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:
(?<month>\d{1,2})\/(?<day>\d{1,2})\/(?<year>\d{4})
result:
month 9
day 28
year 2013
Looking at regex101, the layout of the page looks awfully similar. It seems that the author took Rubular and made it more generic. So yeah, props to Rubular.
They also have a great example:
Screenshot here: http://i.imgur.com/ixyHRde.png