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

[download]

You're expecting the regexp to match the same : twice. Use a zero-width lookahead to check for the trailing : or end of string. 

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

[download]

Better yet, eliminate needless copying. 

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

[download]

Update: Tested. Fixed bug where : would get removed.

Update: If you really need speed, the following is much faster, but it sacrifices readability: 

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

[download]
        Rate orig fast
orig 36251/s   -- -42%
fast 62533/s  72%   --

[download]

In reply to Re: Regex overlap in MAC address by ikegami
in thread Regex overlap in MAC address by superfrink

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