C9)(?=a)abc|def)
2649    .sp
2650    Note that this applies only to assertion conditions, not to other types of
2651    condition.
2652  .P  .P
2653  During matching, when PCRE reaches a callout point, the external function is  During matching, when PCRE reaches a callout point, the external function is
2654  called. It is provided with the number of the callout, the position in the  called. It is provided with the number of the callout, the position in the
# Line 2581  documentation. Line 2667  documentation.
2667  .rs  .rs
2668  .sp  .sp
2669  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2670  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2671  or removal in a future version of Perl". It goes on to say: "Their usage in  change or removal in a future version of Perl". It goes on to say: "Their usage
2672  production code should be noted to avoid problems during upgrades." The same  in production code should be noted to avoid problems during upgrades." The same
2673  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2674  .P  .P
2675    The new verbs make use of what was previously invalid syntax: an opening
2676    parenthesis followed by an asterisk. They are generally of the form
2677    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2678    differently depending on whether or not a name is present. A name is any
2679    sequence of characters that does not include a closing parenthesis. The maximum
2680    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2681    libraries. If the name is empty, that is, if the closing parenthesis
2682    immediately follows the colon, the effect is as if the colon were not there.
2683    Any number of these verbs may occur in a pattern.
2684    .P
2685  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
2686  used only when the pattern is to be matched using one of the traditional  used only when the pattern is to be matched using one of the traditional
2687  matching functions, which use a backtracking algorithm. With the exception of  matching functions, because these use a backtracking algorithm. With the
2688  (*FAIL), which behaves like a failing negative assertion, they cause an error  exception of (*FAIL), which behaves like a failing negative assertion, the
2689  if encountered by a DFA matching function.  backtracking control verbs cause an error if encountered by a DFA matching
2690  .P  function.
 If any of these verbs are used in an assertion or in a subpattern that is  
 called as a subroutine (whether or not recursively), their effect is confined  
 to that subpattern; it does not extend to the surrounding pattern, with one  
 exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in  
 a successful positive assertion \fIis\fP passed back when a match succeeds  
 (compare capturing parentheses in assertions). Note that such subpatterns are  
 processed as anchored at the point where they are tested. Note also that Perl's  
 treatment of subroutines and assertions is different in some cases.  
2691  .P  .P
2692  The new verbs make use of what was previously invalid syntax: an opening  The behaviour of these verbs in
2693  parenthesis followed by an asterisk. They are generally of the form  .\" HTML <a href="#btrepeat">
2694  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  .\" </a>
2695  depending on whether or not an argument is present. A name is any sequence of  repeated groups,
2696  characters that does not include a closing parenthesis. The maximum length of  .\"
2697  name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name  .\" HTML <a href="#btassert">
2698  is empty, that is, if the closing parenthesis immediately follows the colon,  .\" </a>
2699  the effect is as if the colon were not there. Any number of these verbs may  assertions,
2700  occur in a pattern.  .\"
2701    and in
2702    .\" HTML <a href="#btsub">
2703    .\" </a>
2704    subpatterns called as subroutines
2705    .\"
2706    (whether or not recursively) is documented below.
2707  .  .
2708  .  .
2709  .\" HTML <a name="nooptimize"></a>  .\" HTML <a name="nooptimize"></a>
# Line 2619  occur in a pattern. Line 2713  occur in a pattern.
2713  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
2714  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
2715  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2716  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations bypasses the running of a match, any
2717  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2718  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
2719  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
# Line 2633  in the Line 2727  in the
2727  .\" HREF  .\" HREF
2728  \fBpcreapi\fP  \fBpcreapi\fP
2729  .\"  .\"
2730  documentation.  documentation.
2731  .P  .P
2732  Experiments with Perl suggest that it too has similar optimizations, sometimes  Experiments with Perl suggest that it too has similar optimizations, sometimes
2733  leading to anomalous results.  leading to anomalous results.
# Line 2650  followed by a name. Line 2744  followed by a name.
2744  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
2745  pattern. However, when it is inside a subpattern that is called as a  pattern. However, when it is inside a subpattern that is called as a
2746  subroutine, only that subpattern is ended successfully. Matching then continues  subroutine, only that subpattern is ended successfully. Matching then continues
2747  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so  at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2748  far is captured. For example:  assertion succeeds; in a negative assertion, the assertion fails.
