Matching an empty string an unbounded number of times isn't a case of exponential runtime, it's a case of infinite runtime, unless the regexp internals are smart enough to cut the search off (I don't know enough about re's internals to know whether it's smart enough to do so).

So it's worth catching at compile time if it can be caught.  Alas, regexps are so complicated I doubt it's possible to do so without false positives or false negatives short of major effort.

For now, at least the doctest part of the patch should be harmless ;-)