Hi @BurntSushi ,
Sorry to bother you :-) another one!

regex::Regex::new("(?i)(?i)*?");

thread 'main' panicked at 'tried to unwrap expr from HirFrame, got: Concat', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/regex-syntax-0.5.3/src/hir/translate.rs:203:18

playground

@robertswiecki : honggfuzz is really good! libFuzzer has been unable to find those even though I'm sure many people ran it for many hours/days on this code.