Xah Lee has a nice summary of Emacs regular expressions and how they differ from the other familiar varieties. I may have linked to an older version of this a while ago but it’s a worthwhile link for anyone who hasn’t seen it.
If you regularly use Emacs, Perl, and Unix regexs, this is a good resource to help you keep the Emacs differences in mind. You can get much the same information from the Emacs documentation, of course, but it’s handy to have a nice summary like this and you may want to bookmark it.