Annotation of embedaddon/pcre/doc/html/pcrecallout.html, revision 1.1.1.2
1.1 misho 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: <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21: <li><a name="TOC6" href="#SEC6">REVISION</a>
22: </ul>
23: <br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
24: <P>
25: <b>int (*pcre_callout)(pcre_callout_block *);</b>
26: </P>
27: <P>
1.1.1.2 ! misho 28: <b>int (*pcre16_callout)(pcre16_callout_block *);</b>
! 29: </P>
! 30: <P>
1.1 misho 31: PCRE provides a feature called "callout", which is a means of temporarily
32: passing control to the caller of PCRE in the middle of pattern matching. The
33: caller of PCRE provides an external function by putting its entry point in the
1.1.1.2 ! misho 34: global variable <i>pcre_callout</i> (<i>pcre16_callout</i> for the 16-bit
! 35: library). By default, this variable contains NULL, which disables all calling
! 36: out.
1.1 misho 37: </P>
38: <P>
39: Within a regular expression, (?C) indicates the points at which the external
40: function is to be called. Different callout points can be identified by putting
41: a number less than 256 after the letter C. The default value is zero.
42: For example, this pattern has two callout points:
43: <pre>
44: (?C1)abc(?C2)def
45: </pre>
1.1.1.2 ! misho 46: If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
! 47: automatically inserts callouts, all with number 255, before each item in the
! 48: pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
1.1 misho 49: <pre>
50: A(\d{2}|--)
51: </pre>
52: it is processed as if it were
53: <br>
54: <br>
55: (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
56: <br>
57: <br>
58: Notice that there is a callout before and after each parenthesis and
59: alternation bar. Automatic callouts can be used for tracking the progress of
60: pattern matching. The
61: <a href="pcretest.html"><b>pcretest</b></a>
62: command has an option that sets automatic callouts; when it is used, the output
63: indicates how the pattern is matched. This is useful information when you are
64: trying to optimize the performance of a particular pattern.
65: </P>
66: <P>
67: The use of callouts in a pattern makes it ineligible for optimization by the
68: just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
69: option always fails.
70: </P>
71: <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
72: <P>
73: You should be aware that, because of optimizations in the way PCRE matches
74: patterns by default, callouts sometimes do not happen. For example, if the
75: pattern is
76: <pre>
77: ab(?C4)cd
78: </pre>
79: PCRE knows that any matching string must contain the letter "d". If the subject
80: string is "abyz", the lack of "d" means that matching doesn't ever start, and
81: the callout is never reached. However, with "abyd", though the result is still
82: no match, the callout is obeyed.
83: </P>
84: <P>
85: If the pattern is studied, PCRE knows the minimum length of a matching string,
86: and will immediately give a "no match" return without actually running a match
87: if the subject is not long enough, or, for unanchored patterns, if it has
88: been scanned far enough.
89: </P>
90: <P>
91: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
1.1.1.2 ! misho 92: option to the matching function, or by starting the pattern with
! 93: (*NO_START_OPT). This slows down the matching process, but does ensure that
! 94: callouts such as the example above are obeyed.
1.1 misho 95: </P>
96: <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
97: <P>
98: During matching, when PCRE reaches a callout point, the external function
1.1.1.2 ! misho 99: defined by <i>pcre_callout</i> or <i>pcre16_callout</i> is called (if it is set).
! 100: This applies to both normal and DFA matching. The only argument to the callout
! 101: function is a pointer to a <b>pcre_callout</b> or <b>pcre16_callout</b> block.
! 102: These structures contains the following fields:
1.1 misho 103: <pre>
1.1.1.2 ! misho 104: int <i>version</i>;
! 105: int <i>callout_number</i>;
! 106: int *<i>offset_vector</i>;
! 107: const char *<i>subject</i>; (8-bit version)
! 108: PCRE_SPTR16 <i>subject</i>; (16-bit version)
! 109: int <i>subject_length</i>;
! 110: int <i>start_match</i>;
! 111: int <i>current_position</i>;
! 112: int <i>capture_top</i>;
! 113: int <i>capture_last</i>;
! 114: void *<i>callout_data</i>;
! 115: int <i>pattern_position</i>;
! 116: int <i>next_item_length</i>;
! 117: const unsigned char *<i>mark</i>; (8-bit version)
! 118: const PCRE_UCHAR16 *<i>mark</i>; (16-bit version)
1.1 misho 119: </pre>
120: The <i>version</i> field is an integer containing the version number of the
121: block format. The initial version was 0; the current version is 2. The version
122: number will change again in future if additional fields are added, but the
123: intention is never to remove any of the existing fields.
124: </P>
125: <P>
126: The <i>callout_number</i> field contains the number of the callout, as compiled
127: into the pattern (that is, the number after ?C for manual callouts, and 255 for
128: automatically generated callouts).
