This is most certainly due to the Python 2 to Python 3 switch that happened with Sage version 9.0. Invalid escape sequence (such as "\S") will be treated as error in the future. One could either use two backslashes as in "\\S" (the string "\\" is a single character, the backslash symbol) or use the r as you did (in which case all backslashes will be consider as character, even the linebreak "\n").
This deprecation warning is most certainly due to the Python 2 to Python 3 switch that happened with Sage version 9.0. Invalid escape sequence (such as "\S") will be treated as error in the future. One could either use two backslashes as in "\\S" (the string "\\" is a single character, the backslash symbol) or use the r as you did (in which case all backslashes will be consider as character, even the linebreak "\n").