Regular expressions are so powerful. Their syntax can also be quire esoteric. While I consider myself fairly proficient with simple to medium complex expressions, I don’t create them often enough to keep my skills sharp. As such, when I need to figure out what a regex does or need to create one from scratch, there is usually a bit of trial and error as that hard found knowledge works it way through the cobwebs of my mind.
A co-worker pointed me to regexr.com. This site allows you to paste in a regular expression and will show you how it breaks down. Highly recommended.