Lookbehind was by first thought, but it doesn't work unless one changes what is currently allowed. 
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 host  10.11.1.1 192.168.100.0 0.0.0.255'
IP pair: '10.11.1.1' '192.168.100.0'

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In reply to Re^2: Match zero times in regex by ikegami
in thread Match zero times in regex by SomeNetworkGuy

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