trunk/doc/pcrecallout.3

.TH PCRECALLOUT 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE CALLOUTS"
.rs
.sp
.B int (*pcre_callout)(pcre_callout_block *);
.PP
.B int (*pcre16_callout)(pcre16_callout_block *);
.PP
PCRE provides a feature called "callout", which is a means of temporarily
passing control to the caller of PCRE in the middle of pattern matching. The
caller of PCRE provides an external function by putting its entry point in the
global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit 
library). By default, this variable contains NULL, which disables all calling
out.
.P
Within a regular expression, (?C) indicates the points at which the external
function is to be called. Different callout points can be identified by putting
a number less than 256 after the letter C. The default value is zero.
For example, this pattern has two callout points:
.sp
  (?C1)abc(?C2)def
.sp
If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
automatically inserts callouts, all with number 255, before each item in the
pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
.sp
  A(\ed{2}|--)
.sp
it is processed as if it were
.sp
(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
.sp
Notice that there is a callout before and after each parenthesis and
alternation bar. Automatic callouts can be used for tracking the progress of
pattern matching. The
.\" HREF
\fBpcretest\fP
.\"
command has an option that sets automatic callouts; when it is used, the output
indicates how the pattern is matched. This is useful information when you are
trying to optimize the performance of a particular pattern.
.P
The use of callouts in a pattern makes it ineligible for optimization by the
just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
option always fails.
.
.
.SH "MISSING CALLOUTS"
.rs
.sp
You should be aware that, because of optimizations in the way PCRE matches
patterns by default, callouts sometimes do not happen. For example, if the
pattern is
.sp
  ab(?C4)cd
.sp
PCRE knows that any matching string must contain the letter "d". If the subject
string is "abyz", the lack of "d" means that matching doesn't ever start, and
the callout is never reached. However, with "abyd", though the result is still
no match, the callout is obeyed.
.P
If the pattern is studied, PCRE knows the minimum length of a matching string,
and will immediately give a "no match" return without actually running a match
if the subject is not long enough, or, for unanchored patterns, if it has
been scanned far enough.
.P
You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
option to the matching function, or by starting the pattern with
(*NO_START_OPT). This slows down the matching process, but does ensure that
callouts such as the example above are obeyed.
.
.
.SH "THE CALLOUT INTERFACE"
.rs
.sp
During matching, when PCRE reaches a callout point, the external function
defined by \fIpcre_callout\fP or \fIpcre16_callout\fP is called (if it is set).
This applies to both normal and DFA matching. The only argument to the callout
function is a pointer to a \fBpcre_callout\fP or \fBpcre16_callout\fP block.
These structures contains the following fields:
.sp
  int           \fIversion\fP;
  int           \fIcallout_number\fP;
  int          *\fIoffset_vector\fP;
  const char   *\fIsubject\fP;           (8-bit version)
  PCRE_SPTR16   \fIsubject\fP;           (16-bit version) 
  int           \fIsubject_length\fP;
  int           \fIstart_match\fP;
  int           \fIcurrent_position\fP;
  int           \fIcapture_top\fP;
  int           \fIcapture_last\fP;
  void         *\fIcallout_data\fP;
  int           \fIpattern_position\fP;
  int           \fInext_item_length\fP;
  const unsigned char *\fImark\fP;       (8-bit version)
  const PCRE_UCHAR16  *\fImark\fP;       (16-bit version)
.sp
The \fIversion\fP field is an integer containing the version number of the
block format. The initial version was 0; the current version is 2. The version
number will change again in future if additional fields are added, but the
intention is never to remove any of the existing fields.
.P
The \fIcallout_number\fP field contains the number of the callout, as compiled
into the pattern (that is, the number after ?C for manual callouts, and 255 for
automatically generated callouts).
.P
The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
passed by the caller to the matching function. When \fBpcre_exec()\fP or 
\fBpcre16_exec()\fP is used, the contents can be inspected, in order to extract
substrings that have been matched so far, in the same way as for extracting
substrings after a match has completed. For the DFA matching functions, this
field is not useful.
.P
The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
that were passed to the matching function.
.P
The \fIstart_match\fP field normally contains the offset within the subject at
which the current match attempt started. However, if the escape sequence \eK
has been encountered, this value is changed to reflect the modified starting
point. If the pattern is not anchored, the callout function may be called
several times from the same point in the pattern for different starting points
in the subject.
.P
The \fIcurrent_position\fP field contains the offset within the subject of the
current match pointer.
.P
When the \fBpcre_exec()\fP or \fBpcre16_exec()\fP is used, the
\fIcapture_top\fP field contains one more than the number of the highest
numbered captured substring so far. If no substrings have been captured, the
value of \fIcapture_top\fP is one. This is always the case when the DFA
functions are used, because they do not support captured substrings.
.P
The \fIcapture_last\fP field contains the number of the most recently captured
substring. If no substrings have been captured, its value is -1. This is always
the case for the DFA matching functions.
