Code shared from the Rust Playground

playground.rs

fn main() {
    let formula = String::from("K4(ON(Mg2O3)2(Rg)3)2");
    let formula_vec: Vec<char> = formula.chars().collect();

    let result = formula
        .chars()
        .enumerate()
        .fold(String::new(), |mut acc, (i, c)| {
            if i + 1 < formula_vec.len() && (!formula_vec[i + 1].is_ascii_digit() || !formula_vec[i + 1].is_ascii_lowercase()) {
                acc = format!("{}{}{}", acc, c, "1");
            } else {
                acc = format!("{}{}", acc, c);
            }
            acc
        });
    println!("{:?}", result);
}