\u1234 -> u1234 * 5 "Hey\ \u1234!" "C:\\udef2\\"

This is either ambiguous or error-prone.

I agree, and the lexical syntax in the report doesn't give any clue as to how the escape sequences are to be interpreted (indeed, it looks like the \uhhhh sequences were an afterthought, which wasn't followed through into the formal syntax). Cheers, Simon