Challenge: More Compact?

[Bill Bell]

"Tim Peters" <tim.one at home.com> wrote, in part:
> BTW, does it bother anyone that all of these solutions accept an IP
> ending with a newline?

This reminds me of a thread of a few months ago on one of the Perl 
lists. Someone (it might have been me :o) wanted a regex that 
matches the collection of (syntactically correct) URLs exactly.

How do we know that the junk that we write in this quite low-level 
context is correct? How would one check that an IPv4 IP address, 
or a URL for that matter, is correct as specified in an RFC 
somewhere?

(This summer I'm working with one of Parnas' s/e students. Maybe 
that's why my conscience is giving me such grief.)

Can't-think-of-a-way-of-signing-this-ly yours,
etc