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=head1 TITLE
Asignment within a regex
=head1 VERSION
Maintainer: Richard Proctor <[EMAIL PROTECTED]>
Date: 16 Aug 2000
Last Modified: 23 Sep 2000
Mailing List: [EMAIL PROTECTED]
Number: 112
Version: 3
Status: Developing
=head1 ABSTRACT
Provide a simple way of naming and picking out information from a regex
without having to count the brackets.
=head1 DESCRIPTION
If a regex is complex, counting the bracketed sub-expressions to find the
ones you wish to pick out can be messy. It is also prone to maintainability
problems if and when you wish to add to the expression. Using (?:) can be
used to surpress picking up brackets, it helps, but it still gets "complex".
I would sometimes rather just pickout the bits I want within the regex itself.
Suggested syntax: (?$foo= ... ) would assign the string that is matched by
the patten ... to $foo when the patten matches. These assignments would be
made left to right after the match has succeded but before processing a
replacement or other results (or prior to a some (?{...}) or (??{...})
code). There may be whitespace between the $foo and the "=".
Potentially the $foo could be any scalar LHS, as in (?$foo{$bar}= ... )!,
likewise the '=' could be any asignment operator.
The camel and the docs include this example:
if (/Time: (..):(..):(..)/) {
$hours = $1;
$minutes = $2;
$seconds = $3;
}
This then becomes:
/Time: (?$hours=..):(?$minutes=..):(?$seconds=..)/
This is more maintainable than counting the brackets and easier to understand
for a complex regex. And one does not have to worry about the scope of $1 etc.
=head2 Named Backrefs
The first versions of this RFC did not allow for backrefs. I now think this
was a shortcoming. It can be done with (??{quotemeta $foo}), but I find this
clumsy, a better way of using a named back ref might be (?\$foo).
=head2 Scoping
The question of scoping for these assignments has been raised, but I don't
currently have a feel for the "best" way to handle this. Input welcome.
=head2 Brackets
Using this method for capturing wanted content, it might be desirable to
stop ordinary brackets capturing, and needing to use (?:...). I therefore
suggest that as an enhancement to regexes that /b (bracket?) ordinary brackets
just group, without capture - in effect they all behave as (?:...).
=head1 CHANGES
V3 - added bit about backrefs, and brackets.
=head1 IMPLENTATION
Currently all $scalars in regexes are expanded before the main regex compiler
gets to analyse the syntax. This problem also affects several other RFCs
(166 for example). The expansion of variables in regexes needs for these
(and other RFCs) to be driven from within the regex compiler so that the
regex can expand as and where appropriate. Changing this should not affect
any existing behaviour.
=head1 REFERENCES
I brought this up on p5p a couple of years ago, but it was lost in the noise...
RFC 166: Alternative lists and quoting of things
Perlstorm #0040
