> It isn't hard (in Python):

<code deleted>

It's more complex, because the "3" is locale dependent, and is allowed
to be something like "3, then 2, then 3, then 1, repeating until the
start of the string".

See _group() in Lib/locale.py.

In any event, the code needs to be in C (sadly).  But as I said in my
previous comment (which probably crossed paths with yours), this C code
already exists.