This was way too easy for 5kyu, compared to other regex-kata it seems more like 7kyu to me.

Overhauled description

A wider range of sample tests is needed.

assert_equals should be used

It is not explained what is a "valid email address".

First time my code is on par with the top result. I usually feel like a noob coder at this point but not today. Woohoo!

At the very least, I think the description should include the word delegate as a hint if it's going to be 8kyu. Otherwise, no beginner is going to figure that out

The following test case is erroneous:

should return false for 'joe@example'

Though it is discouraged by ICANN, an e-mail address with no dot (using a local domain name) is still a valid e-mail address per RFC.

Needs random tests

nice code