I've used your example strings and it works with Mp3tag 2.37b. No problem.
You have to make sure, to look and write carefully and not to make writing failures, e.g. do not add space chars, and such other wild things.
If you've read the Mp3tag manual, then you should have known, that using a RegExp is a little bit tricky. It is always valueable to know the syntax and how to escape special chars.
You can learn much about using Mp3tag's RegExp machine by surfing through Mp3tag Forums and find user examples.
Like Dano had worked out afterwards by using the built in smart function 'Guess values' there is sometimes more than one way in Mp3tag to solve a problem, especially for such a simple problem.
I did read the manual (the online version - not sure if there is another one?) - the help provided for the actions is rather cryptic (no examples....) - I'm a programmer myself, and still I was battling with it; probably after a while of reading forum posts, using the program itself and playing around I would have been able to get it to work.
It was just that I ran into this this morning, and I wanted a quick solution - all the other great options work very intuitively and are easy to apply - this problem was harder to solve. Maybe it is because not many people run into the same issue? After all, I use MP3Tag for several months now (although not daily), and this was first time I needed something like that.
Anyway, it would be nice to have: some tag-to-tag wizard. Maybe for the wish-list?
BTW, this morning I downloaded V 2.37a - not the beta version you are using.
After some tweaking, I now get 50% result: the artist name and '/' are removed from the title tag, so the title remains, but artist name is not moved to the artist tag.