RFC compliance would require you to allow comments inside email addresses which are contained inside matching parenthesis. As such, it is impossible to validate an Email with a regex as it is impossible do parenthesis matching.

This like many email validation tools available doesn't work for a large subset of valid email addresses. One of my major complaints about this particular example is that it ignores the '+' which is a valid character (even if Microsoft, and bytemycode.com's email vlidation say it isn't). See RFC 822. Proper email validation can be done with the regex at: http://textsnippets.com/posts/show/42