It exists and its flags are exported, but not the direct classes itself. This seems inconsistent to me and fixing it would make things like using static typing on it just a little bit easier.

I think RegexFlag is an implementation detail, but it is true that it isn't prefixed with a _ so putting it in __all__ is not obviously wrong.  However, if we do that we should also document it (currently it is mentioned only in a versionchanged line, which isn't technically documenting it).

I find it curious that static typing is affected by __all___, but I don't really know anything about typing.

I suppose it may be an implementation detail, though I wouldn't be amazed that had enum existed when re was written it'd have been used instead of constant integers at the time. Though I do suppose exposing it fully would add two ways to get the flags which I can see how it would be considered bad.

It's still useful for type checking though, and while I did make a PR to add it to typeshed, that still leaves it in an iffy state and I probably would not be the last person to be confused by it initially.

Well, I consider that they really should be named constants and not an enum, which is why I consider it an implementation detail :)