What would be a globally accepted regular expression to match e-mail addresses

Question

I have seen many examples, with many 'no, you missed something' comments. What is the right way to match an e-mail address?

For Sanity sake, only fully-qualified domain names, no @localhost allowed. (or, both ways)

Subdomains must be allowed (issac@deptartment.company.museum)

Answer 1

This regular expression complies with the grammar described in RFC 2822, it's very long, but the grammar described in the RFC is complex...

Answer 2

It is impossible to do so in a pure regex. Regexen cannot match nested parentheses, which the full RFC spec requires. (The latest RFC on this matter is RFC5322, only released a few months ago.)

Full validation of email addresses requires something along the lines of a CFG, and there are a few more things to be wary of; for example, email addresses can contain '\0', the null character... so you can't use any of C's normal string functions on them.

I actually feel a bit weird answering a question with a link to something I've written, but as it so happens, I have one I prepared earlier: a short and (as far as I can tell) fully-compliant validator, in Haskell; you can see the source code here. I imagine the code could be easily ported to any similar parsing library (perhaps C++’s Boost.Spirit), or just as easily hooked into from another language (Haskell has a very simple way for C to use Haskell code, and everything can use C bindings...)

There are also extensive test cases in the source code (I didn't export them from the module), which are due to Dominic Sayers, who has his own version of an RFC-compliant parser in PHP. (Several of the tests fail, but they are more strict than RFC5322 specifies, so I'm fine with that at the moment.)

Answer 3

That was asked here a couple of weeks ago. What it comes down to is, there are many legal addresses that an easy regex won't match. It takes a truly insane regex to match the majority of legal addresses. And even then, a syntactically legal address doesn't guarantee the existence of an account behind it - take foo@example.invalid, for example.

Answer 4

Not to mention that Chinese/Arabic domain names are to be allowed in the near future. Everyone has to change the email regex used, because those characters are surely not to be covered by [a-z]/i nor \w. They will all fail.

After all, best way to validate the email address is still to actually send an email to the address in question to validate the address. If the email address is part of user authentication (register/login/etc), then you can perfectly combine it with the user activation system. I.e. send an email with a link with an unique activation key to the specified email address and only allow login when the user has activated the newly created account using the link in the email.

If the purpose of the regex is just to quickly inform the user in the UI that the specified email address doesn't look like in the right format, best is still to check if it matches basically the following regex:

([^.@]+)(\\.[^.@]+)*@([^.@]+\\.)+([^.@]+)

Simple as that. Why on earth would you care about the characters used in the name and domain?

Answer 5

Unfortunately, it's not a regex, but... The right way to validate an email address is to send it a message and see if the user replies.

If you really, really want to verify that an address is syntactically-valid/RFC-compliant, then a regex is still unlikely to be the right tool for the job. You could most likely create a parser in fewer characters than the length of a regex with a similar level of RFC compliance and the parser would probably run faster to boot.

Even with a perfect test of RFC compliance, anon@ymo.us is perfectly-formed and refers to an existing domain, so you're not going to know whether the address you're given is actually valid or not unless you send it a message.

Answer 6

There's another discussion about this here: Using a regular expression to validate an email address

Answer 7

General advice is don't

It's probably worth a simple check that they entered "something@somethign.something" just to check they put the email in the right box.
Beyond that most of the official regex allow for so many obscure cases that they have a huge false positive rate (they will allow technically legal but extremely unlikely entries)

Answer 8

From http://www.ex-parrot.com/~pdw/Mail-RFC822-Address.html :

The grammar described in RFC 822 is suprisingly complex. Implementing validation with regular expressions somewhat pushes the limits of what it is sensible to do with regular expressions, although Perl copes well:

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Answer 9

If you're using Perl, I believe the Regex::Common module would be the one you want to use, and in particular, Regex::Common::Email::Address.

Answer 10

I had to recently build some RSS feeds, and part of that included going over the Xml schema, including the items for Webmaster and ManagingEditor, both of which are defined as an e-mail address matching this pattern:

([a-zA-Z0-9_\-])([a-zA-Z0-9_\-\.]*)@(\[((25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]|[0-9])\.){3}|((([a-zA-Z0-9\-]+)\.)+))([a-zA-Z]{2,}|(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]|[0-9])\])