[exim] cut and match help

Hi.

I am trying to cut down the ip address in the sender host address
($sender_host_address) variable -

eg, just say the host address is 192.168.1.50 I would like to cut it
down to 192.168.1

I couldn't see how to do this in the exim specs, maybe I over-looked
something? Any advice greatly appreciated.


Cheers,
Andrew.