It is a bug, I'm reasonably sure. There are other URLs that don't pass -- I made some notes on the bug ticket along with the regexp that I use locally and notes for a 'better' regexp from regexp.org.
Personally, I think the larger regexp is better. The other issue at hand is that a few of the freehosting companies allow _ in the hostname which is invalid as per the RFC, but works on most machines.
Worst case you can patch it locally as I did.
