I had thought of that and the code I did not add would have checked the
first character and set idx to 1 if the first character was not numeric.
This would leave the first element of the array empty, but that is easy to
take care of later. Of course, leaving the empty element means that you will
know that all the odd elements are numbers and all the even ones are
characters.

What is the group variable for? You may want to check your Function
definition and how you are passing the variables back.