0:13:46:b:4:31
^^             First pass
    ^^^^       Second pass
         ^^^   Third pass

##</code><code>##

$mac =~ s/(^|:)([0-9a-fA-F])(?=:|$)/${1}0$2/g;

##</code><code>##

$mac =~ s/(^|:)(?=[0-9a-fA-F](?::|$))/${1}0/g;

##</code><code>##

$mac =~ s/(?<=:)(?=[0-9a-fA-F](?::|$))/0/g;
$mac =~ s/^(?=[0-9a-fA-F](?::|$))/0/;

##</code><code>##

        Rate orig fast
orig 36251/s   -- -42%
fast 62533/s  72%   --