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1  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"  .TH PCREPATTERN 3 "06 September 2013" "PCRE 8.34"
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 20  have copious examples. Jeffrey Friedl's Line 20  have copious examples. Jeffrey Friedl's
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
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    This document discusses the patterns that are supported by PCRE when one its
24    main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25    (16- or 32-bit), is used. PCRE also has alternative matching functions,
26    \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27    different algorithm that is not Perl-compatible. Some of the features discussed
28    below are not available when DFA matching is used. The advantages and
29    disadvantages of the alternative functions, and how they differ from the normal
30    functions, are discussed in the
31    .\" HREF
32    \fBpcrematching\fP
33    .\"
34    page.
35    .
36    .
37    .SH "SPECIAL START-OF-PATTERN ITEMS"
38    .rs
39    .sp
40    A number of options that can be passed to \fBpcre_compile()\fP can also be set
41    by special items at the start of a pattern. These are not Perl-compatible, but
42    are provided to make these options accessible to pattern writers who are not
43    able to change the program that processes the pattern. Any number of these
44    items may appear, but they must all be together right at the start of the
45    pattern string, and the letters must be in upper case.
46    .
47    .
48    .SS "UTF support"
49    .rs
50    .sp
51  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,
52  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
53  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
54    third library that supports 32-bit and UTF-32 character strings. To use these
55  features, PCRE must be built to include appropriate support. When using UTF  features, PCRE must be built to include appropriate support. When using UTF
56  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,
57  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
58  sequences:  these special sequences:
59  .sp  .sp
60    (*UTF8)    (*UTF8)
61    (*UTF16)    (*UTF16)
62      (*UTF32)
63      (*UTF)
64  .sp  .sp
65    (*UTF) is a generic sequence that can be used with any of the libraries.
66  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
67  option. This feature is not Perl-compatible. How setting a UTF mode affects  option. How setting a UTF mode affects pattern matching is mentioned in several
68  pattern matching is mentioned in several places below. There is also a summary  places below. There is also a summary of features in the
 of features in the  
69  .\" HREF  .\" HREF
70  \fBpcreunicode\fP  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73  .P  .P
74  Another special sequence that may appear at the start of a pattern or in  Some applications that allow their users to supply patterns may wish to
75  combination with (*UTF8) or (*UTF16) is:  restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76    option is set at compile time, (*UTF) etc. are not allowed, and their
77    appearance causes an error.
78    .
79    .
80    .SS "Unicode property support"
81    .rs
82    .sp
83    Another special sequence that may appear at the start of a pattern is
84  .sp  .sp
85    (*UCP)    (*UCP)
86  .sp  .sp
# Line 49  This has the same effect as setting the Line 88  This has the same effect as setting the
88  such as \ed and \ew to use Unicode properties to determine character types,  such as \ed and \ew to use Unicode properties to determine character types,
89  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
90  table.  table.
91  .P  .
92    .
93    .SS "Disabling start-up optimizations"
94    .rs
95    .sp
96  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
97  PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are  PCRE_NO_START_OPTIMIZE option either at compile or matching time.
 also some more of these special sequences that are concerned with the handling  
 of newlines; they are described below.  
 .P  
 The remainder of this document discusses the patterns that are supported by  
 PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or  
 \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching  
 functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using  
 a different algorithm that is not Perl-compatible. Some of the features  
 discussed below are not available when DFA matching is used. The advantages and  
 disadvantages of the alternative functions, and how they differ from the normal  
 functions, are discussed in the  
 .\" HREF  
 \fBpcrematching\fP  
 .\"  
 page.  
98  .  .
99  .  .
