Using parser combinator library "nom", this should probably do what you'd want:

    fn parse_abc(input: &str, n: usize) -> IResult<&str, (Vec<char>, Vec<char>, Vec<char>)> {
      let (input, result) = tuple(( many_m_n(n, n, char('a')),
                                  many_m_n(n, n, char('b')),
                                  many_m_n(n, n, char('c'))
                            ))(input)?;
      Ok((input, result)) 
    }
    

It parses (the beginning of) the input, ensuring `n` repetitions of 'a', 'b', and 'c'. Parse errors are reported through the return type, and the remaining characters are returned for the application to deal with as it sees fit.

https://play.rust-lang.org/?version=stable&mode=debug&editio...