nom::is_a!

macro_rules! is_a(
  ($input:expr, $arr:expr) => (
    {
      #[inline(always)]
      fn as_bytes<T: $crate::AsBytes>(b: &T) -> &[u8] {
        b.as_bytes()
      }

      let expected  = $arr;
      let bytes     = as_bytes(&expected);
      let mut index = 0;

      for idx in 0..$input.len() {
        index = idx;
        let mut cont = false;
        for &i in bytes.iter() {
          if $input[idx] == i {
            cont = true;
            break;
          }
        }
        if !cont { break; }
      }
      if index == 0 {
        $crate::IResult::Error($crate::Err::Position($crate::ErrorCode::IsA as u32,$input))
      } else {
        $crate::IResult::Done(&$input[index..], &$input[0..index])
      }
    }
  );
);

is_a!(&[T]) => &[T] -> IResult<&[T], &[T]> returns the longest list of bytes that appear in the provided array

 named!(abcd, is_a!( "abcd" ));

 let r1 = abcd(&b"aaaaefgh"[..]);
 assert_eq!(r1, Done(&b"efgh"[..], &b"aaaa"[..]));

 let r2 = abcd(&b"dcbaefgh"[..]);
 assert_eq!(r2, Done(&b"efgh"[..], &b"dcba"[..]));