The function is powered by Fermat's little theorem, making it potentially faster (though less accurate) than a sieve. Adding tests on 5 and 7 will make this work up to 10^4

Make the sample cases less redundant

I've been in Haskell for a while, so I'm kinda not doing this right. For example, Haskell has no built-in regex, so I didn't even try that.
Also, I'm awful at JS.

Step 1: Cut a 14 x 14 square out of the 20 x 14 rectangle. This leaves a 14 x 6 rectangle.
Step 2: Cut a 9 x 9 square out of the 14 x 6 rectangle. Wait what? How did you do that?

I didn't have a numbers test, but I did have an empty string and there's no specification for empty strings, so that should be an invalid input. (Haskell)

That's... uhh... the exact opposite of the point of the site

don't look at it
it's an abomination

Input is a string. I believe they're therefore immutable in Ruby, which makes things very weird.
OP try printing your answer before returning it and seeing if you are indeed returning the correct answer.