You should bear in mind that a-z, A-Z, 0-9, ., _ and - are not the only valid characters in the start of an email address.
Gmail, for example, lets you put a "+" sign in the address to "fake" a different email (e.g. someone@gmail.com will also get email sent to someone+else@gmail.com).
micky.o'finnagan@wherever.com would not appreciate your code stopping them entering their address ... apostrophes are perfectly valid in email addresses.
The Closure "check" of a valid email address mentioned above is, as it states itself, quite naïve:
http://code.google.com/p/closure-library/source/browse/trunk/closure/goog/format/emailaddress.js#198
I recommend being very open in your client side code, and then much more heavyweight like sending an email with a link to really check that it's "valid" (as in - syntactically valid for their provider, and also not misspelled).
Something like this:
var pattern = /[^@]+@[-a-z\.]\.[a-z\.]{2,6}/
Bearing in mind that theoretically you can have two @ signs in an email address, and I haven't even included characters beyond latin1 in the domain names!
http://www.eurid.eu/en/eu-domain-names/idns-eu
http://haacked.com/archive/2007/08/21/i-knew-how-to-validate-an-email-address-until-i.aspx