.P
The \fIcallout_data\fP field contains a value that is passed to a matching
function specifically so that it can be passed back in callouts. It is passed
in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre16_extra\fP
data structure. If no such data was passed, the value of \fIcallout_data\fP in
a callout block is NULL. There is a description of the \fBpcre_extra\fP
structure in the
.\" HREF
\fBpcreapi\fP
.\"
documentation.
.P
The \fIpattern_position\fP field is present from version 1 of the callout
structure. It contains the offset to the next item to be matched in the pattern
string.
.P
The \fInext_item_length\fP field is present from version 1 of the callout
structure. It contains the length of the next item to be matched in the pattern
string. When the callout immediately precedes an alternation bar, a closing
parenthesis, or the end of the pattern, the length is zero. When the callout
precedes an opening parenthesis, the length is that of the entire subpattern.
.P
The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
help in distinguishing between different automatic callouts, which all have the
same callout number. However, they are set for all callouts.
.P
The \fImark\fP field is present from version 2 of the callout structure. In
callouts from \fBpcre_exec()\fP or \fBpcre16_exec()\fP it contains a pointer to
the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
(*THEN) item in the match, or NULL if no such items have been passed. Instances
of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
callouts from the DFA matching functions this field always contains NULL.
.
.
.SH "RETURN VALUES"
.rs
.sp
The external callout function returns an integer to PCRE. If the value is zero,
matching proceeds as normal. If the value is greater than zero, matching fails
at the current point, but the testing of other matching possibilities goes
ahead, just as if a lookahead assertion had failed. If the value is less than
zero, the match is abandoned, the matching function returns the negative value.
.P
Negative values should normally be chosen from the set of PCRE_ERROR_xxx
values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
it will never be used by PCRE itself.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 08 Janurary 2012
Copyright (c) 1997-2012 University of Cambridge.
.fi
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	.B int (pcre16_callout)(pcre16_callout_block );
10	.PP
11	PCRE provides a feature called "callout", which is a means of temporarily
12	passing control to the caller of PCRE in the middle of pattern matching. The
13	caller of PCRE provides an external function by putting its entry point in the
14	global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit
15	library). By default, this variable contains NULL, which disables all calling
16	out.
17	.P
18	Within a regular expression, (?C) indicates the points at which the external
19	function is to be called. Different callout points can be identified by putting
20	a number less than 256 after the letter C. The default value is zero.
21	For example, this pattern has two callout points:
22	.sp
23	(?C1)abc(?C2)def
24	.sp
25	If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
26	automatically inserts callouts, all with number 255, before each item in the
27	pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
28	.sp
29	A(\ed{2}\|--)
30	.sp
31	it is processed as if it were
32	.sp
33	(?C255)A(?C255)((?C255)\ed{2}(?C255)\|(?C255)-(?C255)-(?C255))(?C255)
34	.sp
35	Notice that there is a callout before and after each parenthesis and
36	alternation bar. Automatic callouts can be used for tracking the progress of
37	pattern matching. The
38	.\" HREF
39	\fBpcretest\fP
40	.\"
41	command has an option that sets automatic callouts; when it is used, the output
42	indicates how the pattern is matched. This is useful information when you are
43	trying to optimize the performance of a particular pattern.
44	.P
45	The use of callouts in a pattern makes it ineligible for optimization by the
46	just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
47	option always fails.
48	.
49	.
50	.SH "MISSING CALLOUTS"
51	.rs
52	.sp
53	You should be aware that, because of optimizations in the way PCRE matches
54	patterns by default, callouts sometimes do not happen. For example, if the
55	pattern is
56	.sp
57	ab(?C4)cd
58	.sp
59	PCRE knows that any matching string must contain the letter "d". If the subject
60	string is "abyz", the lack of "d" means that matching doesn't ever start, and
61	the callout is never reached. However, with "abyd", though the result is still
62	no match, the callout is obeyed.
63	.P
64	If the pattern is studied, PCRE knows the minimum length of a matching string,
65	and will immediately give a "no match" return without actually running a match
66	if the subject is not long enough, or, for unanchored patterns, if it has
67	been scanned far enough.
68	.P
69	You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
70	option to the matching function, or by starting the pattern with
71	(*NO_START_OPT). This slows down the matching process, but does ensure that
72	callouts such as the example above are obeyed.
73	.
74	.
75	.SH "THE CALLOUT INTERFACE"
76	.rs
77	.sp
78	During matching, when PCRE reaches a callout point, the external function
79	defined by \fIpcre_callout\fP or \fIpcre16_callout\fP is called (if it is set).
80	This applies to both normal and DFA matching. The only argument to the callout
81	function is a pointer to a \fBpcre_callout\fP or \fBpcre16_callout\fP block.
