Function nom::hex_digit

pub fn hex_digit<'a, T: ?Sized>(input: &'a T) -> IResult<&'a T, &'a T> where
    T: Index<Range<usize>, Output = T> + Index<RangeFrom<usize>, Output = T>,
    &'a T: IterIndices + InputLength, 

Recognizes hexadecimal numerical characters: 0-9, A-F, a-f