It's a bad idea.
But ...
^([a-zA-Z0-9_\-\.]+)@[^\.]+\.[^\.]+$
Will do it.
You'll be eliminating a heck of a lot of valid emails though! (And letting bad ones though as well ...)
If you are going to use Regular Expressions, then get a copy of
Expresso[
^] - it's free, and it examines and generates Regular expressions.