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

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tomsv
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    Are you trying to validate e-mail (i.e., Internet e-mail messages) or e-mail _addresses_? – pmocek Feb 17 '16 at 22:00

3 Answers3

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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'
Michael
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  • Must see reference QA about using regexp for email validation : [Using a regular expression to validate an email address](http://stackoverflow.com/q/201323/4074148) – Veve May 14 '15 at 11:17
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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.

Peter
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  • this also accepts garbase like 'liar@[999.999.999.999]' – Jasen Aug 29 '17 at 23:33
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    liar@[999.999.999.999] : Square bracket around IP address is considered valid.. A host identified by an Internet Protocol literal address, version 6 [RFC3513] or later, is distinguished by enclosing the IP literal within square brackets ("[" and "]"). This is the only place where square bracket characters are allowed in the URI syntax. – Jry9972 Nov 16 '17 at 12:57
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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

Poorva Pharkya
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