regex fu (if contains : do this)

Hey guys, this is one for any regex grand masters.

I have a field (snort_dst) which contains addresses in both these formats:

1.2.3.4
5.6.7.8:910

where :910 denotes the port number. I have the following search, which splits the address into snort_dst_ip and snort_dst_port fields:

| rex field=snort_dst "(?<snort_dst_ip>[^:]+):(?<snort_dst_port>\S+)"

This properly splits on the : but my values which are just IPs don't get matched, as they don't fit the :portnum regex.

What I need is something which will work in either situation, something like "if contains : do this, else do this", but my regex knowledge is still in its infancy.