Would you like signs with those chars?

isalpha and friends are defined to work on the current locale; you can't trust them to work on just ASCII.

isxdigit is always 0-9A-F, and as far as I can tell from the manpage, isdigit is always 0-9. Barring those, why is it useful to ask "is this an alphabetic/punctuation/whitespace character in ASCII?" Pretty much everything is defined in terms of Unicode now; even if you're processing C code, you should still be prepared for identifiers in Russian, Greek or Chinese. I have a hard time thinking about a case where it's the right thing to check if something is some unspecified alphabetic character, but only those in ASCII.