This has to be one of the best online regex tools. If you even rarely work with regular expressions, you have to at least have a look at this. The visualizer and the debugger are amazing.
If you're serious about regex, you can go ahead and create an account for extra features like being able to save more than 2 unit tests, public expressions, unit testing and more.
Here is a quick tutorial from their team; it's a bit outdated, but it just differs visually, the functionality is basically the same. You can easily make an idea of how it works by watching it.