For curiosity on the performance issue, I have conducted some experiments to compare the performance between a straight regular expression with capture and positive look-ahead with substr & length functions.

The following is my code for the experiment -
use strict;
use Benchmark qw/timethese cmpthese/;
# use re 'debug';

chomp(my @lines = <DATA>);
my $target = shift || 'word';
my $re_positive_lookahead  = qr/^a=<(?=.*?word)/;
my $re_loose = qr/a=<(.*?$target.*?)>/;

cmpthese( timethese(1000000, {
        'Match_L' => '&Match_Loose',
        'Match_P' => '&Match_PLAhead_plus_Substr',
    }) );

sub Match_Loose {
    foreach (@lines) {
        if (/$re_loose/) {
            my $word = $1;
        }
    }
}

sub Match_PLAhead_plus_Substr {
    foreach (@lines) {
        if (/$re_positive_lookahead/) {
            my $word = substr($_, 3, length($_)-4);
        }
    }
}

__DATA__
a=<swords>
a=<wordy>
b=<rappinghood>
a=<thisword>
b=<thatword>
a=<foreword>
b=<junk>
a=<nothing>
b=<word>
b=<wordplay>
b=<end>

[download]
And the result of the experiment is as follows -
Benchmark: timing 1000000 iterations of Match_L, Match_P...
 Match_L: 31 wclock secs (31.16 usr +  0.00 sys = 31.16 CPU) @ 32092.4
+3/s
 Match_P: 27 wclock secs (27.64 usr +  0.00 sys = 27.64 CPU) @ 36179.4
+5/s

           Rate Match_L Match_P
Match_L 32092/s      --    -11%
Match_P 36179/s     13%      --

[download]
The observation is that positive lookahead combined with substr is about 12% faster than straight regexp with capture, which is not an insignificant speed improvement.

In reply to Re: Removing backtracking from a .*? regexp by Roger
in thread Removing backtracking from a .*? regexp by grinder

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