... do a simple count of the x's. ... get a fixed length thing at the beginning.

But I understood that to be what the OP was asking for, at least as a starting point for a larger application. (bopibopi actually seemed to have the counting and extracting part under control, and was asking for help with the subsequent pieces.) Using split may be a good example of doing something slightly different. We may not be so much splitting hairs here as comparing apples and oranges — or perhaps tangerines and oranges since we're not really all that far apart.

Update:

... the -1 limit on split ... an example of a not so common usage ...

As someone addicted to "not so common usage" myself, I can sympathize. (But I have it under control; I haven't used uncommonly in ages!)