I'm wondering if there's a complete and accurate reference for all the various regular expressions in programs like sed, perl, grep, emacs, etc. Currently I just use Google, which usually brings up something that is inaccurate or talks about very basic/general regex. For instance, I'm confused as to why \w matches underscores _ in sed. This is very frustrating, and I haven't found a single resource that describes this. Thanks in advance for any help.