Sed: invalid reference \1 on `s' command's RHS

If you see something like the following error message when you run a sed command:

then it (probably) means that your regex capture group has not been escaped properly.

So, if you are using a command like:

then it needs to be escaped like so:

In other words, round parentheses/brackets need to be escaped while the square brackets do not :|

Hope this helps :)