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revision 63 by nigel, Sat Feb 24 21:40:03 2007 UTC revision 772 by ph10, Wed Nov 30 10:18:07 2011 UTC
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1  .TH PCRE 3  .TH PCRECALLOUT 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH PCRE CALLOUTS  .SH "PCRE CALLOUTS"
5  .rs  .rs
6  .sp  .sp
7  .B int (*pcre_callout)(pcre_callout_block *);  .B int (*pcre_callout)(pcre_callout_block *);
# Line 9  PCRE - Perl-compatible regular expressio Line 9  PCRE - Perl-compatible regular expressio
9  PCRE provides a feature called "callout", which is a means of temporarily  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  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  caller of PCRE provides an external function by putting its entry point in the
12  global variable \fIpcre_callout\fR. By default, this variable contains NULL,  global variable \fIpcre_callout\fP. By default, this variable contains NULL,
13  which disables all calling out.  which disables all calling out.
14    .P
15  Within a regular expression, (?C) indicates the points at which the external  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  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.  a number less than 256 after the letter C. The default value is zero.
18  For example, this pattern has two callout points:  For example, this pattern has two callout points:
19    .sp
20    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
21    .sp
22  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is  If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or
23  set), the external function is called. Its only argument is a pointer to a  \fBpcre_compile2()\fP is called, PCRE automatically inserts callouts, all with
24  \fBpcre_callout\fR block. This contains the following variables:  number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
25    is used with the pattern
26    int          \fIversion\fR;  .sp
27    int          \fIcallout_number\fR;    A(\ed{2}|--)
28    int         *\fIoffset_vector\fR;  .sp
29    const char  *\fIsubject\fR;  it is processed as if it were
30    int          \fIsubject_length\fR;  .sp
31    int          \fIstart_match\fR;  (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
32    int          \fIcurrent_position\fR;  .sp
33    int          \fIcapture_top\fR;  Notice that there is a callout before and after each parenthesis and
34    int          \fIcapture_last\fR;  alternation bar. Automatic callouts can be used for tracking the progress of
35    void        *\fIcallout_data\fR;  pattern matching. The
36    .\" HREF
37  The \fIversion\fR field is an integer containing the version number of the  \fBpcretest\fP
38  block format. The current version is zero. The version number may change in  .\"
39  future if additional fields are added, but the intention is never to remove any  command has an option that sets automatic callouts; when it is used, the output
40  of the existing fields.  indicates how the pattern is matched. This is useful information when you are
41    trying to optimize the performance of a particular pattern.
42  The \fIcallout_number\fR field contains the number of the callout, as compiled  .P
43  into the pattern (that is, the number after ?C).  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  The \fIoffset_vector\fR field is a pointer to the vector of offsets that was  option always fails.
46  passed by the caller to \fBpcre_exec()\fR. The contents can be inspected in  .
47  order to extract substrings that have been matched so far, in the same way as  .
48  for extracting substrings after a match has completed.  .SH "MISSING CALLOUTS"
49    .rs
50  The \fIsubject\fR and \fIsubject_length\fR fields contain copies the values  .sp
51  that were passed to \fBpcre_exec()\fR.  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  The \fIstart_match\fR field contains the offset within the subject at which the  pattern is
54  current match attempt started. If the pattern is not anchored, the callout  .sp
55  function may be called several times for different starting points.    ab(?C4)cd
56    .sp
57  The \fIcurrent_position\fR field contains the offset within the subject of the  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.  current match pointer.
124    .P
125  The \fIcapture_top\fR field contains the number of the highest captured  When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
126  substring so far.  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  The \fIcapture_last\fR field contains the number of the most recently captured  one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
129  substring.  does not support captured substrings.
130    .P
131  The \fIcallout_data\fR field contains a value that is passed to  The \fIcapture_last\fP field contains the number of the most recently captured
132  \fBpcre_exec()\fR by the caller specifically so that it can be passed back in  substring. If no substrings have been captured, its value is -1. This is always
133  callouts. It is passed in the \fIpcre_callout\fR field of the \fBpcre_extra\fR  the case when \fBpcre_dfa_exec()\fP is used.
134  data structure. If no such data was passed, the value of \fIcallout_data\fR in  .P
135  a \fBpcre_callout\fR block is NULL. There is a description of the  The \fIcallout_data\fP field contains a value that is passed to
136  \fBpcre_extra\fR structure in the \fBpcreapi\fR documentation.  \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  .SH RETURN VALUES  \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
140  .rs  description of the \fBpcre_extra\fP structure in the
141  .sp  .\" HREF
142  The callout function returns an integer. If the value is zero, matching  \fBpcreapi\fP
143  proceeds as normal. If the value is greater than zero, matching fails at the  .\"
144  current point, but backtracking to test other possibilities goes ahead, just as  documentation.
145  if a lookahead assertion had failed. If the value is less than zero, the match  .P
146  is abandoned, and \fBpcre_exec()\fR returns the value.  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  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.  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;  The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
182  it will never be used by PCRE itself.  it will never be used by PCRE itself.
183    .
184  .in 0  .
185  Last updated: 21 January 2003  .SH AUTHOR
186  .br  .rs
187  Copyright (c) 1997-2003 University of Cambridge.  .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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