What are best practices for validating email addresses on iOS 2.0

The answer to Using a regular expression to validate an email address explains in great detail that the grammar specified in RFC 5322 is too complicated for primitive regular expressions.

I recommend a real parser approach like MKEmailAddress.

As quick regular expressions solution see this modification of DHValidation:

- (BOOL) validateEmail: (NSString *) candidate {
    NSString *emailRegex =
@"(?:[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])+)\\])"; 
    NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES[c] %@", emailRegex]; 

    return [emailTest evaluateWithObject:candidate];
}

Read the RFC. Almost everyone that thinks they know how to parse/clean/validate an email address is wrong.

https://www.rfc-editor.org/rfc/rfc2822 Section 3.4.1 is very useful. Notice

dtext           =       NO-WS-CTL /     ; Non white space controls

                        %d33-90 /       ; The rest of the US-ASCII
                        %d94-126        ;  characters not including "[",
                                        ;  "]", or "\"

Yes, that means +, ', etc are all legit.

The best solution I have found so far (and the one I ended up going with) is to add RegexKitLite To the project which gives access to regular expressions via NSString Categories.

It is quite painless to add to the project and once in place, any of the regular expression email validation logic will work.

A good start is to decide what do you and do you not want to accept as an email address?

99% of of email addresses look like this: bob.smith@foo.com or fred@bla.edu

However, it's technically legal to have an email address like this: f!#$%&'*+-/=?^_{|}~"ha!"@com

There are probably only a handful of valid emails in the world for top-level domains, and almost nobody uses most of those other characters (especially quotes and backticks), so you might want to assume that these are all invalid things to do. But you should do so as a conscious decision.

Beyond that, do what Paul says and try to match the input to a regular expression like this: ^[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,}$

That one will match pretty much everybody's email address.

While the focus on regular expressions is good, but this is only a first and necessary step. There are other steps that also need to be accounted for a good validation strategy.

Two things on top of my head are :

1. DNS validation to make sure the domain actually exists.

2. After dns validation, you can also choose to do an smtp validation. send a call to the smtp server to see if the user actually exists.

In this way you can catch all kinds of user errors and make sure it is a valid email.

I have found that using a regular expression works quite well to validate an email address.

The major downside to regular expressions of course is maintainability, so comment like you have never commented before. I promise you, if you don't you will wish you did when you go back to the expression after a few weeks.

Here is a link to a good source, http://www.regular-expressions.info/email.html.

Digging up the dirt, but I just stumbled upon SHEmailValidator which does a perfect job and has a nice interface.