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1 <html>
2 <head>
3 <title>pcrecallout specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrecallout man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
17 <li><a name="TOC2" href="#SEC2">MISSING CALLOUTS</a>
18 <li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a>
19 <li><a name="TOC4" href="#SEC4">RETURN VALUES</a>
20 </ul>
21 <br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
22 <P>
23 <b>int (*pcre_callout)(pcre_callout_block *);</b>
24 </P>
25 <P>
26 PCRE provides a feature called "callout", which is a means of temporarily
27 passing control to the caller of PCRE in the middle of pattern matching. The
28 caller of PCRE provides an external function by putting its entry point in the
29 global variable <i>pcre_callout</i>. By default, this variable contains NULL,
30 which disables all calling out.
31 </P>
32 <P>
33 Within a regular expression, (?C) indicates the points at which the external
34 function is to be called. Different callout points can be identified by putting
35 a number less than 256 after the letter C. The default value is zero.
36 For example, this pattern has two callout points:
37 <pre>
38 (?C1)\deabc(?C2)def
39 </pre>
40 If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> is called,
41 PCRE automatically inserts callouts, all with number 255, before each item in
42 the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
43 <pre>
44 A(\d{2}|--)
45 </pre>
46 it is processed as if it were
47 <br>
48 <br>
49 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
50 <br>
51 <br>
52 Notice that there is a callout before and after each parenthesis and
53 alternation bar. Automatic callouts can be used for tracking the progress of
54 pattern matching. The
55 <a href="pcretest.html"><b>pcretest</b></a>
56 command has an option that sets automatic callouts; when it is used, the output
57 indicates how the pattern is matched. This is useful information when you are
58 trying to optimize the performance of a particular pattern.
59 </P>
60 <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
61 <P>
62 You should be aware that, because of optimizations in the way PCRE matches
63 patterns, callouts sometimes do not happen. For example, if the pattern is
64 <pre>
65 ab(?C4)cd
66 </pre>
67 PCRE knows that any matching string must contain the letter "d". If the subject
68 string is "abyz", the lack of "d" means that matching doesn't ever start, and
69 the callout is never reached. However, with "abyd", though the result is still
70 no match, the callout is obeyed.
71 </P>
72 <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
73 <P>
74 During matching, when PCRE reaches a callout point, the external function
75 defined by <i>pcre_callout</i> is called (if it is set). This applies to both
76 the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
77 only argument to the callout function is a pointer to a <b>pcre_callout</b>
78 block. This structure contains the following fields:
79 <pre>
80 int <i>version</i>;
81 int <i>callout_number</i>;
82 int *<i>offset_vector</i>;
83 const char *<i>subject</i>;
84 int <i>subject_length</i>;
85 int <i>start_match</i>;
86 int <i>current_position</i>;
87 int <i>capture_top</i>;
88 int <i>capture_last</i>;
89 void *<i>callout_data</i>;
90 int <i>pattern_position</i>;
91 int <i>next_item_length</i>;
92 </pre>
93 The <i>version</i> field is an integer containing the version number of the
94 block format. The initial version was 0; the current version is 1. The version
95 number will change again in future if additional fields are added, but the
96 intention is never to remove any of the existing fields.
97 </P>
98 <P>
99 The <i>callout_number</i> field contains the number of the callout, as compiled
100 into the pattern (that is, the number after ?C for manual callouts, and 255 for
101 automatically generated callouts).
102 </P>
103 <P>
104 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
105 passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
106 <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
107 substrings that have been matched so far, in the same way as for extracting
108 substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
109 not useful.
110 </P>
111 <P>
112 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
113 that were passed to <b>pcre_exec()</b>.
114 </P>
115 <P>
116 The <i>start_match</i> field contains the offset within the subject at which the
117 current match attempt started. If the pattern is not anchored, the callout
118 function may be called several times from the same point in the pattern for
119 different starting points in the subject.
120 </P>
121 <P>
122 The <i>current_position</i> field contains the offset within the subject of the
123 current match pointer.
124 </P>
125 <P>
126 When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
127 contains one more than the number of the highest numbered captured substring so
128 far. If no substrings have been captured, the value of <i>capture_top</i> is
129 one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
130 does not support captured substrings.
131 </P>
132 <P>
133 The <i>capture_last</i> field contains the number of the most recently captured
134 substring. If no substrings have been captured, its value is -1. This is always
135 the case when <b>pcre_dfa_exec()</b> is used.
136 </P>
137 <P>
138 The <i>callout_data</i> field contains a value that is passed to
139 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
140 passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
141 <b>pcre_extra</b> data structure. If no such data was passed, the value of
142 <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
143 description of the <b>pcre_extra</b> structure in the
144 <a href="pcreapi.html"><b>pcreapi</b></a>
145 documentation.
146 </P>
147 <P>
148 The <i>pattern_position</i> field is present from version 1 of the
149 <i>pcre_callout</i> structure. It contains the offset to the next item to be
150 matched in the pattern string.
151 </P>
152 <P>
153 The <i>next_item_length</i> field is present from version 1 of the
154 <i>pcre_callout</i> structure. It contains the length of the next item to be
155 matched in the pattern string. When the callout immediately precedes an
156 alternation bar, a closing parenthesis, or the end of the pattern, the length
157 is zero. When the callout precedes an opening parenthesis, the length is that
158 of the entire subpattern.
159 </P>
160 <P>
161 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
162 help in distinguishing between different automatic callouts, which all have the
163 same callout number. However, they are set for all callouts.
164 </P>
165 <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
166 <P>
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 <b>pcre_exec()</b> (or <b>pcre_dfa_exec()</b>)
172 returns the negative value.
173 </P>
174 <P>
175 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
176 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
177 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
178 it will never be used by PCRE itself.
179 </P>
180 <P>
181 Last updated: 28 February 2005
182 <br>
183 Copyright &copy; 1997-2005 University of Cambridge.
184 <p>
185 Return to the <a href="index.html">PCRE index page</a>.
186 </p>

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