100  .\" HTML <a name="newlines"></a>  .\" HTML <a name="newlines"></a>
101  .SH "NEWLINE CONVENTIONS"  .SS "Newline conventions"
102  .rs  .rs
103  .sp  .sp
104  PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
# Line 103  example, on a Unix system where LF is th Line 131  example, on a Unix system where LF is th
131    (*CR)a.b    (*CR)a.b
132  .sp  .sp
133  changes the convention to CR. That pattern matches "a\enb" because LF is no  changes the convention to CR. That pattern matches "a\enb" because LF is no
134  longer a newline. Note that these special settings, which are not  longer a newline. If more than one of these settings is present, the last one
 Perl-compatible, are recognized only at the very start of a pattern, and that  
 they must be in upper case. If more than one of them is present, the last one  
135  is used.  is used.
136  .P  .P
137  The newline convention affects the interpretation of the dot metacharacter when  The newline convention affects where the circumflex and dollar assertions are
138  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
139  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
140  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
141    sequence, for Perl compatibility. However, this can be changed; see the
142  description of \eR in the section entitled  description of \eR in the section entitled
143  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
144  .\" </a>  .\" </a>
# Line 121  below. A change of \eR setting can be co Line 148  below. A change of \eR setting can be co
148  convention.  convention.
149  .  .
150  .  .
151    .SS "Setting match and recursion limits"
152    .rs
153    .sp
154    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
155    internal \fBmatch()\fP function is called and on the maximum depth of
156    recursive calls. These facilities are provided to catch runaway matches that
157    are provoked by patterns with huge matching trees (a typical example is a
158    pattern with nested unlimited repeats) and to avoid running out of system stack
159    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
160    gives an error return. The limits can also be set by items at the start of the
161    pattern of the form
162    .sp
163      (*LIMIT_MATCH=d)
164      (*LIMIT_RECURSION=d)
165    .sp
166    where d is any number of decimal digits. However, the value of the setting must
167    be less than the value set by the caller of \fBpcre_exec()\fP for it to have
168    any effect. In other words, the pattern writer can lower the limit set by the
169    programmer, but not raise it. If there is more than one setting of one of these
170    limits, the lower value is used.
171    .
172    .
173    .SH "EBCDIC CHARACTER CODES"
174    .rs
175    .sp
176    PCRE can be compiled to run in an environment that uses EBCDIC as its character
177    code rather than ASCII or Unicode (typically a mainframe system). In the
178    sections below, character code values are ASCII or Unicode; in an EBCDIC
179    environment these characters may have different code values, and there are no
180    code points greater than 255.
181    .
182    .
183  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
184  .rs  .rs
185  .sp  .sp
# Line 246  one of the following escape sequences th Line 305  one of the following escape sequences th
305    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
306    \euhhhh    character with hex code hhhh (JavaScript mode only)    \euhhhh    character with hex code hhhh (JavaScript mode only)
307  .sp  .sp
308  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
309  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
310  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),
311  \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
312  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
313  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.
314  lower case letter is converted to upper case, and then the 0xc0 bits are  .P
315  flipped.)  The \ec facility was designed for use with ASCII characters, but with the
316    extension to Unicode it is even less useful than it once was. It is, however,
317    recognized when PCRE is compiled in EBCDIC mode, where data items are always
318    bytes. In this mode, all values are valid after \ec. If the next character is a
319    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
320    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
321    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
322    characters also generate different values.
323  .P  .P
324  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
325  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 329  between \ex{ and }, but the character co
329    8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint    8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
330    16-bit non-UTF mode   less than 0x10000    16-bit non-UTF mode   less than 0x10000
331    16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint    16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
332      32-bit non-UTF mode   less than 0x80000000
333      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
334  .sp  .sp
335  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
336  "surrogate" codepoints).  "surrogate" codepoints), and 0xffef.
337  .P  .P
338  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
339  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 313  subsequent digits stand for themselves. Line 381  subsequent digits stand for themselves.
381  constrained in the same way as characters specified in hexadecimal.  constrained in the same way as characters specified in hexadecimal.
