184 lines
7.2 KiB
Groff
184 lines
7.2 KiB
Groff
.TH PCRECALLOUT 3
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.SH NAME
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PCRE - Perl-compatible regular expressions
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.SH "PCRE CALLOUTS"
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.rs
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.sp
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.B int (*pcre_callout)(pcre_callout_block *);
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.PP
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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. By default, this variable contains NULL,
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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 \fBpcre_compile()\fP is called,
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PCRE automatically inserts callouts, all with number 255, before each item in
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the 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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.
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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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You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
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option to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. This slows down the
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matching process, but does ensure that callouts such as the example above are
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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 is called (if it is set). This applies to both
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the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
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only argument to the callout function is a pointer to a \fBpcre_callout\fP
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block. This structure 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;
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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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.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 1. 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 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
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\fBpcre_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 \fBpcre_dfa_exec()\fP this field is
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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 \fBpcre_exec()\fP.
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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 function is used, the \fIcapture_top\fP field
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contains one more than the number of the highest numbered captured substring so
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far. If no substrings have been captured, the value of \fIcapture_top\fP is
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one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
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does 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 when \fBpcre_dfa_exec()\fP is used.
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.P
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The \fIcallout_data\fP field contains a value that is passed to
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\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
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passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
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\fBpcre_extra\fP data structure. If no such data was passed, the value of
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\fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
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description of the \fBpcre_extra\fP 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
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\fIpcre_callout\fP structure. It contains the offset to the next item to be
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matched in the pattern string.
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.P
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The \fInext_item_length\fP field is present from version 1 of the
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\fIpcre_callout\fP structure. It contains the length of the next item to be
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matched in the pattern string. When the callout immediately precedes an
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alternation bar, a closing parenthesis, or the end of the pattern, the length
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is zero. When the callout precedes an opening parenthesis, the length is that
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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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.
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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, and \fBpcre_exec()\fP (or \fBpcre_dfa_exec()\fP)
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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: 15 March 2009
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Copyright (c) 1997-2009 University of Cambridge.
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.fi
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