.TH PCRECALLOUT 3

.SH NAME

PCRE - Perl-compatible regular expressions

.SH "PCRE CALLOUTS"

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.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:

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  (?C1)abc(?C2)def

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If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\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

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  A(\ed{2}|--)

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it is processed as if it were

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(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

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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.

.

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.SH "MISSING CALLOUTS"

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You should be aware that, because of optimizations in the way PCRE matches

patterns, callouts sometimes do not happen. For example, if the pattern is

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  ab(?C4)cd

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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.

.

.

.SH "THE CALLOUT INTERFACE"

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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:

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  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;

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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.

.

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.SH "RETURN VALUES"

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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.

.

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.SH AUTHOR

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Philip Hazel

University Computing Service

Cambridge CB2 3QH, England.

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.SH REVISION

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Last updated: 29 May 2007

Copyright (c) 1997-2007 University of Cambridge.

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