A few tips: Only the first + can be omitted, so after that a + or - is a delimiter. if no x is present, use x^0 and if x is present, use x^1.

so I would first split by +I- and then work on each number. If you want to allow x^{-1} then ignore +I- inside {}. Each number can be split at the x and the left part is the number, the right part (if exist) the x^y.

I'll be keeping this challenge on for a bit so you have time to complete it :)