2749    .P
2750    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2751    example:
2752  .sp  .sp
2753    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2754  .sp  .sp
# Line 2684  starting point (see (*SKIP) below). Line 2781  starting point (see (*SKIP) below).
2781  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
2782  (*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.
2783  .P  .P
2784  When a match succeeds, the name of the last-encountered (*MARK) on the matching  When a match succeeds, the name of the last-encountered (*MARK:NAME),
2785  path is passed back to the caller as described in the section entitled  (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2786    caller as described in the section entitled
2787  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2788  .\" </a>  .\" </a>
2789  "Extra data for \fBpcre_exec()\fP"  "Extra data for \fBpcre_exec()\fP"
# Line 2710  indicates which of the two alternatives Line 2808  indicates which of the two alternatives
2808  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2809  capturing parentheses.  capturing parentheses.
2810  .P  .P
2811  If (*MARK) is encountered in a positive assertion, its name is recorded and  If a verb with a name is encountered in a positive assertion that is true, the
2812  passed back if it is the last-encountered. This does not happen for negative  name is recorded and passed back if it is the last-encountered. This does not
2813  assertions.  happen for negative assertions or failing positive assertions.
2814  .P  .P
2815  After a partial match or a failed match, the name of the last encountered  After a partial match or a failed match, the last encountered name in the
2816  (*MARK) in the entire match process is returned. For example:  entire match process is returned. For example:
2817  .sp  .sp
2818      re> /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2819    data> XP    data> XP
# Line 2727  attempts starting at "P" and then with a Line 2825  attempts starting at "P" and then with a
2825  (*MARK) item, but nevertheless do not reset it.  (*MARK) item, but nevertheless do not reset it.
2826  .P  .P
2827  If you are interested in (*MARK) values after failed matches, you should  If you are interested in (*MARK) values after failed matches, you should
2828  probably set the PCRE_NO_START_OPTIMIZE option  probably set the PCRE_NO_START_OPTIMIZE option
2829  .\" HTML <a href="#nooptimize">  .\" HTML <a href="#nooptimize">
2830  .\" </a>  .\" </a>
2831  (see above)  (see above)
2832  .\"  .\"
2833  to ensure that the match is always attempted.  to ensure that the match is always attempted.
2834  .  .
# Line 2741  to ensure that the match is always attem Line 2839  to ensure that the match is always attem
2839  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2840  with what follows, but if there is no subsequent match, causing a backtrack to  with what follows, but if there is no subsequent match, causing a backtrack to
2841  the verb, a failure is forced. That is, backtracking cannot pass to the left of  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2842  the verb. However, when one of these verbs appears inside an atomic group, its  the verb. However, when one of these verbs appears inside an atomic group or an
2843  effect is confined to that group, because once the group has been matched,  assertion that is true, its effect is confined to that group, because once the
2844  there is never any backtracking into it. In this situation, backtracking can  group has been matched, there is never any backtracking into it. In this
2845  "jump back" to the left of the entire atomic group. (Remember also, as stated  situation, backtracking can "jump back" to the left of the entire atomic group
2846  above, that this localization also applies in subroutine calls and assertions.)  or assertion. (Remember also, as stated above, that this localization also
2847    applies in subroutine calls.)
2848  .P  .P
2849  These verbs differ in exactly what kind of failure occurs when backtracking  These verbs differ in exactly what kind of failure occurs when backtracking
2850  reaches them.  reaches them. The behaviour described below is what happens when the verb is
2851    not in a subroutine or an assertion. Subsequent sections cover these special
2852    cases.
2853  .sp  .sp
2854    (*COMMIT)    (*COMMIT)
2855  .sp  .sp
2856  This verb, which may not be followed by a name, causes the whole match to fail  This verb, which may not be followed by a name, causes the whole match to fail
2857  outright if the rest of the pattern does not match. Even if the pattern is  outright if there is a later matching failure that causes backtracking to reach
2858  unanchored, no further attempts to find a match by advancing the starting point  it. Even if the pattern is unanchored, no further attempts to find a match by
2859  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to  advancing the starting point take place. If (*COMMIT) is the only backtracking
2860  finding a match at the current starting point, or not at all. For example:  verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2861    committed to finding a match at the current starting point, or not at all. For
2862    example:
2863  .sp  .sp
2864    a+(*COMMIT)b    a+(*COMMIT)b
2865  .sp  .sp
# Line 2765  dynamic anchor, or "I've started, so I m Line 2868  dynamic anchor, or "I've started, so I m
2868  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2869  match failure.  match failure.
