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Revision 836 - (show annotations) (download)
Wed Dec 28 17:16:11 2011 UTC (2 years, 9 months ago) by ph10
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Merging all the changes from the pcre16 branch into the trunk.

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 .P
43 The use of callouts in a pattern makes it ineligible for optimization by the
44 just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
45 option always fails.
46 .
47 .
48 .SH "MISSING CALLOUTS"
49 .rs
50 .sp
51 You should be aware that, because of optimizations in the way PCRE matches
52 patterns by default, callouts sometimes do not happen. For example, if the
53 pattern is
54 .sp
55 ab(?C4)cd
56 .sp
57 PCRE knows that any matching string must contain the letter "d". If the subject
58 string is "abyz", the lack of "d" means that matching doesn't ever start, and
59 the callout is never reached. However, with "abyd", though the result is still
60 no match, the callout is obeyed.
61 .P
62 If the pattern is studied, PCRE knows the minimum length of a matching string,
63 and will immediately give a "no match" return without actually running a match
64 if the subject is not long enough, or, for unanchored patterns, if it has
65 been scanned far enough.
66 .P
67 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
68 option to \fBpcre_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\fP,
69 or by starting the pattern with (*NO_START_OPT). This slows down the matching
70 process, but does ensure that callouts such as the example above are obeyed.
71 .
72 .
73 .SH "THE CALLOUT INTERFACE"
74 .rs
75 .sp
76 During matching, when PCRE reaches a callout point, the external function
77 defined by \fIpcre_callout\fP is called (if it is set). This applies to both
78 the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
79 only argument to the callout function is a pointer to a \fBpcre_callout\fP
80 block. This structure contains the following fields:
81 .sp
82 int \fIversion\fP;
83 int \fIcallout_number\fP;
84 int *\fIoffset_vector\fP;
85 const char *\fIsubject\fP;
86 int \fIsubject_length\fP;
87 int \fIstart_match\fP;
88 int \fIcurrent_position\fP;
89 int \fIcapture_top\fP;
90 int \fIcapture_last\fP;
91 void *\fIcallout_data\fP;
92 int \fIpattern_position\fP;
93 int \fInext_item_length\fP;
94 const unsigned char *\fImark\fP;
95 .sp
96 The \fIversion\fP field is an integer containing the version number of the
97 block format. The initial version was 0; the current version is 2. The version
98 number will change again in future if additional fields are added, but the
99 intention is never to remove any of the existing fields.
100 .P
101 The \fIcallout_number\fP field contains the number of the callout, as compiled
102 into the pattern (that is, the number after ?C for manual callouts, and 255 for
103 automatically generated callouts).
104 .P
105 The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
106 passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
107 \fBpcre_exec()\fP is used, the contents can be inspected in order to extract
108 substrings that have been matched so far, in the same way as for extracting
109 substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
110 not useful.
111 .P
112 The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
113 that were passed to \fBpcre_exec()\fP.
114 .P
115 The \fIstart_match\fP field normally contains the offset within the subject at
116 which the current match attempt started. However, if the escape sequence \eK
117 has been encountered, this value is changed to reflect the modified starting
118 point. If the pattern is not anchored, the callout function may be called
119 several times from the same point in the pattern for different starting points
120 in the subject.
121 .P
122 The \fIcurrent_position\fP field contains the offset within the subject of the
123 current match pointer.
124 .P
125 When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
126 contains one more than the number of the highest numbered captured substring so
127 far. If no substrings have been captured, the value of \fIcapture_top\fP is
128 one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
129 does not support captured substrings.
130 .P
131 The \fIcapture_last\fP field contains the number of the most recently captured
132 substring. If no substrings have been captured, its value is -1. This is always
133 the case when \fBpcre_dfa_exec()\fP is used.
134 .P
135 The \fIcallout_data\fP field contains a value that is passed to
136 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
137 passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
138 \fBpcre_extra\fP data structure. If no such data was passed, the value of
139 \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
140 description of the \fBpcre_extra\fP structure in the
141 .\" HREF
142 \fBpcreapi\fP
143 .\"
144 documentation.
145 .P
146 The \fIpattern_position\fP field is present from version 1 of the
147 \fIpcre_callout\fP structure. It contains the offset to the next item to be
148 matched in the pattern string.
149 .P
150 The \fInext_item_length\fP field is present from version 1 of the
151 \fIpcre_callout\fP structure. It contains the length of the next item to be
152 matched in the pattern string. When the callout immediately precedes an
153 alternation bar, a closing parenthesis, or the end of the pattern, the length
154 is zero. When the callout precedes an opening parenthesis, the length is that
155 of the entire subpattern.
156 .P
157 The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
158 help in distinguishing between different automatic callouts, which all have the
159 same callout number. However, they are set for all callouts.
160 .P
161 The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP
162 structure. In callouts from \fBpcre_exec()\fP it contains a pointer to the
163 zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
164 item in the match, or NULL if no such items have been passed. Instances of
165 (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
166 callouts from \fBpcre_dfa_exec()\fP this field always contains NULL.
167 .
168 .
169 .SH "RETURN VALUES"
170 .rs
171 .sp
172 The external callout function returns an integer to PCRE. If the value is zero,
173 matching proceeds as normal. If the value is greater than zero, matching fails
174 at the current point, but the testing of other matching possibilities goes
175 ahead, just as if a lookahead assertion had failed. If the value is less than
176 zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
177 returns the negative value.
178 .P
179 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
180 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
181 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
182 it will never be used by PCRE itself.
183 .
184 .
185 .SH AUTHOR
186 .rs
187 .sp
188 .nf
189 Philip Hazel
190 University Computing Service
191 Cambridge CB2 3QH, England.
192 .fi
193 .
194 .
195 .SH REVISION
196 .rs
197 .sp
198 .nf
199 Last updated: 30 November 2011
200 Copyright (c) 1997-2011 University of Cambridge.
201 .fi

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