Except that's not actually correct, because strictly following the ABNF does not yield correct semantics for an email address.

An email address consists of a local-part, a literal @ character, and then a domain name or an IP address literal. The local-part is either a series of dot-separated atoms (/[a-zA-Z0-9!#$%&'+/=?^_`{|}~-]+/ is the syntax for an atom) or a quoted string (/"([^\\"\0-\031\x7f]|\\[^\0-\031\x7f])"/ in regex). If you support EAI, you need to add \u00a0-\u10ffff [i.e., all non-ASCII non-C1 control characters].

According to RFC 822, you can insert whitespace (and comments) arbitrarily into the mailbox production, but that is non-semantic. Seeing "From: John Doe <foo @ example (I ((hate)) CFWS).com>" means that the email address is exactly "[email protected]", and you can reject any claim of the spelling without prejudicing any email addresses.

In practice, you can drop all support for quoted strings and IP address literals in most applications. So a correct email address (pre-EAI) regex in that vein would be:

    [a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+(.[a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+)*@([a-zA-Z0-9]+(-[a-zA-Z0-9]+)*\.)+[a-zA-Z]{2,}

If you insist on quoted-string support, then it looks like this:

    ([a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+(.[a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+)*|"([^\\"\0-\031\x7f]|\\[\\"])*")@([a-zA-Z0-9]+(-[a-zA-Z0-9]+)*\.)+[a-zA-Z]{2,}

[I simplified the quoted string to only accept escapes that are semantically necessary--the strings "a b"@example.com and "a\ b"@example.com correspond to the same email address].

Edit: Sorry, there's quite a few asterisks in the regexes that Hacker News is turning into italicization, and I don't know how to unbork them.

Edit 2: Someone suggested how to unbork the standalone regexes, but the asterisks in the inline regex in the second paragraph are still missing.