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1  .TH PCREPATTERN 3 "17 March 2013" "PCRE 8.33"  .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, an  there is now also support for UTF-8 strings in the original library, an
53  extra library that supports 16-bit and UTF-16 character strings, and a  extra library that supports 16-bit and UTF-16 character strings, and a
# Line 36  these special sequences: Line 64  these special sequences:
64  .sp  .sp
65  (*UTF) is a generic sequence that can be used with any of the libraries.  (*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), (*UTF16), (*UTF32) or (*UTF) 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 53  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.
 .P  
 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the  
 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are  
 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  
 \fBpcre[16|32]_exec()\fP (16- or 32-bit), is used. PCRE also has alternative  
 matching functions, \fBpcre_dfa_exec()\fP and \fBpcre[16|32_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.  
91  .  .
92  .  .
93  .SH "EBCDIC CHARACTER CODES"  .SS "Disabling start-up optimizations"
94  .rs  .rs
95  .sp  .sp
96  PCRE can be compiled to run in an environment that uses EBCDIC as its character  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
97  code rather than ASCII or Unicode (typically a mainframe system). In the  PCRE_NO_START_OPTIMIZE option either at compile or matching time.
 sections below, character code values are ASCII or Unicode; in an EBCDIC  
 environment these characters may have different code values, and there are no  
 code points greater than 255.  
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 117  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 where the circumflex and dollar assertions are  The newline convention affects where the circumflex and dollar assertions are
# Line 136  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 802  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. This is different from  example, \ep{Lu} always matches only upper case letters. This is different from
850  the behaviour of current versions of Perl.  the behaviour of current versions of Perl.
851  .P  .P
852  Matching characters by Unicode property is not fast, because PCRE has to do a  Matching characters by Unicode property is not fast, because PCRE has to do a
# Line 863  the "mark" property always have the "ext Line 907  the "mark" property always have the "ext
907  As well as the standard Unicode properties described above, PCRE supports four  As well as the standard Unicode properties described above, PCRE supports four
908  more that make it possible to convert traditional escape sequences such as \ew  more that make it possible to convert traditional escape sequences such as \ew
909  and \es and POSIX character classes to use Unicode properties. PCRE uses these  and \es and POSIX character classes to use Unicode properties. PCRE uses these
910  non-standard, non-Perl properties internally when PCRE_UCP is set. However,  non-standard, non-Perl properties internally when PCRE_UCP is set. However,
911  they may also be used explicitly. These properties are:  they may also be used explicitly. These properties are:
912  .sp  .sp
913    Xan   Any alphanumeric character    Xan   Any alphanumeric character
# Line 877  carriage return, and any other character Line 921  carriage return, and any other character
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  .P
924  There is another non-standard property, Xuc, which matches any character that  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  can be represented by a Universal Character Name in C++ and other programming
926  languages. These are the characters $, @, ` (grave accent), and all characters  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  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  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  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  where H is a hexadecimal digit. Note that the Xuc property does not match these
931  sequences but the characters that they represent.)  sequences but the characters that they represent.)
932  .  .
933  .  .
# Line 1366  above. There are also the (*UTF8), (*UTF Line 1410  above. There are also the (*UTF8), (*UTF
1410  sequences that can be used to set UTF and Unicode property modes; they are  sequences that can be used to set UTF and Unicode property modes; they are
1411  equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP  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  options, respectively. The (*UTF) sequence is a generic version that can be
1413  used with any of the libraries.  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 1532  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 1554  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 1975  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. (Perl sometimes, but not  capturing is carried out only for positive assertions. (Perl sometimes, but not
2036  always, does do capturing in 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
# Line 2646  explicit callout may also be set at this Line 2703  explicit callout may also be set at this
2703  .sp  .sp
2704    (?(?C9)(?=a)abc|def)    (?(?C9)(?=a)abc|def)
2705  .sp  .sp
2706  Note that this applies only to assertion conditions, not to other types of  Note that this applies only to assertion conditions, not to other types of
2707  condition.  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 2666  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 and 32-bit library.  .\" HTML <a href="#btassert">
2754  If the name is empty, that is, if the closing parenthesis immediately follows  .\" </a>
2755  the colon, the effect is as if the colon were not there. Any number of these  assertions,
2756  verbs may 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 2704  verbs may occur in a pattern. Line 2769  verbs may 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 2735  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 2796  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 a verb with a name is encountered in a positive assertion, its name is  If a verb with a name is encountered in a positive assertion that is true, the
2868  recorded and passed back if it is the last-encountered. This does not happen  name is recorded and passed back if it is the last-encountered. This does not
2869  for negative assertions.  happen for negative assertions or failing positive assertions.
2870  .P  .P
2871  After a partial match or a failed match, the last encountered name in the  After a partial match or a failed match, the last encountered name in the
2872  entire match process is returned. For example:  entire match process is returned. For example:
# Line 2827  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 2851  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 2870  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  In an anchored pattern (*PRUNE) has the same effect as (*COMMIT).  expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2958    as (*COMMIT).
2959  .P  .P
2960  The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).  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  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).  caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2963  .sp  .sp
# Line 2902  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  .P
2990  Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores  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).  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.  second alternative and tries COND2, without backtracking into COND1. If that
3005  If (*THEN) is not inside an alternation, it acts like (*PRUNE).  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3006    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  .P
3009  The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).  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  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).  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 2950  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 2972  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"
# Line 3006  Cambridge CB2 3QH, England. Line 3157  Cambridge CB2 3QH, England.
3157  .rs  .rs
3158  .sp  .sp
3159  .nf  .nf
3160  Last updated: 17 March 2013  Last updated: 06 September 2013
3161  Copyright (c) 1997-2013 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3162  .fi  .fi

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