The problem is that the RFC is amazingly permissive : almost anything goes in email adresses, including whitespace! A RFC-compliant regex is incredibly complex

The problem is that the RFC is amazingly permissive : almost anything goes in email adresses, including whitespace! A RFC-compliant regex is incredibly complex [1]. Furthermore, one would need to read not one, but five RFCs in order to grok everything: http://tools.ietf.org/html/rfc5322 http://tools.ietf.org/html/rfc5322 http://tools.ietf.org/html/rfc3696 http://tools.ietf.org/html/rfc3696 http://tools.ietf.org/html/rfc5321 http://tools.ietf.org/html/rfc5321 http://tools.ietf.org/html/rfc4291#section-2.2 http://tools.ietf.org/html/rfc4291#section-2.2 http://tools.ietf.org/html/rfc1123#section-2.1 http://tools.ietf.org/html/rfc1123#section-2.1 [1] Great overview of the problem, plus a PHP validator. http://www.dominicsayers.com/isemail/ http://www.dominicsayers.com/isemail/