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.
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.