Following up on the thread from last time regexp-split was discussed.

On 8 January 2012 07:05, Andy Wingo <wingo@pobox.com> wrote:
> On Sat 31 Dec 2011 06:54, Daniel Hartwig <mandyke@gmail.com> writes:
>> * [Vanilla `string-split' expanded to support the CHAR_PRED
>>   semantics of `string-index' et al.]
>
> Makes sense to me.
>

Attached, with perhaps too many test cases as well.  Commit message
contains the details on why it's done the way it is.