Today, on #postgresql on IRC, guy (can't contact him now to get his permission to name him), said:
I have a table called problematic_hostnames. It contains a list of banned hostnames in column “hostname" (varchar). I would like to display the top 10 troll ISPs based on this. Does PG have a way of spotting a “pattern"? Some ISPs are example.net while others are foo.bar.example.net, so you can't just regexp the last X.Y (since that would cause “.co.uk" to be one of the top troll ISPs).