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Tue Aug 2 11:00:40 2011 UTC (3 years, 2 months ago) by ph10
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Documentation and general text tidies in preparation for test release.

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_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\fP,
65 or by starting the pattern with (*NO_START_OPT). This slows down the matching
66 process, but does ensure that callouts such as the example above are 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 const unsigned char *\fImark\fP;
91 .sp
92 The \fIversion\fP field is an integer containing the version number of the
93 block format. The initial version was 0; the current version is 2. The version
94 number will change again in future if additional fields are added, but the
95 intention is never to remove any of the existing fields.
96 .P
97 The \fIcallout_number\fP field contains the number of the callout, as compiled
98 into the pattern (that is, the number after ?C for manual callouts, and 255 for
99 automatically generated callouts).
100 .P
101 The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
102 passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
103 \fBpcre_exec()\fP is used, the contents can be inspected in order to extract
104 substrings that have been matched so far, in the same way as for extracting
105 substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
106 not useful.
107 .P
108 The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
109 that were passed to \fBpcre_exec()\fP.
110 .P
111 The \fIstart_match\fP field normally contains the offset within the subject at
112 which the current match attempt started. However, if the escape sequence \eK
113 has been encountered, this value is changed to reflect the modified starting
114 point. If the pattern is not anchored, the callout function may be called
115 several times from the same point in the pattern for different starting points
116 in the subject.
117 .P
118 The \fIcurrent_position\fP field contains the offset within the subject of the
119 current match pointer.
120 .P
121 When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
122 contains one more than the number of the highest numbered captured substring so
123 far. If no substrings have been captured, the value of \fIcapture_top\fP is
124 one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
125 does not support captured substrings.
126 .P
127 The \fIcapture_last\fP field contains the number of the most recently captured
128 substring. If no substrings have been captured, its value is -1. This is always
129 the case when \fBpcre_dfa_exec()\fP is used.
130 .P
131 The \fIcallout_data\fP field contains a value that is passed to
132 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
133 passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
134 \fBpcre_extra\fP data structure. If no such data was passed, the value of
135 \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
136 description of the \fBpcre_extra\fP structure in the
137 .\" HREF
138 \fBpcreapi\fP
139 .\"
140 documentation.
141 .P
142 The \fIpattern_position\fP field is present from version 1 of the
143 \fIpcre_callout\fP structure. It contains the offset to the next item to be
144 matched in the pattern string.
145 .P
146 The \fInext_item_length\fP field is present from version 1 of the
147 \fIpcre_callout\fP structure. It contains the length of the next item to be
148 matched in the pattern string. When the callout immediately precedes an
149 alternation bar, a closing parenthesis, or the end of the pattern, the length
150 is zero. When the callout precedes an opening parenthesis, the length is that
151 of the entire subpattern.
152 .P
153 The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
154 help in distinguishing between different automatic callouts, which all have the
155 same callout number. However, they are set for all callouts.
156 .P
157 The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP
158 structure. In callouts from \fBpcre_exec()\fP it contains a pointer to the
159 zero-terminated name of the most recently passed (*MARK) item in the match, or
160 NULL if there are no (*MARK)s in the current matching path. In callouts from
161 \fBpcre_dfa_exec()\fP this field always contains NULL.
162 .
163 .
164 .SH "RETURN VALUES"
165 .rs
166 .sp
167 The external callout function returns an integer to PCRE. If the value is zero,
168 matching proceeds as normal. If the value is greater than zero, matching fails
169 at the current point, but the testing of other matching possibilities goes
170 ahead, just as if a lookahead assertion had failed. If the value is less than
171 zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
172 returns the negative value.
173 .P
174 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
175 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
176 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
177 it will never be used by PCRE itself.
178 .
179 .
180 .SH AUTHOR
181 .rs
182 .sp
183 .nf
184 Philip Hazel
185 University Computing Service
186 Cambridge CB2 3QH, England.
187 .fi
188 .
189 .
190 .SH REVISION
191 .rs
192 .sp
193 .nf
194 Last updated: 31 July 2011
195 Copyright (c) 1997-2011 University of Cambridge.
196 .fi

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