In bug 250872, I tried adding a related Sourceforce bug URL in the See Also field. However, Bugzilla showed me an error screen instead, saying:
https://sourceforge.net/p/elftoolchain/tickets/597/ is not a valid URL to a bug. See Also URLs should point to one of:
show_bug.cgi in a Bugzilla installation.
A bug on launchpad.net
An issue on code.google.com.
A bug on bugs.debian.org.
An issue in a JIRA installation.
A ticket in a Trac installation.
A bug in a MantisBT installation.
A bug on sourceforge.net.
An issue/pull request on github.com.
A review on reviews.freebsd.org.
A PR on NetBSD GNATS system.
An issue on Google Chromium issue tracker.
An issue on bitbucket.org.
Obviously, that URL is a valid bug on Sourceforge, so I think the regex configured in our Bugzilla is outdated.