Im guessing that the second backslash was deemd unecessary (and in my opinion it is - no need for additional key strokes in the name of some arbitral consistancy). Besides you are probably looking at this wrong - backslash here is equivalent of hash or dolar sign - it denotes begining of expression (just like most languages do). And they literally could not use another one because of backward compatibilty it they want to keep the "no STR"/simple versuon of interpolation possible. They will just add new escape sequence /{ - its kinda brilliant if you think about it.