Search Google for "re documentation" and look at some of the results.

Google is effectively aliasing RE to regular expression. None of the links use "RE" as an acronym. Regex/Regexp has always been the accepted acronym.

They do though. RE is a python library, an Erlang library, there's a reference to perlre, clojure calls it re, etc. Then there's the C lib pcre.

Also Google chooses to alias certain things for a reason.