2870  .P  .P
2871    If there is more than one backtracking verb in a pattern, a different one that
2872    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2873    match does not always guarantee that a match must be at this starting point.
2874    .P
2875  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2876  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
2877  \fBpcretest\fP example:  \fBpcretest\fP example:
# Line 2784  starting points. Line 2891  starting points.
2891    (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
2892  .sp  .sp
2893  This verb causes the match to fail at the current starting position in the  This verb causes the match to fail at the current starting position in the
2894  subject if the rest of the pattern does not match. If the pattern is  subject if there is a later matching failure that causes backtracking to reach
2895  unanchored, the normal "bumpalong" advance to the next starting character then  it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2896  happens. Backtracking can occur as usual to the left of (*PRUNE), before it is  starting character then happens. Backtracking can occur as usual to the left of
2897  reached, or when matching to the right of (*PRUNE), but if there is no match to  (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
2898  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of  if there is no match to the right, backtracking cannot cross (*PRUNE). In
2899  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,  simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
2900  but there are some uses of (*PRUNE) that cannot be expressed in any other way.  possessive quantifier, but there are some uses of (*PRUNE) that cannot be
2901  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an  expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2902  anchored pattern (*PRUNE) has the same effect as (*COMMIT).  as (*COMMIT).
2903    .P
2904    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
2905    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2906    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2907  .sp  .sp
2908    (*SKIP)    (*SKIP)
2909  .sp  .sp
# Line 2813  instead of skipping on to "c". Line 2924  instead of skipping on to "c".
2924  .sp  .sp
2925    (*SKIP:NAME)    (*SKIP:NAME)
2926  .sp  .sp
2927  When (*SKIP) has an associated name, its behaviour is modified. If the  When (*SKIP) has an associated name, its behaviour is modified. When it is
2928  following pattern fails to match, the previous path through the pattern is  triggered, the previous path through the pattern is searched for the most
2929  searched for the most recent (*MARK) that has the same name. If one is found,  recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
2930  the "bumpalong" advance is to the subject position that corresponds to that  is to the subject position that corresponds to that (*MARK) instead of to where
2931  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
2932  matching name is found, the (*SKIP) is ignored.  (*SKIP) is ignored.
2933    .P
2934    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
2935    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2936  .sp  .sp
2937    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2938  .sp  .sp
2939  This verb causes a skip to the next innermost alternative if the rest of the  This verb causes a skip to the next innermost alternative when backtracking
2940  pattern does not match. That is, it cancels pending backtracking, but only  reaches it. That is, it cancels any further backtracking within the current
2941  within the current alternative. Its name comes from the observation that it can  alternative. Its name comes from the observation that it can be used for a
2942  be used for a pattern-based if-then-else block:  pattern-based if-then-else block:
2943  .sp  .sp
2944    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2945  .sp  .sp
2946  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
2947  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
2948  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. If that
2949  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
2950  If (*THEN) is not inside an alternation, it acts like (*PRUNE).  more alternatives, so there is a backtrack to whatever came before the entire
2951    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2952    .P
2953    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
2954    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2955    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2956  .P  .P
2957  Note that a subpattern that does not contain a | character is just a part of  A subpattern that does not contain a | character is just a part of the
2958  the enclosing alternative; it is not a nested alternation with only one  enclosing alternative; it is not a nested alternation with only one
2959  alternative. The effect of (*THEN) extends beyond such a subpattern to the  alternative. The effect of (*THEN) extends beyond such a subpattern to the
2960  enclosing alternative. Consider this pattern, where A, B, etc. are complex  enclosing alternative. Consider this pattern, where A, B, etc. are complex
2961  pattern fragments that do not contain any | characters at this level:  pattern fragments that do not contain any | characters at this level:
# Line 2855  in C, matching moves to (*FAIL), which c Line 2974  in C, matching moves to (*FAIL), which c
2974  because there are no more alternatives to try. In this case, matching does now  because there are no more alternatives to try. In this case, matching does now
2975  backtrack into A.  backtrack into A.
