## Why is this an issue?

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);