Annotation of embedaddon/pcre/doc/html/pcrecallout.html, revision 1.1.1.1
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>
28: PCRE provides a feature called "callout", which is a means of temporarily
29: passing control to the caller of PCRE in the middle of pattern matching. The
30: caller of PCRE provides an external function by putting its entry point in the
31: global variable <i>pcre_callout</i>. By default, this variable contains NULL,
32: which disables all calling out.
33: </P>
34: <P>
35: Within a regular expression, (?C) indicates the points at which the external
36: function is to be called. Different callout points can be identified by putting
37: a number less than 256 after the letter C. The default value is zero.
38: For example, this pattern has two callout points:
39: <pre>
40: (?C1)abc(?C2)def
41: </pre>
42: If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> or
43: <b>pcre_compile2()</b> is called, PCRE automatically inserts callouts, all with
44: number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
45: is used with the pattern
46: <pre>
47: A(\d{2}|--)
48: </pre>
49: it is processed as if it were
50: <br>
51: <br>
52: (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
53: <br>
54: <br>
55: Notice that there is a callout before and after each parenthesis and
56: alternation bar. Automatic callouts can be used for tracking the progress of
57: pattern matching. The
58: <a href="pcretest.html"><b>pcretest</b></a>
59: command has an option that sets automatic callouts; when it is used, the output
60: indicates how the pattern is matched. This is useful information when you are
61: trying to optimize the performance of a particular pattern.
62: </P>
63: <P>
64: The use of callouts in a pattern makes it ineligible for optimization by the
65: just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
66: option always fails.
67: </P>
68: <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
69: <P>
70: You should be aware that, because of optimizations in the way PCRE matches
71: patterns by default, callouts sometimes do not happen. For example, if the
72: pattern is
73: <pre>
74: ab(?C4)cd
75: </pre>
76: PCRE knows that any matching string must contain the letter "d". If the subject
77: string is "abyz", the lack of "d" means that matching doesn't ever start, and
78: the callout is never reached. However, with "abyd", though the result is still
79: no match, the callout is obeyed.
80: </P>
81: <P>
82: If the pattern is studied, PCRE knows the minimum length of a matching string,
83: and will immediately give a "no match" return without actually running a match
84: if the subject is not long enough, or, for unanchored patterns, if it has
85: been scanned far enough.
86: </P>
87: <P>
88: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
89: option to <b>pcre_compile()</b>, <b>pcre_exec()</b>, or <b>pcre_dfa_exec()</b>,
90: or by starting the pattern with (*NO_START_OPT). This slows down the matching
91: process, but does ensure that callouts such as the example above are obeyed.
92: </P>
93: <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
94: <P>
95: During matching, when PCRE reaches a callout point, the external function
96: defined by <i>pcre_callout</i> is called (if it is set). This applies to both
97: the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
98: only argument to the callout function is a pointer to a <b>pcre_callout</b>
99: block. This structure contains the following fields:
100: <pre>
101: int <i>version</i>;
102: int <i>callout_number</i>;
103: int *<i>offset_vector</i>;
104: const char *<i>subject</i>;
105: int <i>subject_length</i>;
106: int <i>start_match</i>;
107: int <i>current_position</i>;
108: int <i>capture_top</i>;
109: int <i>capture_last</i>;
110: void *<i>callout_data</i>;
111: int <i>pattern_position</i>;
112: int <i>next_item_length</i>;
113: const unsigned char *<i>mark</i>;
114: </pre>
115: The <i>version</i> field is an integer containing the version number of the
116: block format. The initial version was 0; the current version is 2. The version
117: number will change again in future if additional fields are added, but the
118: intention is never to remove any of the existing fields.
119: </P>
120: <P>
121: The <i>callout_number</i> field contains the number of the callout, as compiled
122: into the pattern (that is, the number after ?C for manual callouts, and 255 for
123: automatically generated callouts).