129: </P>
130: <P>
131: The <i>offset_vector</i> field is a pointer to the vector of offsets that was
1.1.1.2 ! misho 132: passed by the caller to the matching function. When <b>pcre_exec()</b> or
! 133: <b>pcre16_exec()</b> is used, the contents can be inspected, in order to extract
1.1 misho 134: substrings that have been matched so far, in the same way as for extracting
1.1.1.2 ! misho 135: substrings after a match has completed. For the DFA matching functions, this
! 136: field is not useful.
1.1 misho 137: </P>
138: <P>
139: The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
1.1.1.2 ! misho 140: that were passed to the matching function.
1.1 misho 141: </P>
142: <P>
143: The <i>start_match</i> field normally contains the offset within the subject at
144: which the current match attempt started. However, if the escape sequence \K
145: has been encountered, this value is changed to reflect the modified starting
146: point. If the pattern is not anchored, the callout function may be called
147: several times from the same point in the pattern for different starting points
148: in the subject.
149: </P>
150: <P>
151: The <i>current_position</i> field contains the offset within the subject of the
152: current match pointer.
153: </P>
154: <P>
1.1.1.2 ! misho 155: When the <b>pcre_exec()</b> or <b>pcre16_exec()</b> is used, the
! 156: <i>capture_top</i> field contains one more than the number of the highest
! 157: numbered captured substring so far. If no substrings have been captured, the
! 158: value of <i>capture_top</i> is one. This is always the case when the DFA
! 159: functions are used, because they do not support captured substrings.
1.1 misho 160: </P>
161: <P>
162: The <i>capture_last</i> field contains the number of the most recently captured
163: substring. If no substrings have been captured, its value is -1. This is always
1.1.1.2 ! misho 164: the case for the DFA matching functions.
1.1 misho 165: </P>
166: <P>
1.1.1.2 ! misho 167: The <i>callout_data</i> field contains a value that is passed to a matching
! 168: function specifically so that it can be passed back in callouts. It is passed
! 169: in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre16_extra</b>
! 170: data structure. If no such data was passed, the value of <i>callout_data</i> in
! 171: a callout block is NULL. There is a description of the <b>pcre_extra</b>
! 172: structure in the
1.1 misho 173: <a href="pcreapi.html"><b>pcreapi</b></a>
174: documentation.
175: </P>
176: <P>
1.1.1.2 ! misho 177: The <i>pattern_position</i> field is present from version 1 of the callout
! 178: structure. It contains the offset to the next item to be matched in the pattern
! 179: string.
1.1 misho 180: </P>
181: <P>
1.1.1.2 ! misho 182: The <i>next_item_length</i> field is present from version 1 of the callout
! 183: structure. It contains the length of the next item to be matched in the pattern
! 184: string. When the callout immediately precedes an alternation bar, a closing
! 185: parenthesis, or the end of the pattern, the length is zero. When the callout
! 186: precedes an opening parenthesis, the length is that of the entire subpattern.
1.1 misho 187: </P>
188: <P>
189: The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
190: help in distinguishing between different automatic callouts, which all have the
191: same callout number. However, they are set for all callouts.
192: </P>
193: <P>
1.1.1.2 ! misho 194: The <i>mark</i> field is present from version 2 of the callout structure. In
! 195: callouts from <b>pcre_exec()</b> or <b>pcre16_exec()</b> it contains a pointer to
! 196: the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
! 197: (*THEN) item in the match, or NULL if no such items have been passed. Instances
! 198: of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
! 199: callouts from the DFA matching functions this field always contains NULL.
1.1 misho 200: </P>
201: <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
202: <P>
203: The external callout function returns an integer to PCRE. If the value is zero,
204: matching proceeds as normal. If the value is greater than zero, matching fails
205: at the current point, but the testing of other matching possibilities goes
206: ahead, just as if a lookahead assertion had failed. If the value is less than
1.1.1.2 ! misho 207: zero, the match is abandoned, the matching function returns the negative value.
1.1 misho 208: </P>
209: <P>
210: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
211: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
212: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
213: it will never be used by PCRE itself.
214: </P>
215: <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
216: <P>
217: Philip Hazel
218: <br>
219: University Computing Service
220: <br>
221: Cambridge CB2 3QH, England.
222: <br>
223: </P>
224: <br><a name="SEC6" href="#TOC1">REVISION</a><br>
225: <P>
1.1.1.2 ! misho 226: Last updated: 08 Janurary 2012
1.1 misho 227: <br>
1.1.1.2 ! misho 228: Copyright © 1997-2012 University of Cambridge.
1.1 misho 229: <br>
230: <p>
231: Return to the <a href="index.html">PCRE index page</a>.
232: </p>
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