When a back reference in a regex refers to a capturing group that hasn’t been defined yet (or at all), it can never be matched. Named back
references throw a warning in that case; numeric back references fail silently when they can’t match, simply making the match fail.
When the group is defined before the back reference but on a different control path (like in (.)|\1 for example), this also leads to a
situation where the back reference can never match.
Noncompliant code example
preg_match("/\\1(.)/", $str); // Noncompliant, group 1 is defined after the back reference
preg_match("/(.)\\2/", $str); // Noncompliant, group 2 isn't defined at all
preg_match("/(.)|\\1/", $str); // Noncompliant, group 1 and the back reference are in different branches
preg_match("/(?<x>.)|\k<x>/", $str); // Noncompliant, group x and the back reference are in different branches
Compliant solution
preg_match("/(.)\\1/", $str);
preg_match("/(?<x>.)\\k<x>/", $str);
