These: regex101.com and regexper.com !

Regexper will render your regular expression as a friggin' railroad diagram!! It's extremely handy when debugging your regex or to understand those super complex, multiline regexes, that someone has posted to Stack Overflow.

Both regular-expressions.info and rexegg.com are incredibly good explanations of Regular Expressions.

regular-expressions.info is even available as a PDF e-book against a tip of your choice.

Here is one for Ruby: https://rubular.com/

Time for a shout-out to Lea Verou's terrific fifty-minute tutorial "Regexplained" and accompanying regex playground:

https://youtu.be/EkluES9Rvak

https://regexplained.com/

And for python this one is nice: https://pythex.org/

Regex101 let's you switch mode, and has a python regex one.

pythex works ok but it's UI/UX is worse than regex101 (which as a friend state, has pythonre capabilities as well.

try regex101 and you won't look back.

Thanks for the list, but are they all the same? Why go to one over the other?

It makes me wonder, is there something about Regex that makes people want to create so many tools for it, even after so many other ones already exist?

Some support other varieties of RegEx, and some have interesting features or different interfaces

Not sure if it's completely comprehensive, but the clearest tutorial/reference of regex's that I've used is in the book "The AWK Programming Language" by Aho, Kernighan, Weinberger.

https://ia902309.us.archive.org/25/items/pdfy-MgN0H1joIoDVoI...

That is a nice introduction indeed. Afterwards, to get a taste for more complex matching, maybe the Perl / PCRE docs.

https://perldoc.perl.org/perlre.html

I especially like debuggex for debugging/teaching purposes, here's an example: https://www.debuggex.com/r/SMLRfiyt0Ag2hXu5

Thank you for posting these links. I'm lost whenever I attempt something with Regex.

Are there any online regex testers for ICU regexes?