[exim] REGEX help - Picking IP addresses out of Received Hea…

I'm looking for regular expressions that I can use in Received headers
to pick out IP addresses and use that to look up blacklist information
on mail that has been forwarded to me from other sources. Normally email
comes directly to my servers and that's easy to check the hosts for
blacklists. However when spam that is forwarded from other servers that
send good email the blacklists checks don't work. So I need to pull IP
addresses out of the Received headers to check where the message has been.

So - What I need is a regular expression to grab say the second or third
IP address back in the list and stick that into a variable that I can
use then to look up against blacklists. Or perhaps grab the last, second
to the last IP addresses.

Thanks in advance for your help.