[TezzellEnt]

That's pretty good. I learned from Rails Tutorial to use this one:

    /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i

It's worked for me when I needed to use it.

[PinguTS]

Hope you don't have international customers, because http://äöüß.de is a totally legal domain these days.

[allegory]

Very true. My simplified one on the parent comment of that one makes no attempt to involve Unicode or character ranges for precisely that reason!