If you want to keep using a regexp, then:

$mac =~ s/^(..)(..)(..)(..)(..)(..)$/$1:$2:$3:$4:$5:$6/
  if $mac =~ /^[0-9A-F]{12}$/i;

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In reply to Re: Insert colons into a MAC address by tobyink
in thread Insert colons into a MAC address by mantager

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