Just a side note for 2.7: could I recommend people to be really extra, extra careful when changing what kind of regexps are accepted and what kind of regexps are outright rejected?  I believe the risk of making long-existing and working 2.7 programs suddenly crash on the next 2.7 micro version should *by far* outweight the theoretical advantage of crashing early on sufficiently bogus regexps.

(Fwiw, I believe the same would apply on 3.x too, where rejecting previously-accepted regexps should only be done in a minor version upgrade.)