i know regex support doesn't fit very well with indexed search (similar issue as with wildcard support, just more complex), but for some scenarios it would be nice to have.
Even if slow, it would be better than not supporting it at all - especially for projects using whoosh that had a slow non-indexed search before and rely on regex search in either their code or even in user content (like a wiki page that has a search macro call with a regex search term).
whoosh.query.Wildcard is doing a similar thing, so I guess it could be implemented in a similar way and even an implementation without any "prefix" optimization would help.
I guess it would also need something like a RegexPlugin for the qp, but not sure how a query would look like so it bests fits into whoosh.
For moin 1.x we had "foo.bar" as normal search term and "regex:foo.bar" for regex search (regex: could be shortened as long as it did not get ambiguous, e.g. re:).