Site: https://regex101.com/
This is a website sandbox for learning [or testing] regex expressions. You are able to search the command repository for commands that may be of help for you, and then you are able to see your regex working in real-time, when run on provided test strings. This was invaluable when I was learning how to use RegEx, and I still go to this website when I am working on complex RegEx to validate how it will work.