82	These structures contains the following fields:
83	.sp
84	int \fIversion\fP;
85	int \fIcallout_number\fP;
86	int *\fIoffset_vector\fP;
87	const char *\fIsubject\fP; (8-bit version)
88	PCRE_SPTR16 \fIsubject\fP; (16-bit version)
89	int \fIsubject_length\fP;
90	int \fIstart_match\fP;
91	int \fIcurrent_position\fP;
92	int \fIcapture_top\fP;
93	int \fIcapture_last\fP;
94	void *\fIcallout_data\fP;
95	int \fIpattern_position\fP;
96	int \fInext_item_length\fP;
97	const unsigned char *\fImark\fP; (8-bit version)
98	const PCRE_UCHAR16 *\fImark\fP; (16-bit version)
99	.sp
100	The \fIversion\fP field is an integer containing the version number of the
101	block format. The initial version was 0; the current version is 2. The version
102	number will change again in future if additional fields are added, but the
103	intention is never to remove any of the existing fields.
104	.P
105	The \fIcallout_number\fP field contains the number of the callout, as compiled
106	into the pattern (that is, the number after ?C for manual callouts, and 255 for
107	automatically generated callouts).
108	.P
109	The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
110	passed by the caller to the matching function. When \fBpcre_exec()\fP or
111	\fBpcre16_exec()\fP is used, the contents can be inspected, in order to extract
112	substrings that have been matched so far, in the same way as for extracting
113	substrings after a match has completed. For the DFA matching functions, this
114	field is not useful.
115	.P
116	The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
117	that were passed to the matching function.
118	.P
119	The \fIstart_match\fP field normally contains the offset within the subject at
120	which the current match attempt started. However, if the escape sequence \eK
121	has been encountered, this value is changed to reflect the modified starting
122	point. If the pattern is not anchored, the callout function may be called
123	several times from the same point in the pattern for different starting points
124	in the subject.
125	.P
126	The \fIcurrent_position\fP field contains the offset within the subject of the
127	current match pointer.
128	.P
129	When the \fBpcre_exec()\fP or \fBpcre16_exec()\fP is used, the
130	\fIcapture_top\fP field contains one more than the number of the highest
131	numbered captured substring so far. If no substrings have been captured, the
132	value of \fIcapture_top\fP is one. This is always the case when the DFA
133	functions are used, because they do not support captured substrings.
134	.P
135	The \fIcapture_last\fP field contains the number of the most recently captured
136	substring. If no substrings have been captured, its value is -1. This is always
137	the case for the DFA matching functions.
138	.P
139	The \fIcallout_data\fP field contains a value that is passed to a matching
140	function specifically so that it can be passed back in callouts. It is passed
141	in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre16_extra\fP
142	data structure. If no such data was passed, the value of \fIcallout_data\fP in
143	a callout block is NULL. There is a description of the \fBpcre_extra\fP
144	structure in the
145	.\" HREF
146	\fBpcreapi\fP
147	.\"
148	documentation.
149	.P
150	The \fIpattern_position\fP field is present from version 1 of the callout
151	structure. It contains the offset to the next item to be matched in the pattern
152	string.
153	.P
154	The \fInext_item_length\fP field is present from version 1 of the callout
155	structure. It contains the length of the next item to be matched in the pattern
156	string. When the callout immediately precedes an alternation bar, a closing
157	parenthesis, or the end of the pattern, the length is zero. When the callout
158	precedes an opening parenthesis, the length is that of the entire subpattern.
159	.P
160	The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
161	help in distinguishing between different automatic callouts, which all have the
162	same callout number. However, they are set for all callouts.
163	.P
164	The \fImark\fP field is present from version 2 of the callout structure. In
165	callouts from \fBpcre_exec()\fP or \fBpcre16_exec()\fP it contains a pointer to
166	the zero-terminated name of the most recently passed (MARK), (PRUNE), or
167	(*THEN) item in the match, or NULL if no such items have been passed. Instances
168	of (PRUNE) or (THEN) without a name do not obliterate a previous (*MARK). In
169	callouts from the DFA matching functions this field always contains NULL.
170	.
171	.
172	.SH "RETURN VALUES"
173	.rs
174	.sp
175	The external callout function returns an integer to PCRE. If the value is zero,
176	matching proceeds as normal. If the value is greater than zero, matching fails
177	at the current point, but the testing of other matching possibilities goes
178	ahead, just as if a lookahead assertion had failed. If the value is less than
179	zero, the match is abandoned, the matching function returns the negative value.
180	.P
181	Negative values should normally be chosen from the set of PCRE_ERROR_xxx
182	values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
183	The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
184	it will never be used by PCRE itself.
185	.
186	.
187	.SH AUTHOR
188	.rs
189	.sp
190	.nf
191	Philip Hazel
192	University Computing Service
193	Cambridge CB2 3QH, England.
194	.fi
195	.
196	.
197	.SH REVISION
198	.rs
199	.sp
200	.nf
201	Last updated: 08 Janurary 2012
202	Copyright (c) 1997-2012 University of Cambridge.
203	.fi
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