I am a huge, huge fan of https://regexper.com/ - paste pretty much any regex, and it will generate a visualization of what it does. It's been invaluable even just as a sanity check, it just makes it so easy to trace the flow of what a pattern is doing.

Back when I used Windows, I had an app called Regex Buddy and you could paste in any regex and it would generate a comment block that broke down what was going on. Was super useful when you come back a few months later and need to understand a complex regex