2976  .P  .P
2977  Note also that a conditional subpattern is not considered as having two  Note that a conditional subpattern is not considered as having two
2978  alternatives, because only one is ever used. In other words, the | character in  alternatives, because only one is ever used. In other words, the | character in
2979  a conditional subpattern has a different meaning. Ignoring white space,  a conditional subpattern has a different meaning. Ignoring white space,
2980  consider:  consider:
# Line 2877  starting position, but allowing an advan Line 2996  starting position, but allowing an advan
2996  unanchored pattern). (*SKIP) is similar, except that the advance may be more  unanchored pattern). (*SKIP) is similar, except that the advance may be more
2997  than one character. (*COMMIT) is the strongest, causing the entire match to  than one character. (*COMMIT) is the strongest, causing the entire match to
2998  fail.  fail.
2999    .
3000    .
3001    .SS "More than one backtracking verb"
3002    .rs
3003    .sp
3004    If more than one backtracking verb is present in a pattern, the one that is
3005    backtracked onto first acts. For example, consider this pattern, where A, B,
3006    etc. are complex pattern fragments:
3007    .sp
3008      (A(*COMMIT)B(*THEN)C|ABD)
3009    .sp
3010    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3011    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3012    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3013    not always the same as Perl's. It means that if two or more backtracking verbs
3014    appear in succession, all the the last of them has no effect. Consider this
3015    example:
3016    .sp
3017      ...(*COMMIT)(*PRUNE)...
3018    .sp
3019    If there is a matching failure to the right, backtracking onto (*PRUNE) cases
3020    it to be triggered, and its action is taken. There can never be a backtrack
3021    onto (*COMMIT).
3022    .
3023    .
3024    .\" HTML <a name="btrepeat"></a>
3025    .SS "Backtracking verbs in repeated groups"
3026    .rs
3027    .sp
3028    PCRE differs from Perl in its handling of backtracking verbs in repeated
3029    groups. For example, consider:
3030    .sp
3031      /(a(*COMMIT)b)+ac/
3032    .sp
3033    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3034    the second repeat of the group acts.
3035    .
3036    .
3037    .\" HTML <a name="btassert"></a>
3038    .SS "Backtracking verbs in assertions"
3039    .rs
3040    .sp
3041    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3042    .P
3043    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3044    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3045    fail without any further processing.
3046    .P
3047    The other backtracking verbs are not treated specially if they appear in a
3048    positive assertion. In particular, (*THEN) skips to the next alternative in the
3049    innermost enclosing group that has alternations, whether or not this is within
3050    the assertion.
3051    .P
3052    Negative assertions are, however, different, in order to ensure that changing a
3053    positive assertion into a negative assertion changes its result. Backtracking
3054    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3055    without considering any further alternative branches in the assertion.
3056    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3057    within the assertion (the normal behaviour), but if the assertion does not have
3058    such an alternative, (*THEN) behaves like (*PRUNE).
3059    .
3060    .
3061    .\" HTML <a name="btsub"></a>
3062    .SS "Backtracking verbs in subroutines"
3063    .rs
3064    .sp
3065    These behaviours occur whether or not the subpattern is called recursively.
3066    Perl's treatment of subroutines is different in some cases.
3067    .P
3068    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3069    an immediate backtrack.
3070    .P
3071    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3072    succeed without any further processing. Matching then continues after the
3073    subroutine call.
3074    .P
3075    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3076    the subroutine match to fail.
3077  .P  .P
3078  If more than one such verb is present in a pattern, the "strongest" one wins.  (*THEN) skips to the next alternative in the innermost enclosing group within
3079  For example, consider this pattern, where A, B, etc. are complex pattern  the subpattern that has alternatives. If there is no such group within the
3080  fragments:  subpattern, (*THEN) causes the subroutine match to fail.
 .sp  
   (A(*COMMIT)B(*THEN)C|D)  
 .sp  
 Once A has matched, PCRE is committed to this match, at the current starting  
 position. If subsequently B matches, but C does not, the normal (*THEN) action  
 of trying the next alternative (that is, D) does not happen because (*COMMIT)  
 overrides.  
3081  .  .
3082  .  .
3083  .SH "SEE ALSO"  .SH "SEE ALSO"
3084  .rs  .rs
3085  .sp  .sp
3086  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3087  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3088  .  .
3089  .  .
3090  .SH AUTHOR  .SH AUTHOR
# Line 2911  Cambridge CB2 3QH, England. Line 3101  Cambridge CB2 3QH, England.
3101  .rs  .rs
3102  .sp  .sp
3103  .nf  .nf
3104  Last updated: 01 June 2012  Last updated: 05 April 2013
3105  Copyright (c) 1997-2012 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3106  .fi  .fi

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