slamshuffle/src/mfvi.rs

struct Translation<'a> {
    find: &'a str,
    replacement: &'a str,
}

pub fn replace_chars(input: &str) -> String {
    let trs: Vec<Translation<'_>> = input
        .lines()
        .filter_map(|line| {
            line.strip_prefix("#REPLACE")
                .and_then(|l| l.trim().rsplit_once(' '))
                .and_then(|(first, second)| {
                    let length = std::cmp::min(first.len(), second.len());
                    Some(Translation {
                        find: &first[0..length],
                        replacement: &second[0..length],
                    })
                })
        })
        .collect();

    let mut new = input.to_string();
    for tr in trs {
        new = new.replace(tr.find, tr.replacement);
    }

    new
}