There are a ton of rules which describe if an address conforms to the standard. And there are almost always edge cases which will break the standard.
I use this expression, it's not perfect as there are some edge cases which will slip by but those are easy enough to test by simply sending the test email:
^[_a-z0-9-]+(?:\.[_a-z0-9-]+)*@[a-z0-9-]+(?:\.[a-z0-9-]+)*(?:\.[a-z]{2,4})$
The following regex contains all rules which apply to an email address per the standard. Providing your regex engine handles named back references that regex will work and you can uncomment the commented line.
(?(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] | [\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) | (?"ed_string) )
(?<quoted_string> (?&CFWS)? "" (?: (?&FWS)? (?&qcontent) )* (?&FWS)? "" (?&CFWS)? )
(?<qcontent> (?&qtext) | (?"ed_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) | (?"ed_pair) | (?&comment) )
(?<ccontent> (?&ctext) | (?"ed_pair) )
(?<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) | (?"ed_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))
With a single line commented out to prevent infinite recursion, this expression can be converted into something most any regex engines can handle (be sure to use the x
option):
((?: (?: (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? (?: (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) (?: \. (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) )* ) (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) | (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? "" (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: \x21 | [\x23-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? "" (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) | (?: (?: (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) | (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? "" (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: \x21 | [\x23-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? "" (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) ) (?: \. (?: (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) | (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? "" (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: \x21 | [\x23-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? "" (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) ) )* ) ) @ (?: (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? (?: (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) (?: \. (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) )* ) (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) | (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? \[ (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: [\x21-\x5a] | [\x5e-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \] (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) | (?: (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) (?: \. (?: (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? (?: [a-zA-Z0-9!#$%&'*+/=?^_{|}~-]+ ) (?: (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: \( (?: (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? (?: (?: [\x21-\x27] | [\x2a-\x5b] | [\x5d-\x7e] | (?: (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) ) ) | (?: \\ (?: (?: [\x21-\x7E] ) | (?: [ \t] ) ) | (?: \\ (?: \x00 | (?: [\x01-\x08] | \x0b | \x0c | [\x0e-\x1f] | \x7f ) | \n | \r ) ) ) ) )* (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? \) ) )+ (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) )? | (?: (?: (?: [ \t] )* \r\n )? (?: [ \t] )+ | (?: (?: [ \t] )+ (?: \r\n (?: [ \t] )+ )* ) ) )? ) )* ) ) ))