The web has numerous tutorials about regex, so here is not yet another one.
It's about a tool to design and test your perl regex for PHP or java programming for example.
You dont need to download or install anything, it is online, here: browseAs.com/regex
You type your regex, your optionnal replacement string and the source text and it will perform the test for you. If you want to rework you regex, it's simple, just reedit it, everything is already in the form...
Very easy, very useful.