Function nom::combinator::opt

pub fn opt<I: Clone, O, E: ParseError<I>, F>(
    f: F
) -> impl FnMut(I) -> IResult<I, Option<O>, E> where
    F: Parser<I, O, E>, 

Optional parser: Will return None if not successful.

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

fn parser(i: &str) -> IResult<&str, Option<&str>> {
  opt(alpha1)(i)
}

assert_eq!(parser("abcd;"), Ok((";", Some("abcd"))));
assert_eq!(parser("123;"), Ok(("123;", None)));