I like this answer best, it is concise, eloquent, but very understandable

Haven't seen many/any kyu 4 easier than this one, maybe you overcomplicated it or maybe people's solutions that you looked at were overcomplicated, I think once you get how it works it seem very reasonable

what language?

If anyone can explain this so I don't have to read over it for an hour that'd be lovely

We will not be given input like "3+", there is a space between everything, it doesn't matter what the algorithm does with invalid input because we don't get any

yeah this is really annoying, don't know why the initial function name is wrong

pretty basic regex so you should probably read up on them and practice (I suggest eloquent javascript chapter 9).
The . means any character except newlines

You can't just switch out the . for [a-z], you have to do [a-z||A-Z]

If you don't want to include n in the range you can write (2...n), you don't have to write: && item != n that way