382  For example:  For example:
383  .sp  .sp
384    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
385  .\" JOIN  .\" JOIN
386    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
387              previous capturing subpatterns              previous capturing subpatterns
# Line 471  release 5.10. In contrast to the other s Line 539  release 5.10. In contrast to the other s
539  characters by default, these always match certain high-valued codepoints,  characters by default, these always match certain high-valued codepoints,
540  whether or not PCRE_UCP is set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
541  .sp  .sp
542    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
543    U+0020     Space    U+0020     Space
544    U+00A0     Non-break space    U+00A0     Non-break space
545    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 493  whether or not PCRE_UCP is set. The hori Line 561  whether or not PCRE_UCP is set. The hori
561  .sp  .sp
562  The vertical space characters are:  The vertical space characters are:
563  .sp  .sp
564    U+000A     Linefeed    U+000A     Linefeed (LF)
565    U+000B     Vertical tab    U+000B     Vertical tab (VT)
566    U+000C     Form feed    U+000C     Form feed (FF)
567    U+000D     Carriage return    U+000D     Carriage return (CR)
568    U+0085     Next line    U+0085     Next line (NEL)
569    U+2028     Line separator    U+2028     Line separator
570    U+2029     Paragraph separator    U+2029     Paragraph separator
571  .sp  .sp
# Line 551  change of newline convention; for exampl Line 619  change of newline convention; for exampl
619  .sp  .sp
620    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
621  .sp  .sp
622  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
623  sequences. Inside a character class, \eR is treated as an unrecognized escape  (*UCP) special sequences. Inside a character class, \eR is treated as an
624  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
625  PCRE_EXTRA is set.  causes an error if PCRE_EXTRA is set.
626  .  .
627  .  .
628  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 569  The extra escape sequences are: Line 637  The extra escape sequences are:
637  .sp  .sp
638    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
639    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
640    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
641  .sp  .sp
642  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
643  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
# Line 762  a modifier or "other". Line 830  a modifier or "other".
830  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
831  U+DFFF. Such characters are not valid in Unicode strings and so  U+DFFF. Such characters are not valid in Unicode strings and so
832  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
833  (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
834    PCRE_NO_UTF32_CHECK in the
835  .\" HREF  .\" HREF
836  \fBpcreapi\fP  \fBpcreapi\fP
837  .\"  .\"
# Line 777  Instead, this property is assumed for an Line 846  Instead, this property is assumed for an
846  Unicode table.  Unicode table.
847  .P  .P
848  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
849  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
850    the behaviour of current versions of Perl.
851  .P  .P
852  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
853  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
854  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
855    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
856    PCRE_UCP option or by starting the pattern with (*UCP).
857    .
858    .
859    .SS Extended grapheme clusters
860    .rs
861  .sp  .sp
862  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
863  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  
864  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
865  .\" </a>  .\" </a>
866  (see below).  (see below).
867  .\"  .\"
868  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
869  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
870  8-bit non-UTF-8 mode \eX matches any one character.  .sp
871  .P    (?>\ePM\epM*)
872  Note that recent versions of Perl have changed \eX to match what Unicode calls  .sp
873  an "extended grapheme cluster", which has a more complicated definition.  That is, it matched a character without the "mark" property, followed by zero
874  .P  or more characters with the "mark" property. Characters with the "mark"
875  Matching characters by Unicode property is not fast, because PCRE has to search  property are typically non-spacing accents that affect the preceding character.
876  a structure that contains data for over fifteen thousand characters. That is  .P
877  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
878  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
879  PCRE_UCP option or by starting the pattern with (*UCP).  property, and creating rules that use these properties to define the boundaries
880    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
881    one of these clusters.
882    .P
883    \eX always matches at least one character. Then it decides whether to add
884    additional characters according to the following rules for ending a cluster:
885    .P
886    1. End at the end of the subject string.
887    .P
888    2. Do not end between CR and LF; otherwise end after any control character.
