[maicro]

I can highly recommend regexr.com for testing and developing regex patterns; paste a representative sample in the "Text" area and it'll show you how things match (and usually, crucially, don't match) as you change the regex pattern.

I'm not associated with them, and I understand you don't seem to have any real issues with regex, but thought this would be a good place to mention a useful tool.

[layer8]

Using such testing you can only prove a regex isn’t what you want for specific inputs, and make a number of plausibility checks, but you can’t prove it is what you want for all inputs. For that you need to do the reasoning on the expression as if you had built it yourself.

[mgkimsal]

I've used that and regexbuddy and others over the years. Almost anything the gives you some visual rendering of what's going on is helpful. Personally, I've taken to using the regex stuff in the Jetbrains IDEs, mostly because I'm there already most of the time, and it's 'good enough'. But I'm not always at my own setup, and regexr and similar are always a good tool to have (and to share with others to show them how a regex is working).

If I did this all the time, I might not need tools like this. But complex regex are something I only dive in to a handful of times per year, and it's never the same problem twice.

[lazide]

Nice! I’ve been writing (and reading) regexes for over 20 years now. I can usually ‘read’ them on sight, but I’ve been wanting this tool for when it all goes wrong.

Which of course always happens at least once or twice in any new dataset/usage.

[Rediscover]

Thanks for pointing out regexr.com.

I really appreciate the availability of tools like that.

See also the Emacs built-in M-x re-builder (for elisp-style regexp).