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Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl code to be obeyed in the middle of matching a regular expression. This makes it possible, amongst other things, to extract different substrings that match the same pair of parentheses when there is a repetition.
PCRE provides a similar feature, but of course it cannot obey arbitrary Perl code. The feature is called "callout". The caller of PCRE provides an external function by putting its entry point in the global variable pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit library). By default, this variable contains NULL, which disables all calling out.
Within a regular expression, (?C) indicates the points at which the external function is to be called. If you want to identify different callout points, you can put a number less than 256 after the letter C. The default value is zero. For example, this pattern has two callout points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are automatically installed before each item in the pattern. They are all numbered 255. If there is a conditional group in the pattern whose condition is an assertion, an additional callout is inserted just before the condition. An explicit callout may also be set at this position, as in this example:
(?(?C9)(?=a)abc|def)
Note that this applies only to assertion conditions, not to other types of condition.
During matching, when PCRE reaches a callout point, the external function is called. It is provided with the number of the callout, the position in the pattern, and, optionally, one item of data originally supplied by the caller of the matching function. The callout function may cause matching to proceed, to backtrack, or to fail altogether.
By default, PCRE implements a number of optimizations at compile time and matching time, and one side-effect is that sometimes callouts are skipped. If you need all possible callouts to happen, you need to set options that disable the relevant optimizations.
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 12 November 2013
Copyright © 1997-2013 University of Cambridge.
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