Function combine::parser::byte::letter

pub fn letter<I>() -> Letter<I> where
    I: Stream<Item = u8>,
    I::Error: ParseError<I::Item, I::Range, I::Position>, 

Parses an ASCII alphabet letter (a–z, A–Z).

use combine::Parser;
use combine::parser::byte::letter;
assert_eq!(letter().parse(&b"a"[..]), Ok((b'a', &b""[..])));
assert_eq!(letter().parse(&b"A"[..]), Ok((b'A', &b""[..])));
assert!(letter().parse(&b"9"[..]).is_err());