sed and Mac OS X differences with to upper, to lower and whole capture control sequences

I am trying to take the last two letters out of a filename which are uppercase and append them to the filename in lowercase. I expeceted the command: ``` ls | sed -e "s/.*\([A-Z][A-Z]\)$/\0\/\L\1\E/" ``` to achieve this and on my Ubuntu box it worked fine but on my Mac it simply prints out a `0/LXXE/` where `XX` are the correct letters from the capture. What are the Mac sed equivalents of `\0`, `\L` and `\E`? I've had a look around the web and several people have noticed that Mac OS X sed is different from Ubuntu sed but most threads talk about the `-i` requirement for a file extension or empty string (which has previously tripped me up).