Regexp::MatchContext - Replace (and improve) $MATCH, $PREMATCH, and $POSTMATCH

This document describes Regexp::MatchContext version 0.0.2

    use Regexp::MatchContext -vars;

    $str = m/(?p) \d+ /;

    print "Before:  $PREMATCH\n";
    print "Matched: $MATCH\n";
    print "After:   $POSTMATCH\n";

    $MATCH = 2 * $MATCH;      # substitute into original $str

  

The English.pm module provides named aliases for Perl's built-in $`, $& and $' variables: $PREMATCH, $MATCH, and $POSTMATCH. Unfortunately, those aliases suffer the same problems as their originals: they degrade the performance of every single regex in your program, even if you're only using them to get information about a single match.

This module also provides $PREMATCH, $MATCH, and $POSTMATCH, but in a way that only impacts the performance of matches that you specify. That is, these three variables are only set if the most recently matched regex contained the special (non-standard) meta- flag: (?p).

That is:

    use Regexp::MatchContext -vars;

    $str = 'foobarbaz';

    $str =~ /(?p) foo /x;

    # $PREMATCH contains 'foo'
    # $MATCH contains 'bar'
    # $POSTMATCH contains 'baz'

    $str =~ / foo /x;

    # $PREMATCH, $MATCH, and $POSTMATCH all undef

The (?p) marker can be placed anywhere within the regex and, except for setting the three context variables on a successful match, is otherwise totally ignored.

Unlike the match variables provided in standard Perl, all three match variables provided by Regexp::MatchContext are actually aliases into the original string that the preceding regex matched. So assigning to any of these variables, changes the original string.

This means that, instead of:

    $str =~ s/ foo /bar/;

you could write:

    $str =~ m/ foo (?p)/;

    $MATCH = 'bar';

Or remove everything before a match:

    $str =~ m/ foo (?p)/;

    $PREMATCH = q{};

If you load the module with the argument -subs instead of -vars, then instead of exporting the variables $PREMATCH, $MATCH, and $POSTMATCH, it exports three subroutines: PREMATCH(), MATCH(), and POSTMATCH(). The subroutines are provided for those who prefer not to use package variables in their code.

Calling any of these subroutines returns the same value that the corresponding variable would have yielded:

    use Regexp::MatchContext -subs;

    $str = 'foobarbaz';

    $str =~ /(?p) foo /x;

    # PREMATCH() returns 'foo'
    # MATCH() returns 'bar'
    # POSTMATCH() returns 'baz'

    $str =~ / foo /x;

    # PREMATCH(), MATCH(), and POSTMATCH() all return undef

The three subroutines are also declared :lvalue, so that calls to them can be assigned to, which causes the corresponding part of the matched string to be changed (just as assignments to the equivalent variables does). For example:

    $str =~ m/ foo (?p)/;

    MATCH() = 'bar';

    PREMATCH() = q{};

Can't assign to %s because the preceding match didn't set it.

You tried to assign back via one of the capture variables, but the preceding regex didn't capture anything, so the assignment would do no good. Did you forget to put a (?p) in your regex?

Regexp::MatchContext requires no configuration files or environment variables.

Depends on the Tie::StdScalar and version modules.

None reported.

No bugs have been reported.

Please report any bugs or feature requests to bug-regexp-matchcontext@rt.cpan.org, or through the web interface at http://rt.cpan.org.

Damian Conway <DCONWAY@cpan.org>

Copyright (c) 2005, Damian Conway <DCONWAY@cpan.org>. All rights reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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