nom::filter!

macro_rules! filter {
    ($input:expr, $submac:ident!( $($args:tt)* )) => { ... };
    ($input:expr, $f:expr) => { ... };
}

returns the longest list of bytes until the provided parser fails

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

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