+++ /dev/null
-.TH PCRECALLOUT 3
-.SH NAME
-PCRE - Perl-compatible regular expressions
-.SH "PCRE CALLOUTS"
-.rs
-.sp
-.B int (*pcre_callout)(pcre_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. 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 \fBpcre_compile()\fP or
-\fBpcre_compile2()\fP is called, 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.
-.
-.
-.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 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. 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 is called (if it is set). This applies to both
-the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
-only argument to the callout function is a pointer to a \fBpcre_callout\fP
-block. This structure contains the following fields:
-.sp
- int \fIversion\fP;
- int \fIcallout_number\fP;
- int *\fIoffset_vector\fP;
- const char *\fIsubject\fP;
- 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;
-.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 1. 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 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
-\fBpcre_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 \fBpcre_dfa_exec()\fP this field is
-not useful.
-.P
-The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
-that were passed to \fBpcre_exec()\fP.
-.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 function 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 \fBpcre_dfa_exec()\fP is used, because it
-does 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 when \fBpcre_dfa_exec()\fP is used.
-.P
-The \fIcallout_data\fP field contains a value that is passed to
-\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
-passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
-\fBpcre_extra\fP data structure. If no such data was passed, the value of
-\fIcallout_data\fP in a \fBpcre_callout\fP 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
-\fIpcre_callout\fP 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
-\fIpcre_callout\fP 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.
-.
-.
-.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, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
-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: 29 September 2009
-Copyright (c) 1997-2009 University of Cambridge.
-.fi
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