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1 .TH PCRE 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)\deabc(?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). The only argument is a
62 pointer to a \fBpcre_callout\fP block. This structure contains the following
63 fields:
64 .sp
65 int \fIversion\fP;
66 int \fIcallout_number\fP;
67 int *\fIoffset_vector\fP;
68 const char *\fIsubject\fP;
69 int \fIsubject_length\fP;
70 int \fIstart_match\fP;
71 int \fIcurrent_position\fP;
72 int \fIcapture_top\fP;
73 int \fIcapture_last\fP;
74 void *\fIcallout_data\fP;
75 int \fIpattern_position\fP;
76 int \fInext_item_length\fP;
77 .sp
78 The \fIversion\fP field is an integer containing the version number of the
79 block format. The initial version was 0; the current version is 1. The version
80 number will change again in future if additional fields are added, but the
81 intention is never to remove any of the existing fields.
82 .P
83 The \fIcallout_number\fP field contains the number of the callout, as compiled
84 into the pattern (that is, the number after ?C for manual callouts, and 255 for
85 automatically generated callouts).
86 .P
87 The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
88 passed by the caller to \fBpcre_exec()\fP. The contents can be inspected in
89 order to extract substrings that have been matched so far, in the same way as
90 for extracting substrings after a match has completed.
91 .P
92 The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
93 that were passed to \fBpcre_exec()\fP.
94 .P
95 The \fIstart_match\fP field contains the offset within the subject at which the
96 current match attempt started. If the pattern is not anchored, the callout
97 function may be called several times from the same point in the pattern for
98 different starting points in the subject.
99 .P
100 The \fIcurrent_position\fP field contains the offset within the subject of the
101 current match pointer.
102 .P
103 The \fIcapture_top\fP field contains one more than the number of the highest
104 numbered captured substring so far. If no substrings have been captured,
105 the value of \fIcapture_top\fP is one.
106 .P
107 The \fIcapture_last\fP field contains the number of the most recently captured
108 substring. If no substrings have been captured, its value is -1.
109 .P
110 The \fIcallout_data\fP field contains a value that is passed to
111 \fBpcre_exec()\fP by the caller specifically so that it can be passed back in
112 callouts. It is passed in the \fIpcre_callout\fP field of the \fBpcre_extra\fP
113 data structure. If no such data was passed, the value of \fIcallout_data\fP in
114 a \fBpcre_callout\fP block is NULL. There is a description of the
115 \fBpcre_extra\fP structure in the
116 .\" HREF
117 \fBpcreapi\fP
118 .\"
119 documentation.
120 .P
121 The \fIpattern_position\fP field is present from version 1 of the
122 \fIpcre_callout\fP structure. It contains the offset to the next item to be
123 matched in the pattern string.
124 .P
125 The \fInext_item_length\fP field is present from version 1 of the
126 \fIpcre_callout\fP structure. It contains the length of the next item to be
127 matched in the pattern string. When the callout immediately precedes an
128 alternation bar, a closing parenthesis, or the end of the pattern, the length
129 is zero. When the callout precedes an opening parenthesis, the length is that
130 of the entire subpattern.
131 .P
132 The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
133 help in distinguishing between different automatic callouts, which all have the
134 same callout number. However, they are set for all callouts.
135 .
136 .
137 .SH "RETURN VALUES"
138 .rs
139 .sp
140 The external callout function returns an integer to PCRE. If the value is zero,
141 matching proceeds as normal. If the value is greater than zero, matching fails
142 at the current point, but backtracking to test other matching possibilities
143 goes ahead, just as if a lookahead assertion had failed. If the value is less
144 than zero, the match is abandoned, and \fBpcre_exec()\fP returns the negative
145 value.
146 .P
147 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
148 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
149 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
150 it will never be used by PCRE itself.
151 .P
152 .in 0
153 Last updated: 09 September 2004
154 .br
155 Copyright (c) 1997-2004 University of Cambridge.

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