But at some point you'll want to know whether that code point you read `iswalpha()` or whatever, so you'll have to decode UTF-8 anyway.

At the parser-level, though; not down in the lexer. I intern unique user-defined strings (just with a hashcons or whatever the cool kids call it, these days). That defers the determination of correctness of UTF-kness to "someone else".

Figuring out whether a character should become part of a number or a name, for instance, is typical lexer stuff though. For that you have to classify it.