1PCRECALLOUT(3) Library Functions Manual PCRECALLOUT(3)
2
6 PCRE - Perl-compatible regular expressions
7
9 #include <pcre.h>
11 int (*pcre_callout)(pcre_callout_block *);
13 int (*pcre16_callout)(pcre16_callout_block *);
15 int (*pcre32_callout)(pcre32_callout_block *);
17
19 PCRE provides a feature called "callout", which is a means of temporar‐
21 ily passing control to the caller of PCRE in the middle of pattern
22 matching. The caller of PCRE provides an external function by putting
23 its entry point in the global variable pcre_callout (pcre16_callout for
24 the 16-bit library, pcre32_callout for the 32-bit library). By default,
25 this variable contains NULL, which disables all calling out.
26 Within a regular expression, (?C) indicates the points at which the
28 external function is to be called. Different callout points can be
29 identified by putting a number less than 256 after the letter C. The
30 default value is zero. For example, this pattern has two callout
31 points:
32 (?C1)abc(?C2)def
34 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled,
36 PCRE automatically inserts callouts, all with number 255, before each
37 item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the
38 pattern
39 A(\d{2}|--)
41 it is processed as if it were
43 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
45 Notice that there is a callout before and after each parenthesis and
47 alternation bar. Automatic callouts can be used for tracking the
48 progress of pattern matching. The pcretest command has an option that
49 sets automatic callouts; when it is used, the output indicates how the
50 pattern is matched. This is useful information when you are trying to
51 optimize the performance of a particular pattern.
52 The use of callouts in a pattern makes it ineligible for optimization
54 by the just-in-time compiler. Studying such a pattern with the
55 PCRE_STUDY_JIT_COMPILE option always fails.
56
58 You should be aware that, because of optimizations in the way PCRE
60 matches patterns by default, callouts sometimes do not happen. For
61 example, if the pattern is
62 ab(?C4)cd
64 PCRE knows that any matching string must contain the letter "d". If the
66 subject string is "abyz", the lack of "d" means that matching doesn't
67 ever start, and the callout is never reached. However, with "abyd",
68 though the result is still no match, the callout is obeyed.
69 If the pattern is studied, PCRE knows the minimum length of a matching
71 string, and will immediately give a "no match" return without actually
72 running a match if the subject is not long enough, or, for unanchored
73 patterns, if it has been scanned far enough.
74 You can disable these optimizations by passing the PCRE_NO_START_OPTI‐
76 MIZE option to the matching function, or by starting the pattern with
77 (*NO_START_OPT). This slows down the matching process, but does ensure
78 that callouts such as the example above are obeyed.
79
81 During matching, when PCRE reaches a callout point, the external func‐
83 tion defined by pcre_callout or pcre[16|32]_callout is called (if it is
84 set). This applies to both normal and DFA matching. The only argument
85 to the callout function is a pointer to a pcre_callout or
86 pcre[16|32]_callout block. These structures contains the following
87 fields:
88 int version;
90 int callout_number;
91 int *offset_vector;
92 const char *subject; (8-bit version)
93 PCRE_SPTR16 subject; (16-bit version)
94 PCRE_SPTR32 subject; (32-bit version)
95 int subject_length;
96 int start_match;
97 int current_position;
98 int capture_top;
99 int capture_last;
100 void *callout_data;
101 int pattern_position;
102 int next_item_length;
103 const unsigned char *mark; (8-bit version)
104 const PCRE_UCHAR16 *mark; (16-bit version)
105 const PCRE_UCHAR32 *mark; (32-bit version)
106 The version field is an integer containing the version number of the
108 block format. The initial version was 0; the current version is 2. The
109 version number will change again in future if additional fields are
110 added, but the intention is never to remove any of the existing fields.
111 The callout_number field contains the number of the callout, as com‐
113 piled into the pattern (that is, the number after ?C for manual call‐
114 outs, and 255 for automatically generated callouts).
115 The offset_vector field is a pointer to the vector of offsets that was
117 passed by the caller to the matching function. When pcre_exec() or
118 pcre[16|32]_exec() is used, the contents can be inspected, in order to
119 extract substrings that have been matched so far, in the same way as
120 for extracting substrings after a match has completed. For the DFA
121 matching functions, this field is not useful.
122 The subject and subject_length fields contain copies of the values that
124 were passed to the matching function.
125 The start_match field normally contains the offset within the subject
127 at which the current match attempt started. However, if the escape
128 sequence \K has been encountered, this value is changed to reflect the
129 modified starting point. If the pattern is not anchored, the callout
130 function may be called several times from the same point in the pattern
131 for different starting points in the subject.
132 The current_position field contains the offset within the subject of
134 the current match pointer.
135 When the pcre_exec() or pcre[16|32]_exec() is used, the capture_top
137 field contains one more than the number of the highest numbered cap‐
138 tured substring so far. If no substrings have been captured, the value
139 of capture_top is one. This is always the case when the DFA functions
140 are used, because they do not support captured substrings.
141 The capture_last field contains the number of the most recently cap‐
143 tured substring. If no substrings have been captured, its value is -1.
144 This is always the case for the DFA matching functions.
145 The callout_data field contains a value that is passed to a matching
147 function specifically so that it can be passed back in callouts. It is
148 passed in the callout_data field of a pcre_extra or pcre[16|32]_extra
149 data structure. If no such data was passed, the value of callout_data
150 in a callout block is NULL. There is a description of the pcre_extra
151 structure in the pcreapi documentation.
152 The pattern_position field is present from version 1 of the callout
154 structure. It contains the offset to the next item to be matched in the
155 pattern string.
156 The next_item_length field is present from version 1 of the callout
158 structure. It contains the length of the next item to be matched in the
159 pattern string. When the callout immediately precedes an alternation
160 bar, a closing parenthesis, or the end of the pattern, the length is
161 zero. When the callout precedes an opening parenthesis, the length is
162 that of the entire subpattern.
163 The pattern_position and next_item_length fields are intended to help
165 in distinguishing between different automatic callouts, which all have
166 the same callout number. However, they are set for all callouts.
167 The mark field is present from version 2 of the callout structure. In
169 callouts from pcre_exec() or pcre[16|32]_exec() it contains a pointer
170 to the zero-terminated name of the most recently passed (*MARK),
171 (*PRUNE), or (*THEN) item in the match, or NULL if no such items have
172 been passed. Instances of (*PRUNE) or (*THEN) without a name do not
173 obliterate a previous (*MARK). In callouts from the DFA matching func‐
174 tions this field always contains NULL.
175
177 The external callout function returns an integer to PCRE. If the value
179 is zero, matching proceeds as normal. If the value is greater than
180 zero, matching fails at the current point, but the testing of other
181 matching possibilities goes ahead, just as if a lookahead assertion had
182 failed. If the value is less than zero, the match is abandoned, the
183 matching function returns the negative value.
184 Negative values should normally be chosen from the set of
186 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan‐
187 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
188 reserved for use by callout functions; it will never be used by PCRE
189 itself.
190
192 Philip Hazel
194 University Computing Service
195 Cambridge CB2 3QH, England.
196
198 Last updated: 24 June 2012
200 Copyright (c) 1997-2012 University of Cambridge.
201PCRE 8.30 24 June 2012 PCRECALLOUT(3)