/(?(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)$/xmg