Don't Write Your Own E-mail Validator

Gmail has a nice feature: when delivering e-mail, everything including and after a + in a Gmail address is ignored. For example, mail arriving at all of these addresses would go to the same place if they were Gmail addresses,

Thanks to this feature, when a user acquires a Gmail account, Google is actually providing about a googol (as in the number 10100) different e-mail addresses to that user! Quite appropriate, really.

I have seen other mailers do similar things, like ignoring everything after dashes. A nice advantage to this is when registering at a new website I can customize my e-mail address for them by, say, throwing the website name in it. Because I have a google of e-mail addresses available, it is impossible to run out, so I can give every person I meet their own version of my address. The custom address can come in handy for sorting and filtering, and it will also tell me who is selling out my e-mail address. This, of course, assumes that someone isn't stripping out the extra text in my address to counter the Gmail feature.

However, in my personal experience, most websites will not permit +'s in addresses. This is completely ridiculous, because it means that virtually every website will incorrectly invalidate perfectly valid e-mail addresses. Even major websites, like, screw this up. They see the + in the address and give up.

In fact, if I do a Google search for "email validation regex" right now, 9 of the first 10 results return websites with regular expressions that are complete garbage and will toss out many common, valid addresses. The only useful result was at the fifth spot (linked below).

For the love of Stallman's beard, stop writing your own e-mail address validators!

Why shouldn't you even bother writing your own? Because the proper Perl regular expression for RFC822 is over 6 kilobytes in length! Follow that link and look at that. This is the smallest regular expression you would need to get it right.

If you really insist on having a nice short one and don't want to use a validation library, which, again, is a stupid idea and you should be using a library, then use the dumbest, most liberal expression you can. (Just don't forget the security issues.) Like this,


Seriously, if you add anything else you most almost surely make it incorrectly reject valid addresses. Note that e-mail addresses can contain spaces, and even more than one @! These are valid addresses,

"John Doe"

I have not yet found a website that will accept either of these, even though both are completely valid addresses. Even MS Outlook, which I use at work (allowing me to verify this), will refuse to send e-mail to these addresses (Gmail accepts it just fine). Hmmm... maybe having an address like these is a good anti-spam measure!

So if your e-mail address is "John Doe" no one using Outlook can send you e-mail, which sounds like a feature to me, really.

So, everyone, please stop writing e-mail validation regular expressions. The work has been done, and you will only get it wrong, guaranteed.

This is a similar rant I came across while writing mine: Stop Doing Email Validation the Wrong Way.

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Chris Wellons (PGP)
~skeeto/ (view)