IntelliJ IDEA has a built in Regex Tester

This idea is just too freaking awesome. Only just discovered it today,
so I don't know how long it's been around. I know it wasn't there the
last time I worked on a lot of Regex Java code.

A built-in Regex tester within the editing window of IntelliJ IDEA. So
you write the regex in your code, you can test it right there!