Well, I think I'll choose this:

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

which can be also spelled as:

$mac =~ s/(\p{Hex}{2})\B/$1:/g;

The [:xdigit:] class does not work (I don't know why).

@Not_a_Number: probably there isn't a reason to prefer a regex over 'unpack' or the use of a module, in this case, since this regex is not really more readable than other methods.
Anyway I'm more familiar with regexes than with unpack or the Net::MAC module, so there is a chanche that I will recognize and understand that sooner when looking again at my code in the future.
As for modules, I often avoid (or completely overlook) them, because I usually have little or no control on the systems I work onto (so installing a module can be a tricky or impossible task). I see that "use a module" vs "reinvent the wheel" is an ever discussed problem. I'm on the "reinvent the wheel" side, but it's not my fault :)

Thanks to all, bye.

Edit:
jut in case someone come here in the future.

[:xdigit:]

actually works:

$mac =~ s/([[:xdigit:]]{2})\B/$1:/g;

I just didn't put enough square brackets. [:xdigit:] is equivalent to [:xdigt] (literally), while [[:xdigit:]] is equivalent to [0-9a-fA-F]. Reading the f. manual helps as usual :)

In reply to Re^3: Insert colons into a MAC address by mantager
in thread Insert colons into a MAC address by mantager

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