889    .P
890    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
891    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
892    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
893    character; an LVT or T character may be follwed only by a T character.
894    .P
895    4. Do not end before extending characters or spacing marks. Characters with
896    the "mark" property always have the "extend" grapheme breaking property.
897    .P
898    5. Do not end after prepend characters.
899    .P
900    6. Otherwise, end the cluster.
901  .  .
902  .  .
903  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
904  .SS PCRE's additional properties  .SS PCRE's additional properties
905  .rs  .rs
906  .sp  .sp
907  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described above, PCRE supports four
908  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
909  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
910  properties. PCRE uses these non-standard, non-Perl properties internally when  non-standard, non-Perl properties internally when PCRE_UCP is set. However,
911  PCRE_UCP is set. They are:  they may also be used explicitly. These properties are:
912  .sp  .sp
913    Xan   Any alphanumeric character    Xan   Any alphanumeric character
914    Xps   Any POSIX space character    Xps   Any POSIX space character
# Line 825  property. Xps matches the characters tab Line 920  property. Xps matches the characters tab
920  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
921  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
922  same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
923    .P
924    There is another non-standard property, Xuc, which matches any character that
925    can be represented by a Universal Character Name in C++ and other programming
926    languages. These are the characters $, @, ` (grave accent), and all characters
927    with Unicode code points greater than or equal to U+00A0, except for the
928    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
929    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
930    where H is a hexadecimal digit. Note that the Xuc property does not match these
931    sequences but the characters that they represent.)
932  .  .
933  .  .
934  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 930  regular expression. Line 1034  regular expression.
1034  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1035  .rs  .rs
1036  .sp  .sp
1037    The circumflex and dollar metacharacters are zero-width assertions. That is,
1038    they test for a particular condition being true without consuming any
1039    characters from the subject string.
1040    .P
1041  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1042  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
1043  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
1044  \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
1045  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1046  meaning  meaning
# Line 949  constrained to match only at the start o Line 1057  constrained to match only at the start o
1057  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1058  to be anchored.)  to be anchored.)
1059  .P  .P
1060  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
1061  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
1062  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
1063  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
1064  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
1065  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1066  .P  .P
1067  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
1068  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 1015  name; PCRE does not support this. Line 1123  name; PCRE does not support this.
1123  .sp  .sp
1124  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,
1125  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
1126  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
1127    a 32-bit unit. Unlike a dot, \eC always
1128  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
1129  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
1130  used. Because \eC breaks up characters into individual data units, matching one  used. Because \eC breaks up characters into individual data units, matching one
1131  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
1132  malformed UTF character. This has undefined results, because PCRE assumes that  malformed UTF character. This has undefined results, because PCRE assumes that
1133  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
1134  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
1135  is used).  PCRE_NO_UTF32_CHECK option is used).
1136  .P  .P
1137  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1138  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 1082  circumflex is not an assertion; it still Line 1191  circumflex is not an assertion; it still
1191  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
1192  string.  string.
1193  .P  .P
1194  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)
1195  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
1196  escaping mechanism.  \ex{ escaping mechanism.
1197  .P  .P
1198  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
1199  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 1297  the section entitled Line 1406  the section entitled
1406  .\" </a>  .\" </a>
1407  "Newline sequences"  "Newline sequences"
1408  .\"  .\"
1409  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1410  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
1411  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
1412    options, respectively. The (*UTF) sequence is a generic version that can be
1413    used with any of the libraries. However, the application can set the
1414    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1415  .  .
1416  .  .
1417  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1465  for the first (and in this example, the Line 1577  for the first (and in this example, the
1577  matched. This saves searching to find which numbered subpattern it was.  matched. This saves searching to find which numbered subpattern it was.
