"Mastering Regular Expressions" from O'Reilly covers regular expressions to validate email. The resulting regular expression is dozens of lines long.

Consider RFC 3696, "Application Techniques for Checking and Transformation of Names". The author provides some example valid email addresses:

    Abc\@def@example.com
    Fred\ Bloggs@example.com
    Joe.\\Blow@example.com
    "Abc@def"@example.com
    "Fred Bloggs"@example.com
    customer/department=shipping@example.com
    $A12345@example.com
    !def!xyz%abc@example.com

A one line regular expression won't do much but block valid users. As has been said often in this thread, the best way to validate an email is to send to the address and confirm delivery through user interaction. Everything else is half-measures, often doing more harm than good.