Annotation of embedaddon/pcre/doc/pcrecallout.3, revision 1.1.1.4
1.1.1.4 ! misho 1: .TH PCRECALLOUT 3 "03 March 2013" "PCRE 8.33"
1.1 misho 2: .SH NAME
3: PCRE - Perl-compatible regular expressions
1.1.1.4 ! misho 4: .SH SYNOPSIS
1.1 misho 5: .rs
6: .sp
1.1.1.4 ! misho 7: .B #include <pcre.h>
! 8: .PP
! 9: .SM
1.1 misho 10: .B int (*pcre_callout)(pcre_callout_block *);
11: .PP
1.1.1.2 misho 12: .B int (*pcre16_callout)(pcre16_callout_block *);
13: .PP
1.1.1.4 ! misho 14: .B int (*pcre32_callout)(pcre32_callout_block *);
! 15: .
! 16: .SH DESCRIPTION
! 17: .rs
! 18: .sp
1.1 misho 19: PCRE provides a feature called "callout", which is a means of temporarily
20: passing control to the caller of PCRE in the middle of pattern matching. The
21: caller of PCRE provides an external function by putting its entry point in the
1.1.1.2 misho 22: global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit
1.1.1.4 ! misho 23: library, \fIpcre32_callout\fP for the 32-bit library). By default, this
! 24: variable contains NULL, which disables all calling out.
1.1 misho 25: .P
26: Within a regular expression, (?C) indicates the points at which the external
27: function is to be called. Different callout points can be identified by putting
28: a number less than 256 after the letter C. The default value is zero.
29: For example, this pattern has two callout points:
30: .sp
31: (?C1)abc(?C2)def
32: .sp
1.1.1.2 misho 33: If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
34: automatically inserts callouts, all with number 255, before each item in the
35: pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
1.1 misho 36: .sp
37: A(\ed{2}|--)
38: .sp
39: it is processed as if it were
40: .sp
41: (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
42: .sp
43: Notice that there is a callout before and after each parenthesis and
1.1.1.4 ! misho 44: alternation bar. If the pattern contains a conditional group whose condition is
! 45: an assertion, an automatic callout is inserted immediately before the
! 46: condition. Such a callout may also be inserted explicitly, for example:
! 47: .sp
! 48: (?(?C9)(?=a)ab|de)
! 49: .sp
! 50: This applies only to assertion conditions (because they are themselves
! 51: independent groups).
! 52: .P
! 53: Automatic callouts can be used for tracking the progress of pattern matching.
! 54: The
1.1 misho 55: .\" HREF
56: \fBpcretest\fP
57: .\"
58: command has an option that sets automatic callouts; when it is used, the output
59: indicates how the pattern is matched. This is useful information when you are
60: trying to optimize the performance of a particular pattern.
61: .
62: .
63: .SH "MISSING CALLOUTS"
64: .rs
65: .sp
66: You should be aware that, because of optimizations in the way PCRE matches
67: patterns by default, callouts sometimes do not happen. For example, if the
68: pattern is
69: .sp
70: ab(?C4)cd
71: .sp
72: PCRE knows that any matching string must contain the letter "d". If the subject
73: string is "abyz", the lack of "d" means that matching doesn't ever start, and
74: the callout is never reached. However, with "abyd", though the result is still
75: no match, the callout is obeyed.
76: .P
77: If the pattern is studied, PCRE knows the minimum length of a matching string,
78: and will immediately give a "no match" return without actually running a match
79: if the subject is not long enough, or, for unanchored patterns, if it has
80: been scanned far enough.
81: .P
82: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
1.1.1.2 misho 83: option to the matching function, or by starting the pattern with
84: (*NO_START_OPT). This slows down the matching process, but does ensure that
85: callouts such as the example above are obeyed.
1.1 misho 86: .
87: .
88: .SH "THE CALLOUT INTERFACE"
89: .rs
90: .sp
91: During matching, when PCRE reaches a callout point, the external function
1.1.1.4 ! misho 92: defined by \fIpcre_callout\fP or \fIpcre[16|32]_callout\fP is called
! 93: (if it is set). This applies to both normal and DFA matching. The only
! 94: argument to the callout function is a pointer to a \fBpcre_callout\fP
! 95: or \fBpcre[16|32]_callout\fP block.
1.1.1.2 misho 96: These structures contains the following fields:
97: .sp
98: int \fIversion\fP;
99: int \fIcallout_number\fP;
100: int *\fIoffset_vector\fP;
101: const char *\fIsubject\fP; (8-bit version)
102: PCRE_SPTR16 \fIsubject\fP; (16-bit version)
1.1.1.4 ! misho 103: PCRE_SPTR32 \fIsubject\fP; (32-bit version)
1.1.1.2 misho 104: int \fIsubject_length\fP;
105: int \fIstart_match\fP;
106: int \fIcurrent_position\fP;
107: int \fIcapture_top\fP;
108: int \fIcapture_last\fP;
109: void *\fIcallout_data\fP;
110: int \fIpattern_position\fP;
111: int \fInext_item_length\fP;
112: const unsigned char *\fImark\fP; (8-bit version)
113: const PCRE_UCHAR16 *\fImark\fP; (16-bit version)
1.1.1.4 ! misho 114: const PCRE_UCHAR32 *\fImark\fP; (32-bit version)
1.1 misho 115: .sp
116: The \fIversion\fP field is an integer containing the version number of the
117: block format. The initial version was 0; the current version is 2. The version
118: number will change again in future if additional fields are added, but the
119: intention is never to remove any of the existing fields.
