Annotation of embedaddon/pcre/doc/pcrecallout.3, revision 1.1.1.1
1.1 misho 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: .P
43: The use of callouts in a pattern makes it ineligible for optimization by the
44: just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
45: option always fails.
46: .
47: .
48: .SH "MISSING CALLOUTS"
49: .rs
50: .sp
51: You should be aware that, because of optimizations in the way PCRE matches
52: patterns by default, callouts sometimes do not happen. For example, if the
53: pattern is
54: .sp
55: ab(?C4)cd
56: .sp
57: PCRE knows that any matching string must contain the letter "d". If the subject
58: string is "abyz", the lack of "d" means that matching doesn't ever start, and
59: the callout is never reached. However, with "abyd", though the result is still
60: no match, the callout is obeyed.
61: .P
62: If the pattern is studied, PCRE knows the minimum length of a matching string,
63: and will immediately give a "no match" return without actually running a match
64: if the subject is not long enough, or, for unanchored patterns, if it has
65: been scanned far enough.
66: .P
67: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
68: option to \fBpcre_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\fP,
69: or by starting the pattern with (*NO_START_OPT). This slows down the matching
70: process, but does ensure that callouts such as the example above are obeyed.
71: .
72: .
73: .SH "THE CALLOUT INTERFACE"
74: .rs
75: .sp
76: During matching, when PCRE reaches a callout point, the external function
77: defined by \fIpcre_callout\fP is called (if it is set). This applies to both
78: the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
79: only argument to the callout function is a pointer to a \fBpcre_callout\fP
80: block. This structure contains the following fields:
81: .sp
82: int \fIversion\fP;
83: int \fIcallout_number\fP;
84: int *\fIoffset_vector\fP;
85: const char *\fIsubject\fP;
86: int \fIsubject_length\fP;
87: int \fIstart_match\fP;
88: int \fIcurrent_position\fP;
89: int \fIcapture_top\fP;
90: int \fIcapture_last\fP;
91: void *\fIcallout_data\fP;
92: int \fIpattern_position\fP;
93: int \fInext_item_length\fP;
94: const unsigned char *\fImark\fP;
95: .sp
96: The \fIversion\fP field is an integer containing the version number of the
97: block format. The initial version was 0; the current version is 2. The version
98: number will change again in future if additional fields are added, but the
99: intention is never to remove any of the existing fields.
100: .P
101: The \fIcallout_number\fP field contains the number of the callout, as compiled
102: into the pattern (that is, the number after ?C for manual callouts, and 255 for
103: automatically generated callouts).
104: .P
105: The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
106: passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
107: \fBpcre_exec()\fP is used, the contents can be inspected in order to extract
108: substrings that have been matched so far, in the same way as for extracting
109: substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
110: not useful.
111: .P
112: The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
113: that were passed to \fBpcre_exec()\fP.
114: .P
115: The \fIstart_match\fP field normally contains the offset within the subject at
116: which the current match attempt started. However, if the escape sequence \eK
117: has been encountered, this value is changed to reflect the modified starting
118: point. If the pattern is not anchored, the callout function may be called
119: several times from the same point in the pattern for different starting points
120: in the subject.
121: .P
122: The \fIcurrent_position\fP field contains the offset within the subject of the
123: current match pointer.
124: .P
125: When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
126: contains one more than the number of the highest numbered captured substring so
127: far. If no substrings have been captured, the value of \fIcapture_top\fP is
128: one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
129: does not support captured substrings.
130: .P
131: The \fIcapture_last\fP field contains the number of the most recently captured
132: substring. If no substrings have been captured, its value is -1. This is always
133: the case when \fBpcre_dfa_exec()\fP is used.
134: .P
135: The \fIcallout_data\fP field contains a value that is passed to
136: \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
137: passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
138: \fBpcre_extra\fP data structure. If no such data was passed, the value of
139: \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
140: description of the \fBpcre_extra\fP structure in the
141: .\" HREF
142: \fBpcreapi\fP
143: .\"
144: documentation.
145: .P
146: The \fIpattern_position\fP field is present from version 1 of the
147: \fIpcre_callout\fP structure. It contains the offset to the next item to be
148: matched in the pattern string.
149: .P
150: The \fInext_item_length\fP field is present from version 1 of the
151: \fIpcre_callout\fP structure. It contains the length of the next item to be
152: matched in the pattern string. When the callout immediately precedes an
153: alternation bar, a closing parenthesis, or the end of the pattern, the length
154: is zero. When the callout precedes an opening parenthesis, the length is that
155: of the entire subpattern.
156: .P
157: The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
158: help in distinguishing between different automatic callouts, which all have the
159: same callout number. However, they are set for all callouts.
160: .P
161: The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP
162: structure. In callouts from \fBpcre_exec()\fP it contains a pointer to the
163: zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
164: item in the match, or NULL if no such items have been passed. Instances of
165: (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
166: callouts from \fBpcre_dfa_exec()\fP this field always contains NULL.
167: .
168: .
169: .SH "RETURN VALUES"
170: .rs
171: .sp
172: The external callout function returns an integer to PCRE. If the value is zero,
173: matching proceeds as normal. If the value is greater than zero, matching fails
174: at the current point, but the testing of other matching possibilities goes
175: ahead, just as if a lookahead assertion had failed. If the value is less than
176: zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
177: returns the negative value.
178: .P
179: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
180: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
181: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
182: it will never be used by PCRE itself.
183: .
184: .
185: .SH AUTHOR
186: .rs
187: .sp
188: .nf
189: Philip Hazel
190: University Computing Service
191: Cambridge CB2 3QH, England.
192: .fi
193: .
194: .
195: .SH REVISION
196: .rs
197: .sp
198: .nf
199: Last updated: 30 November 2011
200: Copyright (c) 1997-2011 University of Cambridge.
201: .fi
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