Comment by hathawsh

I tried using that expression for a while, but then a user with a valid email address containing upper unicode characters showed up. I switched to a simpler expression:

    ^[^@\s\x00-\x1f]+@[^@\s\x00-\x1f.]+(:?\.[^@\s\x00-\x1f.]+)*$

It requires exactly one "@", disallows whitespace and control characters, prevents repeated dots in the domain name, and ensures the domain doesn't end with a dot. It catches a few typos and I think it allows every real email address I've heard of.