RegEx101.com now offers a debugger

Would be ultra-cool if you could propose a "generate a matching sample" button.

I have thought about this, but in cases where it would be useful, it's impossible to generate a sample match string. For example, creating a match string for /(?:a|[bc])efg?/ is super simple, but for something like: /(ab(?1)*)/ it becomes much harder. Not to mention the performance hit you would see for these more complex expressions. (These are just dummy expressions for illustrative purposes, but I'm sure you get my gist.)

http://rubular.com/ is also really great! I use it often when programming ruby regular expressions. Fill in a batch of test strings into the box, then run your regular expression against it, and instantly see (visually) what is happening. This is a BIG plus coming from perl regex 10 years ago ;) This is not a dig a perl regex, but I just remember it was a trial and error loop, where I would continually be iterating the script to see if it worked, where as with this webpage, you just iterate, more quickly.

They also have a great example:

  test string:
  Today's date is: 9/28/2013.

  regex:
  (?\d{1,2})\/(?\d{1,2})\/(?\d{4})

  result:
  month	9
  day	28
  year	2013

Screenshot here: http://i.imgur.com/ixyHRde.png

And a couple of alternatives:

https://www.debuggex.com/

(for Python flavor:

https://www.debuggex.com/?flavor=python )

and one more Python + regex:

http://www.pyregex.com/

http://txt2re.com/ is my favorite RegEx tool. It goes beyond just testing your regular expressions against a sample. It builds the pattern for you and generates the code in different languages.