RFC 5322 email format validation

Question

How can I check if emails that are generated by my code a valid according to RFC 5322 ?

Answer 1

Here's a PCRE regular expression (taken from a PHP library) that will validate according to RFC 5322:

'/^(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){255,})(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){65,}@)((?>(?>(?>((?>(?>(?>\x0D\x0A)?[\t ])+|(?>[\t ]*\x0D\x0A)?[\t ]+)?)(\((?>(?2)(?>[\x01-\x08\x0B\x0C\x0E-\'*-\[\]-\x7F]|\\\[\x00-\x7F]|(?3)))*(?2)\)))+(?2))|(?2))?)([!#-\'*+\/-9=?^-~-]+|"(?>(?2)(?>[\x01-\x08\x0B\x0C\x0E-!#-\[\]-\x7F]|\\\[\x00-\x7F]))*(?2)")(?>(?1)\.(?1)(?4))*(?1)@(?!(?1)[a-z\d-]{64,})(?1)(?>([a-z\d](?>[a-z\d-]*[a-z\d])?)(?>(?1)\.(?!(?1)[a-z\d-]{64,})(?1)(?5)){0,126}|\[(?:(?>IPv6:(?>([a-f\d]{1,4})(?>:(?6)){7}|(?!(?:.*[a-f\d][:\]]){8,})((?6)(?>:(?6)){0,6})?::(?7)?))|(?>(?>IPv6:(?>(?6)(?>:(?6)){5}:|(?!(?:.*[a-f\d]:){6,})(?8)?::(?>((?6)(?>:(?6)){0,4}):)?))?(25[0-5]|2[0-4]\d|1\d{2}|[1-9]?\d)(?>\.(?9)){3}))\])(?1)$/isD'

Unlike Peter's answer it does allow for single-label domain names (which are syntactically valid) and IPv6 address literals.

However, I'd strongly suggest to instead validate according to RFC 5321 which doesn't allow for comments or folding white space (which are semantically invisible and so not actually a part of the email address) or for obsolete local parts (which can just be re-written as non-obsolete quoted strings):

'/^(?!(?>"?(?>\\\[ -~]|[^"])"?){255,})(?!"?(?>\\\[ -~]|[^"]){65,}"?@)(?>([!#-\'*+\/-9=?^-~-]+)(?>\.(?1))*|"(?>[ !#-\[\]-~]|\\\[ -~])*")@(?!.*[^.]{64,})(?>([a-z\d](?>[a-z\d-]*[a-z\d])?)(?>\.(?2)){0,126}|\[(?:(?>IPv6:(?>([a-f\d]{1,4})(?>:(?3)){7}|(?!(?:.*[a-f\d][:\]]){8,})((?3)(?>:(?3)){0,6})?::(?4)?))|(?>(?>IPv6:(?>(?3)(?>:(?3)){5}:|(?!(?:.*[a-f\d]:){6,})(?5)?::(?>((?3)(?>:(?3)){0,4}):)?))?(25[0-5]|2[0-4]\d|1\d{2}|[1-9]?\d)(?>\.(?6)){3}))\])$/iD'

Answer 2

Email Regex as per RFC 5322 Policy After so much struggle I made the regex validating all the cases as per 5322 except one: (1)admin@mailserver1 (local domain name with no TLD, although ICANN highly discourages dot less email addresses)

^(?=.{1,64}@)((?:[A-Za-z0-9!#$%&'*+-/=?^\{\|\}~]+|"(?:\\"|\\\\|[A-Za-z0-9\.!#\$%&'\*\+\-/=\?\^_{|}~ (),:;<>@[].])+")(?:.(?:[A-Za-z0-9!#$%&'*+-/=?^\{\|\}~]+|"(?:\\"|\\\\|[A-Za-z0-9\.!#\$%&'\*\+\-/=\?\^_{|}~ (),:;<>@[].])+")))@(?=.{1,255}.)((?:[A-Za-z0-9]+(?:(?:[A-Za-z0-9-][A-Za-z0-9])?).)+[A-Za-z]{2,})|(((0|[1-9A-Fa-f][0-9A-Fa-f]{0,3}):){0,6}(0|)])$

Please click here to get a clear idea about this regex https://regex101.com/r/7u0dze/1

Answer 3

Using this regex its like 98% valid. It doesn't validate the following:

postbox@com

admin@mailserver1

user@[IPv6:2001:db8:1ff::a0b:dbd0]

But it covers everything else

^(([^<>()[\\]\\.,;:\\s@\"]+(\\.[^<>()[\\]\\.,;:\\s@\"]+)*)|(\".+\"))@((\\[[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\])|(([a-zA-Z\\-0-9]+\\.)+[a-zA-Z]{2,}))$

Note: This is transported directly from some production Golang code so slashes are added.