The true power of regular expressions (2012)

/
    (?(DEFINE)
        (?<addr_spec> (?&local_part) @ (?&domain) )
        (?<local_part> (?&dot_atom) | (?&quoted_string) | (?&obs_local_part) )
        (?<domain> (?&dot_atom) | (?&domain_literal) | (?&obs_domain) )
        (?<domain_literal> (?&CFWS)? \[ (?: (?&FWS)? (?&dtext) )* (?&FWS)? \] (?&CFWS)? )
        (?<dtext> [\x21-\x5a] | [\x5e-\x7e] | (?&obs_dtext) )
        (?<quoted_pair> \\ (?: (?&VCHAR) | (?&WSP) ) | (?&obs_qp) )
        (?<dot_atom> (?&CFWS)? (?&dot_atom_text) (?&CFWS)? )
        (?<dot_atom_text> (?&atext) (?: \. (?&atext) )* )
        (?<atext> [a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+ )
        (?<atom> (?&CFWS)? (?&atext) (?&CFWS)? )
        (?<word> (?&atom) | (?&quoted_string) )
        (?<quoted_string> (?&CFWS)? " (?: (?&FWS)? (?&qcontent) )* (?&FWS)? " (?&CFWS)? )
        (?<qcontent> (?&qtext) | (?&quoted_pair) )
        (?<qtext> \x21 | [\x23-\x5b] | [\x5d-\x7e] | (?&obs_qtext) )

        # comments and whitespace
        (?<FWS> (?: (?&WSP)* \r\n )? (?&WSP)+ | (?&obs_FWS) )
        (?<CFWS> (?: (?&FWS)? (?&comment) )+ (?&FWS)? | (?&FWS) )
        (?<comment> \( (?: (?&FWS)? (?&ccontent) )* (?&FWS)? \) )
        (?<ccontent> (?&ctext) | (?&quoted_pair) | (?&comment) )
        (?<ctext> [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?&obs_ctext) )

        # obsolete tokens
        (?<obs_domain> (?&atom) (?: \. (?&atom) )* )
        (?<obs_local_part> (?&word) (?: \. (?&word) )* )
        (?<obs_dtext> (?&obs_NO_WS_CTL) | (?&quoted_pair) )
        (?<obs_qp> \\ (?: \x00 | (?&obs_NO_WS_CTL) | \n | \r ) )
        (?<obs_FWS> (?&WSP)+ (?: \r\n (?&WSP)+ )* )
        (?<obs_ctext> (?&obs_NO_WS_CTL) )
        (?<obs_qtext> (?&obs_NO_WS_CTL) )
        (?<obs_NO_WS_CTL> [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f )

        # character class definitions
        (?<VCHAR> [\x21-\x7E] )
        (?<WSP> [ \t] )
    )
    ^(?&addr_spec)$
/x