RSPEC PR: https://github.com/SonarSource/rspec/pull/654

The rule should raise whenever a reference to a nonexistent group is made in the $replacement argument passed to preg_replace().

To note:
The backreferences can be of the form \n, $n, or ${n}. Where n is the group we are referring to.
Named backreferences do not seem possible. However, named groups can still be referenced to by their number.