my $foo = "0:0A:0C:B:B8:F";
$foo =~ s/\A([0-9A-F]){1}:/0$&/g;  #take care of the start
$foo =~ s/:([0-9A-F]{1}):/:0$1:/g; #take care of the middle
$foo =~ s/:([0-9A-F]{1})\Z/:0$1/g; #take care of the end
$foo =~ s/\://g;                   #remove the colon