1578  .P  .P
1579  If you make a back reference to a non-unique named subpattern from elsewhere in  If you make a back reference to a non-unique named subpattern from elsewhere in
1580  the pattern, the one that corresponds to the first occurrence of the name is  the pattern, the subpatterns to which the name refers are checked in the order
1581  used. In the absence of duplicate numbers (see the previous section) this is  in which they appear in the overall pattern. The first one that is set is used
1582  the one with the lowest number. If you use a named reference in a condition  for the reference. For example, this pattern matches both "foofoo" and
1583    "barbar" but not "foobar" or "barfoo":
1584    .sp
1585      (?:(?<n>foo)|(?<n>bar))\k<n>
1586    .sp
1587    .P
1588    If you make a subroutine call to a non-unique named subpattern, the one that
1589    corresponds to the first occurrence of the name is used. In the absence of
1590    duplicate numbers (see the previous section) this is the one with the lowest
1591    number.
1592    .P
1593    If you use a named reference in a condition
1594  test (see the  test (see the
1595  .\"  .\"
1596  .\" HTML <a href="#conditions">  .\" HTML <a href="#conditions">
# Line 1487  documentation. Line 1610  documentation.
1610  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1611  subpatterns with the same number because PCRE uses only the numbers when  subpatterns with the same number because PCRE uses only the numbers when
1612  matching. For this reason, an error is given at compile time if different names  matching. For this reason, an error is given at compile time if different names
1613  are given to subpatterns with the same number. However, you can give the same  are given to subpatterns with the same number. However, you can always give the
1614  name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.  same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1615    set.
1616  .  .
1617  .  .
1618  .SH REPETITION  .SH REPETITION
# Line 1534  quantifier, but a literal string of four Line 1658  quantifier, but a literal string of four
1658  In UTF modes, quantifiers apply to characters rather than to individual data  In UTF modes, quantifiers apply to characters rather than to individual data
1659  units. Thus, for example, \ex{100}{2} matches two characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1660  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,
1661  \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
1662  data units long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1663  .P  .P
1664  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
1665  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 1621  In cases where it is known that the subj Line 1745  In cases where it is known that the subj
1745  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1746  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1747  .P  .P
1748  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1749  is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
1750  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
1751  succeeds. Consider, for example:  succeeds. Consider, for example:
# Line 1631  succeeds. Consider, for example: Line 1755  succeeds. Consider, for example:
1755  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
1756  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1757  .P  .P
1758    Another case where implicit anchoring is not applied is when the leading .* is
1759    inside an atomic group. Once again, a match at the start may fail where a later
1760    one succeeds. Consider this pattern:
1761    .sp
1762      (?>.*?a)b
1763    .sp
1764    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1765    (*PRUNE) and (*SKIP) also disable this optimization.
1766    .P
1767  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1768  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1769  .sp  .sp
# Line 1899  except that it does not cause the curren Line 2032  except that it does not cause the curren
2032  Assertion subpatterns are not capturing subpatterns. If such an assertion  Assertion subpatterns are not capturing subpatterns. If such an assertion
2033  contains capturing subpatterns within it, these are counted for the purposes of  contains capturing subpatterns within it, these are counted for the purposes of
2034  numbering the capturing subpatterns in the whole pattern. However, substring  numbering the capturing subpatterns in the whole pattern. However, substring
2035  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
2036  sense for negative assertions.  always, does do capturing in negative assertions.)
2037  .P  .P
2038  For compatibility with Perl, assertion subpatterns may be repeated; though  For compatibility with Perl, assertion subpatterns may be repeated; though
2039  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 2552  same pair of parentheses when there is a Line 2685  same pair of parentheses when there is a
2685  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
2686  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
2687  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
2688  (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).
2689  variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2690  .P  .P
2691  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2692  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 2564  For example, this pattern has two callou Line 2697  For example, this pattern has two callou
2697  .sp  .sp
2698  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
2699  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2700  255.  255. If there is a conditional group in the pattern whose condition is an
2701    assertion, an additional callout is inserted just before the condition. An
2702    explicit callout may also be set at this position, as in this example:
2703    .sp
2704      (?(?C9)(?=a)abc|def)
2705    .sp
2706    Note that this applies only to assertion conditions, not to other types of
2707    condition.
