Function abnf_core::is_hexdig

pub fn is_hexdig(c: impl AsChar) -> bool

HEXDIG = DIGIT / “A” / “B” / “C” / “D” / “E” / “F”

Note: ABNF strings are case-insensitive so a / … / f are allowed, too. Issue: https://github.com/duesee/abnf-core/issues/12