124: </P>
125: <P>
126: The <i>offset_vector</i> field is a pointer to the vector of offsets that was
127: passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
128: <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
129: substrings that have been matched so far, in the same way as for extracting
130: substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
131: not useful.
132: </P>
133: <P>
134: The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
135: that were passed to <b>pcre_exec()</b>.
136: </P>
137: <P>
138: The <i>start_match</i> field normally contains the offset within the subject at
139: which the current match attempt started. However, if the escape sequence \K
140: has been encountered, this value is changed to reflect the modified starting
141: point. If the pattern is not anchored, the callout function may be called
142: several times from the same point in the pattern for different starting points
143: in the subject.
144: </P>
145: <P>
146: The <i>current_position</i> field contains the offset within the subject of the
147: current match pointer.
148: </P>
149: <P>
150: When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
151: contains one more than the number of the highest numbered captured substring so
152: far. If no substrings have been captured, the value of <i>capture_top</i> is
153: one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
154: does not support captured substrings.
155: </P>
156: <P>
157: The <i>capture_last</i> field contains the number of the most recently captured
158: substring. If no substrings have been captured, its value is -1. This is always
159: the case when <b>pcre_dfa_exec()</b> is used.
160: </P>
161: <P>
162: The <i>callout_data</i> field contains a value that is passed to
163: <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
164: passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
165: <b>pcre_extra</b> data structure. If no such data was passed, the value of
166: <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
167: description of the <b>pcre_extra</b> structure in the
168: <a href="pcreapi.html"><b>pcreapi</b></a>
169: documentation.
170: </P>
171: <P>
172: The <i>pattern_position</i> field is present from version 1 of the
173: <i>pcre_callout</i> structure. It contains the offset to the next item to be
174: matched in the pattern string.
175: </P>
176: <P>
177: The <i>next_item_length</i> field is present from version 1 of the
178: <i>pcre_callout</i> structure. It contains the length of the next item to be
179: matched in the pattern string. When the callout immediately precedes an
180: alternation bar, a closing parenthesis, or the end of the pattern, the length
181: is zero. When the callout precedes an opening parenthesis, the length is that
182: of the entire subpattern.
183: </P>
184: <P>
185: The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
186: help in distinguishing between different automatic callouts, which all have the
187: same callout number. However, they are set for all callouts.
188: </P>
189: <P>
190: The <i>mark</i> field is present from version 2 of the <i>pcre_callout</i>
191: structure. In callouts from <b>pcre_exec()</b> it contains a pointer to the
192: zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
193: item in the match, or NULL if no such items have been passed. Instances of
194: (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
195: callouts from <b>pcre_dfa_exec()</b> this field always contains NULL.
196: </P>
197: <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
198: <P>
199: The external callout function returns an integer to PCRE. If the value is zero,
200: matching proceeds as normal. If the value is greater than zero, matching fails
201: at the current point, but the testing of other matching possibilities goes
202: ahead, just as if a lookahead assertion had failed. If the value is less than
203: zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
204: returns the negative value.
205: </P>
206: <P>
207: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
208: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
209: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
210: it will never be used by PCRE itself.
211: </P>
212: <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
213: <P>
214: Philip Hazel
215: <br>
216: University Computing Service
217: <br>
218: Cambridge CB2 3QH, England.
219: <br>
220: </P>
221: <br><a name="SEC6" href="#TOC1">REVISION</a><br>
222: <P>
223: Last updated: 30 November 2011
224: <br>
225: Copyright © 1997-2011 University of Cambridge.
226: <br>
227: <p>
228: Return to the <a href="index.html">PCRE index page</a>.
229: </p>
FreeBSD-CVSweb <freebsd-cvsweb@FreeBSD.org>
![[BACK]](/icons/cvsweb/back.gif){kind=icon}
Return to pcrecallout.html CVS log ![[TXT]](/icons/cvsweb/text.gif){kind=icon}
![[DIR]](/icons/cvsweb/dir.gif){kind=icon}
Up to [ELWIX - Embedded LightWeight unIX -] / embedaddon / pcre / doc / html