I don't believe the claim made by bortzmeyer, above, that "The grammar (specified in RFC 5322) is too complicated for that" (to be handled by a regular expression).
Here is the grammar: (from http://tools.ietf.org/html/rfc5322#section-3.4.1)
addr-spec = local-part "@" domainlocal-part = dot-atom / quoted-string / obs-local-partdomain = dot-atom / domain-literal / obs-domaindomain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]dtext = %d33-90 / ; Printable US-ASCII %d94-126 / ; characters not including obs-dtext ; "[", "]", or "\"Assuming that dot-atom, quoted-string, obs-local-part, obs-domain are themselves regular languages, this is a very simple grammar. Just replace the local-part and domain in the addr-spec production with their respective productions, and you have a regular language, directly translatable to a regular expression.
