Regex overlap in MAC address

superfrink has asked for the wisdom of the Perl Monks concerning the following question:

I have a list of MAC addresses from two sources that I want to unique and sort. I discovered that some of the address do not have zero padding and some do. I ran into a problem trying to use a regex to zero pad a MAC address.

My sample MAC:

0:13:46:b:4:31    # input
00:13:46:0b:04:31 # expected output
[download]

The regex I tried was:

$mac =~ s/(^|:)([\da-f])(:|$)/${1}0${2}${3}/g;
[download]

This gave me:

0:13:46:b:4:31    # input
00:13:46:0b:4:31  # produced
00:13:46:0b:04:31 # expected
[download]

My guess is there is an overlap problem because the regex matches and replaces the : charater and so the : is not available to be mactched for the "4" because it was already used by the "b".

I thought about it for a bit and got around the problem without using a regex.

my @parts = split /:/ , $mac;
foreach (@parts) { if (length == 1) { $_ = "0" . $_; } }
$mac = join ":" , @parts;
[download]

Can someone explain why the regex did not work as I expected or how to make it work as desired?

Comment on Regex overlap in MAC address Select or Download Code

Replies are listed 'Best First'.
Re: Regex overlap in MAC address by ikegami (Patriarch) on Sep 11, 2006 at 19:23 UTC
`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]	[reply] [d/l] [select]
Re^2: Regex overlap in MAC address by Skeeve (Parson) on Sep 11, 2006 at 20:06 UTC
Hi ikegami could you please rate this too? `$mac= join ":",map substr("00$_",-2),split/:/,$mac;` [download] `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]
Re^3: Regex overlap in MAC address by ikegami (Patriarch) on Sep 11, 2006 at 20:48 UTC
Sure thing use Benchmark qw( cmpthese ); our $data = '0:13:46:b:4:31'; cmpthese(-3, { ike_orig => 'my $mac = $data; $mac =~ s/(^\|:)(?=[0-9a-fA-F](?::\|$)) +/${1}0/g; $mac;', ike_fast => 'my $mac = $data; $mac =~ s/(?<=:)(?=[0-9a-fA-F](?::\|$) +)/0/g; $mac =~ s/^(?=[0-9a-fA-F](?::\|$))/0/; + $mac;', skeeve => 'my $mac = $data; $mac = join ":", map substr("0$_",-2) +, split/:/, $mac; $mac;', jwkrahn1 => 'my $mac = $data; $mac =~ s/([^:]+)/ sprintf "%02s", $1 + /eg; $mac;', jwkrahn2 => 'my $mac = $data; $mac = join ":", map sprintf( "%02s", + $_ ), split /:/, $mac; $mac;', }); [download] `Rate ike_orig jwkrahn1 skeeve jwkrahn2 ike_fast ike_orig 37908/s -- -3% -29% -31% -43% jwkrahn1 38957/s 3% -- -27% -29% -41% skeeve 53318/s 41% 37% -- -3% -20% jwkrahn2 54879/s 45% 41% 3% -- -18% ike_fast 66551/s 76% 71% 25% 21% --` [download]	[reply] [d/l] [select]
Re^4: Regex overlap in MAC address by Skeeve (Parson) on Sep 12, 2006 at 07:11 UTC
Re^4: Regex overlap in MAC address by Anonymous Monk on Sep 12, 2006 at 08:53 UTC
Re^5: Regex overlap in MAC address by ikegami (Patriarch) on Sep 12, 2006 at 19:11 UTC
Re: Regex overlap in MAC address by VSarkiss (Monsignor) on Sep 11, 2006 at 20:56 UTC
Personally, I prefer your second approach over the regex approach. I would make it a little more brief with sprintf, something like this: `my $mac_pad = join(':' => map { sprintf "%02s", $_ } split(/:/, $mac));` [download] You could argue that it's too compact, but I think the join-map-split idiom is fairly common. YMMV. Do not rebuke them with harsh words ... but rather lead them gently - with URLs - so that they may learn wisdom.	[reply] [d/l]
Re: Regex overlap in MAC address by jwkrahn (Abbot) on Sep 11, 2006 at 20:12 UTC
Update: Thanks Skeeve for pointing out that error, it is now fixed. `$ perl -le' $mac = "0:13:46:b:4:31"; $mac =~ s/([^:]+)/ sprintf "%02x", hex $1 /eg; print $mac; ' 00:13:46:0b:04:31 $ perl -le' $mac = "0:13:46:b:4:31"; $mac = join ":", map sprintf( "%02x", hex ), split /:/, $mac; print $mac; ' 00:13:46:0b:04:31` [download]	[reply] [d/l]
Re^2: Regex overlap in MAC address by Skeeve (Parson) on Sep 11, 2006 at 20:28 UTC
Didn't you notice that it fails for hex digits a-f? use "%02s" instead. `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]