IP address matching is a bit tricky, so it's often helpful to turn to accumulated wisdom. Regexp::Common and Regexp::Common::net can help. (To see the trickiness, change one of the 255s in the example data of the OP to 256 – or even 666 or 66666 – and see the result using the OP regex.) 

>perl -wMstrict -le
"use Regexp::Common qw(net);
 my $IPv4 = qr{ (?<! \d) $RE{net}{IPv4} (?! \d) }xms;
 ;;
 my @strs = (
   'permit ip host 10.11.1.1 192.168.100.0 0.0.0.255',
   'permit ip 10.11.1.0  0.0.0.255 192.168.100.0 0.0.0.255',
   );
 ;;
 for my $s (@strs) {
   print qq{from: '$s'};
   while ($s =~ m{ (?<! host) \s+ ($IPv4) \s+ ($IPv4) }xmsg) {
     print qq{IP pair: '$1' '$2'};
     }
   }
"
from: 'permit ip host 10.11.1.1 192.168.100.0 0.0.0.255'
IP pair: '192.168.100.0' '0.0.0.255'
from: 'permit ip 10.11.1.0  0.0.0.255 192.168.100.0 0.0.0.255'
IP pair: '10.11.1.0' '0.0.0.255'
IP pair: '192.168.100.0' '0.0.0.255'

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Update: Unfortunately, this solution has a bug. See Re^2: Match zero times in regex for counter-example data demonstrating it.

In reply to Re: Match zero times in regex by AnomalousMonk
in thread Match zero times in regex by SomeNetworkGuy

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