Another example that won't match that particular case is something like "3-1/2", which I've seen used fairly often to convey the notion of "3.5" rather than "2.5"; things like this can be accommodated, of course, but you really need to be clear about what the conventions are for a given input source (and take the trouble to enforce these conventions by rejecting violations).

Um, the string "2/3" no more matches the op's stated case than "-1/2" does. If you really only want to match the given case you could just test for string equality with "1/2" and return 0.5 or "undefined". Anything else is a generalization. It is a fallacy to say that one thing matches the particular case while another doesn't because it fits your first approach. And taking such assumptions and running with them is a sure way to wind up miscommunicating.

We agree that you need to generalize somehow, and errors should be explicit. But I think that you stopped with too limited a generalization, and I would have liked to see the limits of your approach stated up front.