Frankly, I don't really like that idea; I think it muddles up the RE
syntax to have such a group-modifying operator, and seems rather
unpythonic: the existing way to do this --  use .upper(), .lower() or
.title() to format the groups in a match object as necessary -- seems to
be much more readable and reasonable in this sense.

I think the proposed changes look good, but I agree that the focus
should be on breaking up the megapatch into more digestible feature
additions, starting from the barebones engine. Until that's done, I
doubt *anyone* will want to review it, let alone merge it into the main
Python distribution. So, I think we should hold off on any new features
until this raft of changes can be properly broken up, reviewed and
(hopefully) merged in.