Saxon

I have initially implemented a fairly modest fix, to minimize risk. The change is that OpSequence (which implements a regular expression corresponding to a sequence of subexpressions (a) knows statically whether it contains any capturing subexpressions, and (b) if so, when it starts matching, it takes a copy of the state of the matcher (specifically the current captured groups), and if the sequence as a whole does not match, then it resets the state to the original.

I will experiment with improvements to this solution on the 9.8 branch. For example, I think we can improve the data structure for holding this state information.

Bug fix applied in the Saxon 9.7.0.18 maintenance release.