Function nom::combinator::peek

pub fn peek<I: Clone, O, E: ParseError<I>, F>(
    f: F
) -> impl Fn(I) -> IResult<I, O, E> where
    F: Fn(I) -> IResult<I, O, E>, 

tries to apply its parser without consuming the input

use nom::combinator::peek;
use nom::character::complete::alpha1;

let parser = peek(alpha1);

assert_eq!(parser("abcd;"), Ok(("abcd;", "abcd")));
assert_eq!(parser("123;"), Err(Err::Error(("123;", ErrorKind::Alpha))));