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Documentation update
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 or
23 \fBpcre_compile2()\fP is called, PCRE automatically inserts callouts, all with
24 number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
25 is used with the pattern
26 .sp
27 A(\ed{2}|--)
28 .sp
29 it is processed as if it were
30 .sp
31 (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
32 .sp
33 Notice that there is a callout before and after each parenthesis and
34 alternation bar. Automatic callouts can be used for tracking the progress of
35 pattern matching. The
36 .\" HREF
37 \fBpcretest\fP
38 .\"
39 command has an option that sets automatic callouts; when it is used, the output
40 indicates how the pattern is matched. This is useful information when you are
41 trying to optimize the performance of a particular pattern.
42 .
43 .
44 .SH "MISSING CALLOUTS"
45 .rs
46 .sp
47 You should be aware that, because of optimizations in the way PCRE matches
48 patterns by default, callouts sometimes do not happen. For example, if the
49 pattern is
50 .sp
51 ab(?C4)cd
52 .sp
53 PCRE knows that any matching string must contain the letter "d". If the subject
54 string is "abyz", the lack of "d" means that matching doesn't ever start, and
55 the callout is never reached. However, with "abyd", though the result is still
56 no match, the callout is obeyed.
57 .P
58 If the pattern is studied, PCRE knows the minimum length of a matching string,
59 and will immediately give a "no match" return without actually running a match
60 if the subject is not long enough, or, for unanchored patterns, if it has
61 been scanned far enough.
62 .P
63 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
64 option to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. This slows down the
65 matching process, but does ensure that callouts such as the example above are
66 obeyed.
67 .
68 .
69 .SH "THE CALLOUT INTERFACE"
70 .rs
71 .sp
72 During matching, when PCRE reaches a callout point, the external function
73 defined by \fIpcre_callout\fP is called (if it is set). This applies to both
74 the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
75 only argument to the callout function is a pointer to a \fBpcre_callout\fP
76 block. This structure contains the following fields:
77 .sp
78 int \fIversion\fP;
79 int \fIcallout_number\fP;
80 int *\fIoffset_vector\fP;
81 const char *\fIsubject\fP;
82 int \fIsubject_length\fP;
83 int \fIstart_match\fP;
84 int \fIcurrent_position\fP;
85 int \fIcapture_top\fP;
86 int \fIcapture_last\fP;
87 void *\fIcallout_data\fP;
88 int \fIpattern_position\fP;
89 int \fInext_item_length\fP;
90 .sp
91 The \fIversion\fP field is an integer containing the version number of the
92 block format. The initial version was 0; the current version is 1. The version
93 number will change again in future if additional fields are added, but the
94 intention is never to remove any of the existing fields.
95 .P
96 The \fIcallout_number\fP field contains the number of the callout, as compiled
97 into the pattern (that is, the number after ?C for manual callouts, and 255 for
98 automatically generated callouts).
99 .P
100 The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
101 passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
102 \fBpcre_exec()\fP is used, the contents can be inspected in order to extract
103 substrings that have been matched so far, in the same way as for extracting
104 substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
105 not useful.
106 .P
107 The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
108 that were passed to \fBpcre_exec()\fP.
109 .P
110 The \fIstart_match\fP field normally contains the offset within the subject at
111 which the current match attempt started. However, if the escape sequence \eK
112 has been encountered, this value is changed to reflect the modified starting
113 point. If the pattern is not anchored, the callout function may be called
114 several times from the same point in the pattern for different starting points
115 in the subject.
116 .P
117 The \fIcurrent_position\fP field contains the offset within the subject of the
118 current match pointer.
119 .P
120 When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
121 contains one more than the number of the highest numbered captured substring so
122 far. If no substrings have been captured, the value of \fIcapture_top\fP is
123 one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
124 does not support captured substrings.
125 .P
126 The \fIcapture_last\fP field contains the number of the most recently captured
127 substring. If no substrings have been captured, its value is -1. This is always
128 the case when \fBpcre_dfa_exec()\fP is used.
129 .P
130 The \fIcallout_data\fP field contains a value that is passed to
131 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
132 passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
133 \fBpcre_extra\fP data structure. If no such data was passed, the value of
134 \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
135 description of the \fBpcre_extra\fP structure in the
136 .\" HREF
137 \fBpcreapi\fP
138 .\"
139 documentation.
140 .P
141 The \fIpattern_position\fP field is present from version 1 of the
142 \fIpcre_callout\fP structure. It contains the offset to the next item to be
143 matched in the pattern string.
144 .P
145 The \fInext_item_length\fP field is present from version 1 of the
146 \fIpcre_callout\fP structure. It contains the length of the next item to be
147 matched in the pattern string. When the callout immediately precedes an
148 alternation bar, a closing parenthesis, or the end of the pattern, the length
149 is zero. When the callout precedes an opening parenthesis, the length is that
150 of the entire subpattern.
151 .P
152 The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
153 help in distinguishing between different automatic callouts, which all have the
154 same callout number. However, they are set for all callouts.
155 .
156 .
157 .SH "RETURN VALUES"
158 .rs
159 .sp
160 The external callout function returns an integer to PCRE. If the value is zero,
161 matching proceeds as normal. If the value is greater than zero, matching fails
162 at the current point, but the testing of other matching possibilities goes
163 ahead, just as if a lookahead assertion had failed. If the value is less than
164 zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
165 returns the negative value.
166 .P
167 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
168 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
169 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
170 it will never be used by PCRE itself.
171 .
172 .
173 .SH AUTHOR
174 .rs
175 .sp
176 .nf
177 Philip Hazel
178 University Computing Service
179 Cambridge CB2 3QH, England.
180 .fi
181 .
182 .
183 .SH REVISION
184 .rs
185 .sp
186 .nf
187 Last updated: 29 September 2009
188 Copyright (c) 1997-2009 University of Cambridge.
189 .fi

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