Panic in regex_syntax, found by fuzzing

[PaulGrandperrin]

While fuzzing the regex_syntax target from https://github.com/rust-fuzz/targets, I hit this panic:

thread 'main' panicked at 'tried to unwrap expr from HirFrame, got: Group { old_flags: None }', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/regex-syntax-0.6.2/src/hir/translate.rs:203:18

Here is the regex:

regex::Regex::new("(((?x)))");

playground

[BurntSushi]

Nice! It looks like ((?x)) also panics. (?:(?x)) does as well. As does ((?x))x.