fn regexp_last_match(vm: &mut VM, _: Value, args: &Args2) -> VMResult {
    args.check_args_num(1)?;
    let nth = vm[0].coerce_to_fixnum("1st arg")?;
    if nth == 0 {
        return Ok(vm.get_special_var(0));
    }
    if nth < 0 {
        return Err(RubyError::argument("1st arg must not be sub zero."));
    };
    let str = vm.get_special_matches(nth as usize);
    Ok(str)
}
(not supported) Regexp.last_match -> MatchData
Regexp.last_match(nth) -> String | nil
https://docs.ruby-lang.org/ja/latest/method/Regexp/s/last_match.html
ruruby/src/builtin/regexp.rs