I highly recommend man 7 regex
as a compact, concise reference. Do check out other links already mentioned, but keep the section 7 manpage open as a reference to unify concepts as you learn.
I've posted on this topic before, so I'll probably add a little more to this post later.
Insead of making a lot of copypasta in this thread I'll just post the link to the last related thread I participated in on the subject of regular expressions.
Here are the LQ tags I use for regular expressions.
, regular expression