Sites like THIS are really helpful. they let you see what you are currently matching with your expression, give you a little cheat sheet to reference, etc.
And nothing beats good old fashion practice
As others above have mentioned, there are many sites to help you.
These three come to mind:
regexone as already mentioned. Great hands on tutorial!
regex101 is a nice sandbox to play around with regular expressions.
regexr this one is my favourite but somehow doesn’t seem to work in some browsers.
Of course MDN offers an excellent guide for how regular expressions work.