2708  .P  .P
2709  During matching, when PCRE reaches a callout point, the external function is  During matching, when PCRE reaches a callout point, the external function is
2710  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 2583  documentation. Line 2723  documentation.
2723  .rs  .rs
2724  .sp  .sp
2725  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2726  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2727  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
2728  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
2729  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2730  .P  .P
2731    The new verbs make use of what was previously invalid syntax: an opening
2732    parenthesis followed by an asterisk. They are generally of the form
2733    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2734    differently depending on whether or not a name is present. A name is any
2735    sequence of characters that does not include a closing parenthesis. The maximum
2736    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2737    libraries. If the name is empty, that is, if the closing parenthesis
2738    immediately follows the colon, the effect is as if the colon were not there.
2739    Any number of these verbs may occur in a pattern.
2740    .P
2741  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
2742  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
2743  matching functions, which use a backtracking algorithm. With the exception of  matching functions, because these use a backtracking algorithm. With the
2744  (*FAIL), which behaves like a failing negative assertion, they cause an error  exception of (*FAIL), which behaves like a failing negative assertion, the
2745  if encountered by a DFA matching function.  backtracking control verbs cause an error if encountered by a DFA matching
2746  .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.  
2747  .P  .P
2748  The new verbs make use of what was previously invalid syntax: an opening  The behaviour of these verbs in
2749  parenthesis followed by an asterisk. They are generally of the form  .\" HTML <a href="#btrepeat">
2750  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  .\" </a>
2751  depending on whether or not an argument is present. A name is any sequence of  repeated groups,
2752  characters that does not include a closing parenthesis. The maximum length of  .\"
2753  name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name  .\" HTML <a href="#btassert">
2754  is empty, that is, if the closing parenthesis immediately follows the colon,  .\" </a>
2755  the effect is as if the colon were not there. Any number of these verbs may  assertions,
2756  occur in a pattern.  .\"
2757    and in
2758    .\" HTML <a href="#btsub">
2759    .\" </a>
2760    subpatterns called as subroutines
2761    .\"
2762    (whether or not recursively) is documented below.
2763  .  .
2764  .  .
2765  .\" HTML <a name="nooptimize"></a>  .\" HTML <a name="nooptimize"></a>
# Line 2621  occur in a pattern. Line 2769  occur in a pattern.
2769  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
2770  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
2771  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2772  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
2773  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2774  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
2775  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 2652  followed by a name. Line 2800  followed by a name.
2800  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
2801  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
2802  subroutine, only that subpattern is ended successfully. Matching then continues  subroutine, only that subpattern is ended successfully. Matching then continues
2803  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
2804  far is captured. For example:  assertion succeeds; in a negative assertion, the assertion fails.
2805    .P
2806    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2807    example:
2808  .sp  .sp
2809    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2810  .sp  .sp
# Line 2686  starting point (see (*SKIP) below). Line 2837  starting point (see (*SKIP) below).
2837  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
2838  (*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.
2839  .P  .P
2840  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),
2841  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
2842    caller as described in the section entitled
2843  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2844  .\" </a>  .\" </a>
2845  "Extra data for \fBpcre_exec()\fP"  "Extra data for \fBpcre_exec()\fP"
# Line 2712  indicates which of the two alternatives Line 2864  indicates which of the two alternatives
2864  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2865  capturing parentheses.  capturing parentheses.
2866  .P  .P
2867  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
2868  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
2869  assertions.  happen for negative assertions or failing positive assertions.
