Rust's regex crate has fuzzing running against it, and that has lead to a lot of useful small fixes: https://github.com/rust-lang/regex/pull/262

pcre has been fuzzed by quite a few people a while ago, I remember also having reported a couple of issues. When I tested re2 nothing showed up. oniguruma also had quite a few issues, I tested it with libfuzzer lately. But everything should be fixed now.