Reproducing rust-lang/regex#779

Reproduction script (Rust)

// src/main.rs (excerpt — see this directory for the full crate)
let haystack = "a\naaa\n";
let re_plus     = Regex::new("(?m)(^|a)+").unwrap();
let re_expanded = Regex::new("(?m)(^|a)(^|a)*").unwrap();

let matches_plus     = re_plus.find_iter(haystack)
    .map(|m| (m.start(), m.end())).collect::<Vec<_>>();
let matches_expanded = re_expanded.find_iter(haystack)
    .map(|m| (m.start(), m.end())).collect::<Vec<_>>();

let reproduced = matches_plus != matches_expanded;