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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). The only argument is a
76 pointer to a <b>pcre_callout</b> block. This structure contains the following
77 fields:
78 <pre>
79 int <i>version</i>;
80 int <i>callout_number</i>;
81 int *<i>offset_vector</i>;
82 const char *<i>subject</i>;
83 int <i>subject_length</i>;
84 int <i>start_match</i>;
85 int <i>current_position</i>;
86 int <i>capture_top</i>;
87 int <i>capture_last</i>;
88 void *<i>callout_data</i>;
89 int <i>pattern_position</i>;
90 int <i>next_item_length</i>;
91 </pre>
92 The <i>version</i> field is an integer containing the version number of the
93 block format. The initial version was 0; the current version is 1. 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 <P>
98 The <i>callout_number</i> field contains the number of the callout, as compiled
99 into the pattern (that is, the number after ?C for manual callouts, and 255 for
100 automatically generated callouts).
101 </P>
102 <P>
103 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
104 passed by the caller to <b>pcre_exec()</b>. The contents can be inspected in
105 order to extract substrings that have been matched so far, in the same way as
106 for extracting substrings after a match has completed.
107 </P>
108 <P>
109 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
110 that were passed to <b>pcre_exec()</b>.
111 </P>
112 <P>
113 The <i>start_match</i> field contains the offset within the subject at which the
114 current match attempt started. If the pattern is not anchored, the callout
115 function may be called several times from the same point in the pattern for
116 different starting points in the subject.
117 </P>
118 <P>
119 The <i>current_position</i> field contains the offset within the subject of the
120 current match pointer.
121 </P>
122 <P>
123 The <i>capture_top</i> field contains one more than the number of the highest
124 numbered captured substring so far. If no substrings have been captured,
125 the value of <i>capture_top</i> is one.
126 </P>
127 <P>
128 The <i>capture_last</i> field contains the number of the most recently captured
129 substring. If no substrings have been captured, its value is -1.
130 </P>
131 <P>
132 The <i>callout_data</i> field contains a value that is passed to
133 <b>pcre_exec()</b> by the caller specifically so that it can be passed back in
134 callouts. It is passed in the <i>pcre_callout</i> field of the <b>pcre_extra</b>
135 data structure. If no such data was passed, the value of <i>callout_data</i> in
136 a <b>pcre_callout</b> block is NULL. There is a description of the
137 <b>pcre_extra</b> structure in the
138 <a href="pcreapi.html"><b>pcreapi</b></a>
139 documentation.
140 </P>
141 <P>
142 The <i>pattern_position</i> field is present from version 1 of the
143 <i>pcre_callout</i> structure. It contains the offset to the next item to be
144 matched in the pattern string.
145 </P>
146 <P>
147 The <i>next_item_length</i> field is present from version 1 of the
148 <i>pcre_callout</i> structure. It contains the length of the next item to be
149 matched in the pattern string. When the callout immediately precedes an
150 alternation bar, a closing parenthesis, or the end of the pattern, the length
151 is zero. When the callout precedes an opening parenthesis, the length is that
152 of the entire subpattern.
153 </P>
154 <P>
155 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
156 help in distinguishing between different automatic callouts, which all have the
157 same callout number. However, they are set for all callouts.
158 </P>
159 <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
160 <P>
161 The external callout function returns an integer to PCRE. If the value is zero,
162 matching proceeds as normal. If the value is greater than zero, matching fails
163 at the current point, but backtracking to test other matching possibilities
164 goes ahead, just as if a lookahead assertion had failed. If the value is less
165 than zero, the match is abandoned, and <b>pcre_exec()</b> returns the negative
166 value.
167 </P>
168 <P>
169 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
170 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
171 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
172 it will never be used by PCRE itself.
173 </P>
174 <P>
175 Last updated: 09 September 2004
176 <br>
177 Copyright &copy; 1997-2004 University of Cambridge.
178 <p>
179 Return to the <a href="index.html">PCRE index page</a>.
180 </p>

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