It explains what each character does just by hovering over a regex. Best tool to learn or to fine tune your regular expression (with testing included).
Love RegexBuddy. That feature along with the "use" tab which generates code for whatever language you select. I don't have to remember the specifics in all the languages I work in. I just select from the dropdowns, for example, "JavaScript (Chrome)" then "Use regex object to get the part of a string matched by a numbered group". Replace the placeholder variable names, and you're good to go!
It will also do things like warn you if you use named groups if your selected language doesn't support them, and the "Use" dropdown won't provide that option.
Hah wow... I've had this app for over a decade and never noticed that feature... right next to the Debug button which I have used numerous times on really gnarly regexes
https://www.debuggex.com/ is a neat one for showing
a syntax chart (or railroad chart) visualization.
I generally use https://regex101.com for its display of matched groups (when I'm dealing with complex groups and/or replacement backreferences), and http://regexstorm.net/tester when I specifically need to check a regex that will be running in .NET or Powershell.
For advanced users (and those who want to become regex gurus), the most helpful regex site for me is http://www.rexegg.com/. Also the O'Reilly book "Mastering Regular Expressions" is probably worth gold.
It explains what each character does just by hovering over a regex. Best tool to learn or to fine tune your regular expression (with testing included).