The problem I have with all of these regex websites and tools is that they usually only use the regex dialect of whatever language they’re written in. Or at best, a very incomplete list of dialects. Subtle differences between dialects matter a lot. Personally, I would need at least Perl, Python, GNU utils (with and without -E), BSD utils (or whatever comes with macOS, again with and without-E), and vim with the various settings for “magic”. Ideally, they would also be able to convert between all these dialects.