120: .P
121: The \fIcallout_number\fP 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: The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
1.1.1.2 misho 126: passed by the caller to the matching function. When \fBpcre_exec()\fP or
1.1.1.4 ! misho 127: \fBpcre[16|32]_exec()\fP is used, the contents can be inspected, in order to
! 128: extract substrings that have been matched so far, in the same way as for
! 129: extracting substrings after a match has completed. For the DFA matching
! 130: functions, this field is not useful.
1.1 misho 131: .P
132: The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
1.1.1.2 misho 133: that were passed to the matching function.
1.1 misho 134: .P
135: The \fIstart_match\fP field normally contains the offset within the subject at
136: which the current match attempt started. However, if the escape sequence \eK
137: has been encountered, this value is changed to reflect the modified starting
138: point. If the pattern is not anchored, the callout function may be called
139: several times from the same point in the pattern for different starting points
140: in the subject.
141: .P
142: The \fIcurrent_position\fP field contains the offset within the subject of the
143: current match pointer.
144: .P
1.1.1.4 ! misho 145: When the \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP is used, the
1.1.1.2 misho 146: \fIcapture_top\fP field contains one more than the number of the highest
147: numbered captured substring so far. If no substrings have been captured, the
148: value of \fIcapture_top\fP is one. This is always the case when the DFA
149: functions are used, because they do not support captured substrings.
1.1 misho 150: .P
151: The \fIcapture_last\fP field contains the number of the most recently captured
1.1.1.4 ! misho 152: substring. However, when a recursion exits, the value reverts to what it was
! 153: outside the recursion, as do the values of all captured substrings. If no
! 154: substrings have been captured, the value of \fIcapture_last\fP is -1. This is
! 155: always the case for the DFA matching functions.
1.1 misho 156: .P
1.1.1.2 misho 157: The \fIcallout_data\fP field contains a value that is passed to a matching
158: function specifically so that it can be passed back in callouts. It is passed
1.1.1.4 ! misho 159: in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre[16|32]_extra\fP
1.1.1.2 misho 160: data structure. If no such data was passed, the value of \fIcallout_data\fP in
161: a callout block is NULL. There is a description of the \fBpcre_extra\fP
162: structure in the
1.1 misho 163: .\" HREF
164: \fBpcreapi\fP
165: .\"
166: documentation.
167: .P
1.1.1.2 misho 168: The \fIpattern_position\fP field is present from version 1 of the callout
169: structure. It contains the offset to the next item to be matched in the pattern
170: string.
171: .P
172: The \fInext_item_length\fP field is present from version 1 of the callout
173: structure. It contains the length of the next item to be matched in the pattern
174: string. When the callout immediately precedes an alternation bar, a closing
175: parenthesis, or the end of the pattern, the length is zero. When the callout
176: precedes an opening parenthesis, the length is that of the entire subpattern.
1.1 misho 177: .P
178: The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
179: help in distinguishing between different automatic callouts, which all have the
180: same callout number. However, they are set for all callouts.
181: .P
1.1.1.2 misho 182: The \fImark\fP field is present from version 2 of the callout structure. In
1.1.1.4 ! misho 183: callouts from \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP it contains a
! 184: pointer to the zero-terminated name of the most recently passed (*MARK),
! 185: (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been
! 186: passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
! 187: previous (*MARK). In callouts from the DFA matching functions this field always
! 188: contains NULL.
1.1 misho 189: .
190: .
191: .SH "RETURN VALUES"
192: .rs
193: .sp
194: The external callout function returns an integer to PCRE. If the value is zero,
195: matching proceeds as normal. If the value is greater than zero, matching fails
196: at the current point, but the testing of other matching possibilities goes
197: ahead, just as if a lookahead assertion had failed. If the value is less than
1.1.1.2 misho 198: zero, the match is abandoned, the matching function returns the negative value.
1.1 misho 199: .P
200: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
201: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
202: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
203: it will never be used by PCRE itself.
204: .
205: .
206: .SH AUTHOR
207: .rs
208: .sp
209: .nf
210: Philip Hazel
211: University Computing Service
212: Cambridge CB2 3QH, England.
213: .fi
214: .
215: .
216: .SH REVISION
217: .rs
218: .sp
219: .nf
1.1.1.4 ! misho 220: Last updated: 03 March 2013
! 221: Copyright (c) 1997-2013 University of Cambridge.
1.1 misho 222: .fi
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