Is regular expression recognition of an email address hard?

Question

I recently read somewhere that writing a regexp to match an email address, taking into account all the variations and possibilities of the standard is extremely hard and is significantly more complicated than what one would initially assume.

Why is that?

Are there any known and proven regexps that actually do this fully?

What are some good alternatives to using regexps for matching email addresses?

Answer 1

For the formal e-mail spec, yes, it is technically impossible via Regex due to the recursion of things like comments (especially if you don't remove comments to whitespace first), and the various different formats (an e-mail address isn't always [email protected]). You can get close (with some massive and incomprehensible Regex patterns), but a far better way of checking an e-mail is to do the very familiar handshake:

• they tell you their e-mail
• you e-mail them a confimation link with a Guid

• when they click on the link you know that:

  1. the e-mail is correct
  2. it exists
  3. they own it

Far better than blindly accepting an e-mail address.

Answer 2

There are a number of Perl modules (for example) that do this. Don't try and write your own regexp to do it. Look at

Mail::VRFY will do syntax and network checks (does and SMTP server somewhere accept this address)

https://metacpan.org/pod/Mail::VRFY

RFC::RFC822::Address - a recursive descent email address parser.

https://metacpan.org/pod/RFC::RFC822::Address

Mail::RFC822::Address - regexp-based address validation, worth looking at just for the insane regexp

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

Similar tools exist for other languages. Insane regexp below...

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