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1 .TH PCRECALLOUT 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "PCRE CALLOUTS"
5 .rs
6 .sp
7 .B int (*pcre_callout)(pcre_callout_block *);
8 .PP
9 PCRE provides a feature called "callout", which is a means of temporarily
10 passing control to the caller of PCRE in the middle of pattern matching. The
11 caller of PCRE provides an external function by putting its entry point in the
12 global variable \fIpcre_callout\fP. By default, this variable contains NULL,
13 which disables all calling out.
14 .P
15 Within a regular expression, (?C) indicates the points at which the external
16 function is to be called. Different callout points can be identified by putting
17 a number less than 256 after the letter C. The default value is zero.
18 For example, this pattern has two callout points:
19 .sp
20 (?C1)abc(?C2)def
21 .sp
22 If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP is called,
23 PCRE automatically inserts callouts, all with number 255, before each item in
24 the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
25 .sp
26 A(\ed{2}|--)
27 .sp
28 it is processed as if it were
29 .sp
30 (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
31 .sp
32 Notice that there is a callout before and after each parenthesis and
33 alternation bar. Automatic callouts can be used for tracking the progress of
34 pattern matching. The
35 .\" HREF
36 \fBpcretest\fP
37 .\"
38 command has an option that sets automatic callouts; when it is used, the output
39 indicates how the pattern is matched. This is useful information when you are
40 trying to optimize the performance of a particular pattern.
41 .
42 .
43 .SH "MISSING CALLOUTS"
44 .rs
45 .sp
46 You should be aware that, because of optimizations in the way PCRE matches
47 patterns, callouts sometimes do not happen. For example, if the pattern is
48 .sp
49 ab(?C4)cd
50 .sp
51 PCRE knows that any matching string must contain the letter "d". If the subject
52 string is "abyz", the lack of "d" means that matching doesn't ever start, and
53 the callout is never reached. However, with "abyd", though the result is still
54 no match, the callout is obeyed.
55 .
56 .
57 .SH "THE CALLOUT INTERFACE"
58 .rs
59 .sp
60 During matching, when PCRE reaches a callout point, the external function
61 defined by \fIpcre_callout\fP is called (if it is set). This applies to both
62 the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
63 only argument to the callout function is a pointer to a \fBpcre_callout\fP
64 block. This structure contains the following fields:
65 .sp
66 int \fIversion\fP;
67 int \fIcallout_number\fP;
68 int *\fIoffset_vector\fP;
69 const char *\fIsubject\fP;
70 int \fIsubject_length\fP;
71 int \fIstart_match\fP;
72 int \fIcurrent_position\fP;
73 int \fIcapture_top\fP;
74 int \fIcapture_last\fP;
75 void *\fIcallout_data\fP;
76 int \fIpattern_position\fP;
77 int \fInext_item_length\fP;
78 .sp
79 The \fIversion\fP field is an integer containing the version number of the
80 block format. The initial version was 0; the current version is 1. The version
81 number will change again in future if additional fields are added, but the
82 intention is never to remove any of the existing fields.
83 .P
84 The \fIcallout_number\fP field contains the number of the callout, as compiled
85 into the pattern (that is, the number after ?C for manual callouts, and 255 for
86 automatically generated callouts).
87 .P
88 The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
89 passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
90 \fBpcre_exec()\fP is used, the contents can be inspected in order to extract
91 substrings that have been matched so far, in the same way as for extracting
92 substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
93 not useful.
94 .P
95 The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
96 that were passed to \fBpcre_exec()\fP.
97 .P
98 The \fIstart_match\fP field contains the offset within the subject at which the
99 current match attempt started. If the pattern is not anchored, the callout
100 function may be called several times from the same point in the pattern for
101 different starting points in the subject.
102 .P
103 The \fIcurrent_position\fP field contains the offset within the subject of the
104 current match pointer.
105 .P
106 When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
107 contains one more than the number of the highest numbered captured substring so
108 far. If no substrings have been captured, the value of \fIcapture_top\fP is
109 one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
110 does not support captured substrings.
111 .P
112 The \fIcapture_last\fP field contains the number of the most recently captured
113 substring. If no substrings have been captured, its value is -1. This is always
114 the case when \fBpcre_dfa_exec()\fP is used.
115 .P
116 The \fIcallout_data\fP field contains a value that is passed to
117 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
118 passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
119 \fBpcre_extra\fP data structure. If no such data was passed, the value of
120 \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
121 description of the \fBpcre_extra\fP structure in the
122 .\" HREF
123 \fBpcreapi\fP
124 .\"
125 documentation.
126 .P
127 The \fIpattern_position\fP field is present from version 1 of the
128 \fIpcre_callout\fP structure. It contains the offset to the next item to be
129 matched in the pattern string.
130 .P
131 The \fInext_item_length\fP field is present from version 1 of the
132 \fIpcre_callout\fP structure. It contains the length of the next item to be
133 matched in the pattern string. When the callout immediately precedes an
134 alternation bar, a closing parenthesis, or the end of the pattern, the length
135 is zero. When the callout precedes an opening parenthesis, the length is that
136 of the entire subpattern.
137 .P
138 The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
139 help in distinguishing between different automatic callouts, which all have the
140 same callout number. However, they are set for all callouts.
141 .
142 .
143 .SH "RETURN VALUES"
144 .rs
145 .sp
146 The external callout function returns an integer to PCRE. If the value is zero,
147 matching proceeds as normal. If the value is greater than zero, matching fails
148 at the current point, but the testing of other matching possibilities goes
149 ahead, just as if a lookahead assertion had failed. If the value is less than
150 zero, the match is abandoned, and \fBpcre_exec()\fP (or \fBpcre_dfa_exec()\fP)
151 returns the negative value.
152 .P
153 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
154 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
155 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
156 it will never be used by PCRE itself.
157 .
158 .
159 .SH AUTHOR
160 .rs
161 .sp
162 .nf
163 Philip Hazel
164 University Computing Service
165 Cambridge CB2 3QH, England.
166 .fi
167 .
168 .
169 .SH REVISION
170 .rs
171 .sp
172 .nf
173 Last updated: 06 March 2007
174 Copyright (c) 1997-2007 University of Cambridge.
175 .fi

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