Write a function or program to validate an e-mail address against RFC 5321 (some grammar rules found in 5322) with the relaxation that you can ignore comments and folding whitespace (CFWS) and generalised address literals. This gives the grammar

Mailbox = Local-part "@" ( Domain / address-literal ) Local-part = Dot-string / Quoted-string Dot-string = Atom *("." Atom) Atom = 1*atext atext = ALPHA / DIGIT / ; Printable US-ASCII "!" / "#" / ; characters not including "$" / "%" / ; specials. Used for atoms. "&" / "'" / "*" / "+" / "-" / "/" / "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~" Quoted-string = DQUOTE *QcontentSMTP DQUOTE QcontentSMTP = qtextSMTP / quoted-pairSMTP qtextSMTP = %d32-33 / %d35-91 / %d93-126 quoted-pairSMTP = %d92 %d32-126 Domain = sub-domain *("." sub-domain) sub-domain = Let-dig [Ldh-str] Let-dig = ALPHA / DIGIT Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig address-literal = "[" ( IPv4-address-literal / IPv6-address-literal ) "]" IPv4-address-literal = Snum 3("." Snum) IPv6-address-literal = "IPv6:" IPv6-addr Snum = 1*3DIGIT ; representing a decimal integer value in the range 0 through 255 

Note: I've skipped the definition of IPv6-addr because this particular RFC gets it wrong and disallows e.g. ::1. The correct spec is in RFC 2373.

Restrictions

You may not use any existing e-mail validation library calls. However, you may use existing network libraries to check IP addresses.

If you write a function/method/operator/equivalent it should take a string and return a boolean or truthy/falsy value, as appropriate for your language. If you write a program it should take a single line from stdin and indicate valid or invalid via the exit code.

Test cases

The following test cases are listed in blocks for compactness. The first block are cases which should pass:

[email protected] [email protected] [email protected] [email protected] [email protected] [email protected] email@[123.123.123.123] "email"@domain.com [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] ""@domain.com "e"@domain.com "\@"@domain.com email@domain "Abc\@def"@example.com "Fred Bloggs"@example.com "Joe\\Blow"@example.com "Abc@def"@example.com customer/[email protected] [email protected] !def!xyz%[email protected] [email protected] _somename@[IPv6:::1] [email protected] [email protected] [email protected] 

The following test cases should not pass:

plainaddress #@%^%#$@#$@#.com @domain.com Joe Smith <[email protected]> email.domain.com email@[email protected] [email protected] [email protected] [email protected] [email protected] [email protected] (Joe Smith) [email protected] [email protected] email@[IPv6:127.0.0.1] email@[127.0.0] email@[.127.0.0.1] email@[127.0.0.1.] email@IPv6:::1] [email protected]] email@[256.123.123.123]