How to match exactly one byte using nom? - parsing

Question

I want to match exactly one alphabetic character (a-zA-Z) with nom.
I know I can match greedily using take_while! with something like this:
// match one or more alphabetical characters
pub fn alpha_many(input: &[u8]) -> IResult<&[u8], &[u8]> {
take_while!(input, |c| {
(c >= 0x41 && c <= 0x5a) || (c >= 0x61 && c <= 0x7a)
})
}
But I can't find how to match only one byte. There is one_of!, but I can't use a closure, I have to pass a whole slice:
// match exactly one alphabetical character
pub fn alpha_one(input: &[u8]) -> IResult<&[u8], u8> {
one_of!(
input,
[
0x41, 0x42, 0x43,
// etc until 0x5a and then from 0x61 to 0x7a
// ...
].as_ref()
)
}

Answer 1

I've come up with this. I'll mark this as the accepted answer tomorrow if nobody comes up with a better solution:
use nom::{self, ErrorKind, IResult, Needed};
/// Alphabetical characters ([RFC5234 appendix B.1])
///
/// [RFC5234 appendix B.1]: https://tools.ietf.org/html/rfc5234#appendix-B.1
///
/// ```no_rust
/// ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
/// ```
pub struct Alpha;
impl Alpha {
/// Return true if the given byte represents an alphabetical character
pub fn is_alpha(c: u8) -> bool {
(c >= 0x41 && c <= 0x5a) || (c >= 0x61 && c <= 0x7a)
}
/// Parse one or more alphabetical characters
pub fn parse_many(input: &[u8]) -> IResult<&[u8], &[u8]> {
take_while!(input, Self::is_alpha)
}
/// Parse one alphabetical character
pub fn parse_one(input: &[u8]) -> IResult<&[u8], u8> {
Self::parse_n(input, 1).map(|res| res[0])
}
/// Parse n alphabetical characters
pub fn parse_n(input: &[u8], n: usize) -> IResult<&[u8], &[u8]> {
Self::parse_m_n(input, n, n)
}
/// Parse between m and n alphabetical characters
pub fn parse_m_n(input: &[u8], m: usize, n: usize) -> IResult<&[u8], &[u8]> {
if input.len() < m {
return IResult::Incomplete(Needed::Size(input.len() - m));
}
for i in 0..n {
if !Self::is_alpha(input[i]) {
// We were supposed to have at least m printable bytes
if i < m {
return IResult::Error(error_position!(ErrorKind::ManyMN, &input[..]));
} else {
return IResult::Done(&input[i..], &input[0..i]);
}
}
}
return IResult::Done(&input[n..], &input[0..n]);
}
}