nom::filter!

macro_rules! filter(
  ($input:expr, $submac:ident!( $($args:tt)* )) => (

    {
      let mut index = 0;
      let mut found = false;
      for idx in 0..$input.len() {
        index = idx;
        if !$submac!($input[idx], $($args)*) {
          found = true;
          break;
        }
      }
      if index == 0 {
        $crate::IResult::Error($crate::Err::Position($crate::ErrorCode::Filter as u32,$input))
      } else if found {
        $crate::IResult::Done(&$input[index..], &$input[0..index])
      } else {
        $crate::IResult::Done(&b""[..], $input)
      }
    }
  );
  ($input:expr, $f:expr) => (
    filter!($input, call!($f));
  );
);

filter!(&[T] -> bool) => &[T] -> IResult<&[T], &[T]> returns the longest list of bytes until the provided function fails.

The argument is either a function &[T] -> bool or a macro returning a `bool

 named!( alpha, filter!( is_alphanumeric ) );

 let r = alpha(&b"abcd\nefgh"[..]);
 assert_eq!(r, Done(&b"\nefgh"[..], &b"abcd"[..]));