Best practices for email address validation (including the + in gmail addresses)

Question

I know there are a lot of questions on here about email validation and specific RegEx's. I'd like to know what the best practices are for validating emails with respect to having the [email protected] trick (details here). My current RegExp for JavaScript validation is as follows, but it doesn't support the extra + in the handle:

/^([a-zA-Z0-9_.-])+@(([a-zA-Z0-9-])+.)+([a-zA-Z0-9]{2,4})+$/

Are there any other services that support the extra +? Should I allow a + in the address or should I alter the RegEx to only allow it for an email with gmail.com or googlemail.com as the domain? If so, what would be the altered RegEx?

UPDATE: Thanks to everyone for pointing out that + is valid per the spec. I didn't know that and now do for the future. For those of you saying that its bad to even use a RegEx to validate it, my reason is completely based on a creative design I'm building to. Our client's design places a green check or a red X next to the email address input on blur of it. That icon indicates whether or not its a valid email address so I must use some JS to validate it then.

Answer 1

+ is a valid character in an email address. It doesn't matter if the domain isn't gmail.com or googlemail.com

Regexes aren't actually a very good way of validating emails, but if you just want to modify your regex to handle the plus, change it to the following:

/^([a-zA-Z0-9_.-\+])+@(([a-zA-Z0-9-])+.)+([a-zA-Z0-9]{2,4})+$/

As an example of how this regex doesn't validate against the spec: The email [email protected] is valid according to it.

Answer 2

If you need to validate emails via regexp, then read the standard or at least this article.

The standard suggests to use this regexp:

(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])

If that doesn't scare you, it should :)