2870  .P  .P
2871  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
2872  (*MARK) in the entire match process is returned. For example:  entire match process is returned. For example:
2873  .sp  .sp
2874      re> /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2875    data> XP    data> XP
# Line 2743  to ensure that the match is always attem Line 2895  to ensure that the match is always attem
2895  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2896  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
2897  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
2898  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
2899  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
2900  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
2901  "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
2902  above, that this localization also applies in subroutine calls and assertions.)  or assertion. (Remember also, as stated above, that this localization also
2903    applies in subroutine calls.)
2904  .P  .P
2905  These verbs differ in exactly what kind of failure occurs when backtracking  These verbs differ in exactly what kind of failure occurs when backtracking
2906  reaches them.  reaches them. The behaviour described below is what happens when the verb is
2907    not in a subroutine or an assertion. Subsequent sections cover these special
2908    cases.
2909  .sp  .sp
2910    (*COMMIT)    (*COMMIT)
2911  .sp  .sp
2912  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
2913  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
2914  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
2915  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
2916  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
2917    committed to finding a match at the current starting point, or not at all. For
2918    example:
2919  .sp  .sp
2920    a+(*COMMIT)b    a+(*COMMIT)b
2921  .sp  .sp
# Line 2767  dynamic anchor, or "I've started, so I m Line 2924  dynamic anchor, or "I've started, so I m
2924  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
2925  match failure.  match failure.
2926  .P  .P
2927    If there is more than one backtracking verb in a pattern, a different one that
2928    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2929    match does not always guarantee that a match must be at this starting point.
2930    .P
2931  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,
2932  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
2933  \fBpcretest\fP example:  \fBpcretest\fP example:
# Line 2786  starting points. Line 2947  starting points.
2947    (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
2948  .sp  .sp
2949  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
2950  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
2951  unanchored, the normal "bumpalong" advance to the next starting character then  it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2952  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
2953  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
2954  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
2955  (*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
2956  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
2957  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
2958  anchored pattern (*PRUNE) has the same effect as (*COMMIT).  as (*COMMIT).
2959    .P
2960    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
2961    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2962    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2963  .sp  .sp
2964    (*SKIP)    (*SKIP)
2965  .sp  .sp
# Line 2815  instead of skipping on to "c". Line 2980  instead of skipping on to "c".
2980  .sp  .sp
2981    (*SKIP:NAME)    (*SKIP:NAME)
2982  .sp  .sp
2983  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
2984  following pattern fails to match, the previous path through the pattern is  triggered, the previous path through the pattern is searched for the most
2985  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
2986  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
2987  (*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
2988  matching name is found, the (*SKIP) is ignored.  (*SKIP) is ignored.
2989    .P
2990    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
2991    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2992  .sp  .sp
2993    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2994  .sp  .sp
2995  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
2996  pattern does not match. That is, it cancels pending backtracking, but only  reaches it. That is, it cancels any further backtracking within the current
2997  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
2998  be used for a pattern-based if-then-else block:  pattern-based if-then-else block:
2999  .sp  .sp
3000    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3001  .sp  .sp
3002  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
3003  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
3004  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. If that
3005  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
3006  If (*THEN) is not inside an alternation, it acts like (*PRUNE).  more alternatives, so there is a backtrack to whatever came before the entire
3007    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3008    .P
3009    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3010    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3011    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3012  .P  .P
3013  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
3014  the enclosing alternative; it is not a nested alternation with only one  enclosing alternative; it is not a nested alternation with only one
3015  alternative. The effect of (*THEN) extends beyond such a subpattern to the  alternative. The effect of (*THEN) extends beyond such a subpattern to the
3016  enclosing alternative. Consider this pattern, where A, B, etc. are complex  enclosing alternative. Consider this pattern, where A, B, etc. are complex
3017  pattern fragments that do not contain any | characters at this level:  pattern fragments that do not contain any | characters at this level:
# Line 2857  in C, matching moves to (*FAIL), which c Line 3030  in C, matching moves to (*FAIL), which c
3030  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
3031  backtrack into A.  backtrack into A.
