215 lines
8.7 KiB
Groff
215 lines
8.7 KiB
Groff
.TH PCRECALLOUT 3 "24 June 2012" "PCRE 8.30"
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.SH NAME
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PCRE - Perl-compatible regular expressions
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.SH SYNOPSIS
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.rs
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.sp
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.B #include <pcre.h>
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.PP
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.SM
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.B int (*pcre_callout)(pcre_callout_block *);
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.PP
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.B int (*pcre16_callout)(pcre16_callout_block *);
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.PP
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.B int (*pcre32_callout)(pcre32_callout_block *);
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.
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.SH DESCRIPTION
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.rs
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.sp
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PCRE provides a feature called "callout", which is a means of temporarily
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passing control to the caller of PCRE in the middle of pattern matching. The
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caller of PCRE provides an external function by putting its entry point in the
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global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit
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library, \fIpcre32_callout\fP for the 32-bit library). By default, this
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variable contains NULL, which disables all calling out.
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.P
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Within a regular expression, (?C) indicates the points at which the external
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function is to be called. Different callout points can be identified by putting
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a number less than 256 after the letter C. The default value is zero.
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For example, this pattern has two callout points:
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.sp
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(?C1)abc(?C2)def
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.sp
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If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
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automatically inserts callouts, all with number 255, before each item in the
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pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
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.sp
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A(\ed{2}|--)
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.sp
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it is processed as if it were
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.sp
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(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
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.sp
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Notice that there is a callout before and after each parenthesis and
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alternation bar. Automatic callouts can be used for tracking the progress of
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pattern matching. The
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.\" HREF
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\fBpcretest\fP
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.\"
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command has an option that sets automatic callouts; when it is used, the output
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indicates how the pattern is matched. This is useful information when you are
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trying to optimize the performance of a particular pattern.
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.P
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The use of callouts in a pattern makes it ineligible for optimization by the
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just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
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option always fails.
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.
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.
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.SH "MISSING CALLOUTS"
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.rs
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.sp
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You should be aware that, because of optimizations in the way PCRE matches
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patterns by default, callouts sometimes do not happen. For example, if the
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pattern is
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.sp
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ab(?C4)cd
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.sp
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PCRE knows that any matching string must contain the letter "d". If the subject
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string is "abyz", the lack of "d" means that matching doesn't ever start, and
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the callout is never reached. However, with "abyd", though the result is still
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no match, the callout is obeyed.
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.P
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If the pattern is studied, PCRE knows the minimum length of a matching string,
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and will immediately give a "no match" return without actually running a match
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if the subject is not long enough, or, for unanchored patterns, if it has
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been scanned far enough.
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.P
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You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
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option to the matching function, or by starting the pattern with
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(*NO_START_OPT). This slows down the matching process, but does ensure that
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callouts such as the example above are obeyed.
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.
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.
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.SH "THE CALLOUT INTERFACE"
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.rs
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.sp
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During matching, when PCRE reaches a callout point, the external function
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defined by \fIpcre_callout\fP or \fIpcre[16|32]_callout\fP is called
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(if it is set). This applies to both normal and DFA matching. The only
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argument to the callout function is a pointer to a \fBpcre_callout\fP
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or \fBpcre[16|32]_callout\fP block.
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These structures contains the following fields:
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.sp
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int \fIversion\fP;
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int \fIcallout_number\fP;
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int *\fIoffset_vector\fP;
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const char *\fIsubject\fP; (8-bit version)
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PCRE_SPTR16 \fIsubject\fP; (16-bit version)
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PCRE_SPTR32 \fIsubject\fP; (32-bit version)
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int \fIsubject_length\fP;
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int \fIstart_match\fP;
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int \fIcurrent_position\fP;
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int \fIcapture_top\fP;
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int \fIcapture_last\fP;
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void *\fIcallout_data\fP;
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int \fIpattern_position\fP;
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int \fInext_item_length\fP;
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const unsigned char *\fImark\fP; (8-bit version)
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const PCRE_UCHAR16 *\fImark\fP; (16-bit version)
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const PCRE_UCHAR32 *\fImark\fP; (32-bit version)
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.sp
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The \fIversion\fP field is an integer containing the version number of the
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block format. The initial version was 0; the current version is 2. The version
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number will change again in future if additional fields are added, but the
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intention is never to remove any of the existing fields.
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.P
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The \fIcallout_number\fP field contains the number of the callout, as compiled
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into the pattern (that is, the number after ?C for manual callouts, and 255 for
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automatically generated callouts).
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.P
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The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
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passed by the caller to the matching function. When \fBpcre_exec()\fP or
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\fBpcre[16|32]_exec()\fP is used, the contents can be inspected, in order to extract
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substrings that have been matched so far, in the same way as for extracting
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substrings after a match has completed. For the DFA matching functions, this
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field is not useful.
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.P
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The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
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that were passed to the matching function.
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.P
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The \fIstart_match\fP field normally contains the offset within the subject at
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which the current match attempt started. However, if the escape sequence \eK
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has been encountered, this value is changed to reflect the modified starting
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point. If the pattern is not anchored, the callout function may be called
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several times from the same point in the pattern for different starting points
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in the subject.
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.P
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The \fIcurrent_position\fP field contains the offset within the subject of the
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current match pointer.
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.P
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When the \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP is used, the
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\fIcapture_top\fP field contains one more than the number of the highest
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numbered captured substring so far. If no substrings have been captured, the
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value of \fIcapture_top\fP is one. This is always the case when the DFA
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functions are used, because they do not support captured substrings.
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.P
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The \fIcapture_last\fP field contains the number of the most recently captured
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substring. If no substrings have been captured, its value is -1. This is always
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the case for the DFA matching functions.
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.P
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The \fIcallout_data\fP field contains a value that is passed to a matching
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function specifically so that it can be passed back in callouts. It is passed
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in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre[16|32]_extra\fP
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data structure. If no such data was passed, the value of \fIcallout_data\fP in
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a callout block is NULL. There is a description of the \fBpcre_extra\fP
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structure in the
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.\" HREF
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\fBpcreapi\fP
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.\"
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documentation.
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.P
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The \fIpattern_position\fP field is present from version 1 of the callout
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structure. It contains the offset to the next item to be matched in the pattern
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string.
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.P
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The \fInext_item_length\fP field is present from version 1 of the callout
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structure. It contains the length of the next item to be matched in the pattern
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string. When the callout immediately precedes an alternation bar, a closing
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parenthesis, or the end of the pattern, the length is zero. When the callout
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precedes an opening parenthesis, the length is that of the entire subpattern.
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.P
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The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
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help in distinguishing between different automatic callouts, which all have the
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same callout number. However, they are set for all callouts.
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.P
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The \fImark\fP field is present from version 2 of the callout structure. In
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callouts from \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP it contains a pointer to
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the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
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(*THEN) item in the match, or NULL if no such items have been passed. Instances
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of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
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callouts from the DFA matching functions this field always contains NULL.
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.
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.
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.SH "RETURN VALUES"
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.rs
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.sp
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The external callout function returns an integer to PCRE. If the value is zero,
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matching proceeds as normal. If the value is greater than zero, matching fails
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at the current point, but the testing of other matching possibilities goes
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ahead, just as if a lookahead assertion had failed. If the value is less than
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zero, the match is abandoned, the matching function returns the negative value.
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.P
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Negative values should normally be chosen from the set of PCRE_ERROR_xxx
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values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
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The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
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it will never be used by PCRE itself.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge CB2 3QH, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 24 June 2012
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Copyright (c) 1997-2012 University of Cambridge.
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.fi
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