What I'm saying is that you want to make a loop with $regsubex, //g match before and after each character, which means: for N characters, you have N + 1 matches.
So you are always making a extra match, an extra evaluation of $ulist() with a N parameter that is invalid: it returns $null and you don't see the problem, but since //g doesn't match characters but position, you have one useless comma left, which is why you did $left().
I guess it's ok, but having that extra evaluation could be a problem, using a while loop would be, anyways, good practice, probably faster and it would make the code readable
