[dmurray]

> I also don’t want to allow every possible technically valid URL — quite the opposite.

Well, that should make things a lot easier. What does he mean here? The rest of the text doesn't make it clear to me, unless it's meant to be "every possibly valid HTTP, HTTPS, or FTP URL" which isn't exactly "the opposite".

[lelandfe]

The next paragraph might be that clarification, although I agree it isn't totally clear what he meant there:

> Assume that this regex will be used for a public URL shortener written in PHP, so URLs like http://localhost/, //foo.bar/, ://foo.bar/, data:text/plain;charset=utf-8,OHAI and tel:[phone] shouldn’t pass (even though they’re technically valid). Also, in this case I only want to allow the HTTP, HTTPS and FTP protocols.