Function nom::combinator::verify

pub fn verify<I: Clone, O1, O2, E: ParseError<I>, F, G>(
    first: F, 
    second: G
) -> impl Fn(I) -> IResult<I, O1, E> where
    F: Fn(I) -> IResult<I, O1, E>,
    G: Fn(&O2) -> bool,
    O1: Borrow<O2>,
    O2: ?Sized, 

returns the result of the child parser if it satisfies a verification function

the verification function takes as argument a reference to the output of the parser

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

let parser = verify(alpha1, |s: &str| s.len() == 4);

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