/
(?(DEFINE)
(?<addr_spec> (?&local_part) @ (?&domain) )
(?<local_part> (?&dot_atom) | (?"ed_string) | (?&obs_local_part) )
(?<domain> (?&dot_atom) | (?&domain_literal) | (?&obs_domain) )
(?<domain_literal> (?&CFWS)? \[ (?: (?&FWS)? (?&dtext) )* (?&FWS)? \] (?&CFWS)? )
(?<dtext> [\x21-\x5a]