error: sed: -e expression #1, char 81: invalid reference \1 on `s' command's RHS

[StarrKiss]

When I run this command in the setup procedure:

sudo sed -i 's/^(#?greeter)-session\s*=\s*(.*)/greeter-session = lightdm-webkit2-greeter #\1/ #\2g' /etc/lightdm/lightdm.conf

To get lightdm set up, it gives me this error:

sed: -e expression #1, char 81: invalid reference \1 on `s' command's RHS

[NoiSek]

For anyone who runs into this in the future, all this command does is set greeter-session = lightdm-webkit2-greeter in /etc/lightdm/lightdm.conf. You can do this manually yourself to achieve the same endgoal.