My memory must be playing tricks (or I misunderstood some email). They were pretty extensive, yes. Basically the problem is IIRC that the code in regex.c that tries to save/restore "register" values during backtracking does it at the wrong time/place, which causes all kinds of difficulties and bugs which are partly patched over with various hacks/workarounds. I bumped into this problem when I tried to add support for shy-groups to Emacs and after spending some time trying to understand what was going on I finally realized what was the root cause of the problem. So my extensive changes make the code simpler. Yes, I remember taking a look at it and noticing that merging my changes can't be done with diff3 at all :-( But this is getting further away from the issue at hand with M-x grep. After all, I have no idea if the OP's problem is even related to those regexp.c bugs. Stefan