test.pl

#!/usr/bin/perl
use Benchmark qw(:all);

$a = "foo[bar]moo";

cmpthese(
    10000000, {
        capture => sub {
            $a =~ /\[(\w)\]/;
            $1;
        },
        zerowidth => sub { 
            $a =~ /(?<=\[)(\d+)(?=\])/;
            $1;
        },
        split => sub{
            $b = (split( /\[|\]/, $a))[1];
            $b;
            
        },
        twosplit => sub{
            $b = (split( /\[/, (split( /\]/, $a ))[0]))[1];
            $b;
        },
    }
);

__END__
               Rate     split  twosplit   capture zerowidth
split      233209/s        --      -58%      -84%      -85%
twosplit   559597/s      140%        --      -61%      -64%
capture   1428571/s      513%      155%        --       -7%
zerowidth 1536098/s      559%      175%        8%        --