I believe it's not really regex. So no anchors or any of that fun stuff.
Pretty sure the only wildcards that work here are "*" and "?" (standard OS path wildcards).
The behavior you encountered does seem like a bug though, or at the very least an inconsistency and undesirable feature.
I mean, if you just use "something", it will actually leave "something-else" visible. But, it will hide all "something" dirs too, for instance "foo/something", which makes sense but is not good in your first post case.
But I see no logical reason why "bar/something" should hide "bar/something-else" as well. If I wanted that, I would expect to use "bar/something*"
So unless that gets fixed/changed, you have two options - 1) adopt a better folder naming scheme so you don't have this problem (for example, prepend . to folders you want to hide, Linux style), 2) decide on the least of the two negatives and roll with that.