($a =~ /(\d+)/)[0] <=> ($b =~ /(\d+)/)[0]

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;

sub compare;

my @l;
for (10, 100, 1000)
{
    @l = map { "a$_" } 1..$_;
    compare "Pre-sorted";
    
    @l = reverse @l;
    compare "Reverse-sorted";
    
    use List::Util qw(shuffle);
    @l = shuffle @l;
    compare "Shuffled";
}

my %regexes;
sub brute_force
{
    my $c = 0;
    my @s = sort {
        $c += 2;
        ($a =~ /(\d+)/)[0] <=> ($b =~ /(\d+)/)[0]
    } @l;
    push @{$regexes{force}}, $c;
}

sub ST
{
    my $c = 0;
    my @s = map { $_->[1] } sort { $a->[0] <=> $b->[0] } map {
        $c += 1;
        /(\d+)/;
        [ $1, $_ ]
    } @l;
    push @{$regexes{ST}}, $c;
}

sub compare
{
    use Benchmark qw(cmpthese);

    my $title = shift;
    my $len   = @l;
    $title = "$title ($len).";
    print $title, "\n";
    print '-' x length $title, "\n";
    %regexes = ();

    cmpthese(-1,
             {
                 force => \&brute_force,
                 ST    => \&ST,
             }
            );
    for (qw(force ST))
    {
        use List::Util qw(sum);
        my $avg = sum(@{$regexes{$_}}) / @{$regexes{$_}};
        printf "%10s %d regexes\n", $_, $avg;
    }
    print "\n";
}

##</code><code>##

Pre-sorted (10).
----------------
         Rate force    ST
force 34461/s    --  -10%
ST    38399/s   11%    --
     force 38 regexes
        ST 10 regexes

Reverse-sorted (10).
--------------------
         Rate    ST force
ST    39097/s    --   -5%
force 41353/s    6%    --
     force 30 regexes
        ST 10 regexes

Shuffled (10).
--------------
         Rate force    ST
force 26795/s    --  -25%
ST    35544/s   33%    --
     force 50 regexes
        ST 10 regexes

Pre-sorted (100).
-----------------
        Rate    ST force
ST    4696/s    --  -30%
force 6698/s   43%    --
     force 198 regexes
        ST 100 regexes

Reverse-sorted (100).
---------------------
        Rate    ST force
ST    4745/s    --  -30%
force 6761/s   43%    --
     force 198 regexes
        ST 100 regexes

Shuffled (100).
---------------
        Rate force    ST
force 1347/s    --  -59%
ST    3257/s  142%    --
     force 1096 regexes
        ST 100 regexes

Pre-sorted (1000).
------------------
       Rate    ST force
ST    439/s    --  -35%
force 673/s   53%    --
     force 1998 regexes
        ST 1000 regexes

Reverse-sorted (1000).
----------------------
       Rate    ST force
ST    427/s    --  -37%
force 673/s   58%    --
     force 1998 regexes
        ST 1000 regexes

Shuffled (1000).
----------------
        Rate force    ST
force 86.3/s    --  -64%
ST     243/s  181%    --
     force 17440 regexes
        ST 1000 regexes