3032  .P  .P
3033  Note also that a conditional subpattern is not considered as having two  Note that a conditional subpattern is not considered as having two
3034  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
3035  a conditional subpattern has a different meaning. Ignoring white space,  a conditional subpattern has a different meaning. Ignoring white space,
3036  consider:  consider:
# Line 2879  starting position, but allowing an advan Line 3052  starting position, but allowing an advan
3052  unanchored pattern). (*SKIP) is similar, except that the advance may be more  unanchored pattern). (*SKIP) is similar, except that the advance may be more
3053  than one character. (*COMMIT) is the strongest, causing the entire match to  than one character. (*COMMIT) is the strongest, causing the entire match to
3054  fail.  fail.
3055    .
3056    .
3057    .SS "More than one backtracking verb"
3058    .rs
3059    .sp
3060    If more than one backtracking verb is present in a pattern, the one that is
3061    backtracked onto first acts. For example, consider this pattern, where A, B,
3062    etc. are complex pattern fragments:
3063    .sp
3064      (A(*COMMIT)B(*THEN)C|ABD)
3065    .sp
3066    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3067    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3068    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3069    not always the same as Perl's. It means that if two or more backtracking verbs
3070    appear in succession, all the the last of them has no effect. Consider this
3071    example:
3072    .sp
3073      ...(*COMMIT)(*PRUNE)...
3074    .sp
3075    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3076    it to be triggered, and its action is taken. There can never be a backtrack
3077    onto (*COMMIT).
3078    .
3079    .
3080    .\" HTML <a name="btrepeat"></a>
3081    .SS "Backtracking verbs in repeated groups"
3082    .rs
3083    .sp
3084    PCRE differs from Perl in its handling of backtracking verbs in repeated
3085    groups. For example, consider:
3086    .sp
3087      /(a(*COMMIT)b)+ac/
3088    .sp
3089    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3090    the second repeat of the group acts.
3091    .
3092    .
3093    .\" HTML <a name="btassert"></a>
3094    .SS "Backtracking verbs in assertions"
3095    .rs
3096    .sp
3097    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3098    .P
3099    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3100    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3101    fail without any further processing.
3102    .P
3103    The other backtracking verbs are not treated specially if they appear in a
3104    positive assertion. In particular, (*THEN) skips to the next alternative in the
3105    innermost enclosing group that has alternations, whether or not this is within
3106    the assertion.
3107    .P
3108    Negative assertions are, however, different, in order to ensure that changing a
3109    positive assertion into a negative assertion changes its result. Backtracking
3110    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3111    without considering any further alternative branches in the assertion.
3112    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3113    within the assertion (the normal behaviour), but if the assertion does not have
3114    such an alternative, (*THEN) behaves like (*PRUNE).
3115    .
3116    .
3117    .\" HTML <a name="btsub"></a>
3118    .SS "Backtracking verbs in subroutines"
3119    .rs
3120    .sp
3121    These behaviours occur whether or not the subpattern is called recursively.
3122    Perl's treatment of subroutines is different in some cases.
3123    .P
3124    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3125    an immediate backtrack.
3126    .P
3127    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3128    succeed without any further processing. Matching then continues after the
3129    subroutine call.
3130    .P
3131    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3132    the subroutine match to fail.
3133  .P  .P
3134  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
3135  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
3136  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.  
3137  .  .
3138  .  .
3139  .SH "SEE ALSO"  .SH "SEE ALSO"
3140  .rs  .rs
3141  .sp  .sp
3142  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3143  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3144  .  .
3145  .  .
3146  .SH AUTHOR  .SH AUTHOR
# Line 2913  Cambridge CB2 3QH, England. Line 3157  Cambridge CB2 3QH, England.
3157  .rs  .rs
3158  .sp  .sp
3159  .nf  .nf
3160  Last updated: 17 June 2012  Last updated: 06 September 2013
3161  Copyright (c) 1997-2012 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3162  .fi  .fi

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