        +44 1234 45665
        +49 (1234) 44444
        +44 12344 55538";

        let re = Regex::new(r"(\+[\d]{1,4})").unwrap();

        let prefixes = re
            .captures_iter(&phone_numbers)
            .map(|match_| match_.get(1))
            .filter(|m| m.is_some())
            .fold(RefCell::new(counter), |c, prefix| {