The tutorial I learned regular expressions from, which is longer but more detailed than this one, was http://www.regular-expressions.info/. It’s free, thorough and well-organized. Its only flaw is that its section on support in various languages is out of date. It was written to sell a Windows-only regex tool, but it’s very non-pushy with the advertisements.
You can see a list of online regular expression testers for various languages at the Stack Overflow “regex” tag wiki, in the “Online sandboxes” section: http://stackoverflow.com/tags/regex/info. For JavaScript regexes, http://regexpal.com/ is easy to use.
regular-expressions.info has been a good reference site for me. Another useful tool is an online regex checker. Out of a handful or so options, I find this the best : http://regex101.com/?
You can see a list of online regular expression testers for various languages at the Stack Overflow “regex” tag wiki, in the “Online sandboxes” section: http://stackoverflow.com/tags/regex/info. For JavaScript regexes, http://regexpal.com/ is easy to use.