The String.substring(int) method should permit negative indeces. myString.substring(-3) should return the same as myString.substring(myString.length()-3) i.e. the last three characters.
It's only somewhat annoying with named Strings to have to write the name twice, but with unnamed calculated Strings it's really bothersome sometimes. One has to assign the String to a temporary variable just to be able to get the last n characters.
The same applies to String.substring(int,int).