First sentence in [lex.ext]/1:

If a token matches both user-defined-literal and another literal kind, it is treated as the latter.

I don't think this is necessary as the non-terminal user-defined-literal is already the last item in the definition of the literal grammar production.

Thus, I think this paragraph could be simplified as follows:

1. The syntactic non-terminal preceding the ud-suffix in a user-defined-literal is taken to be the longest sequence of characters that could match that non-terminal. [ Example: 123_­km is a user-defined-literal, but 12LL is an integer-literal. — end example ]