Well the answer for your question: "Is the separator always converted into a regular expression?" is:
It depends solely on the implementation. For example if you look at WebKit implementation http://svn.webkit.org/repository/webkit/trunk/Source/JavaScriptCore/runtime/StringPrototype.cpp (find stringProtoFuncSplit) then you see it is not always converted to RegEx. However, this does not imply anything, it is just a matter of implementation