## 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 `PatternSyntaxException` 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

    Pattern.compile("\\1(.)"); // Noncompliant, group 1 is defined after the back reference
    Pattern.compile("(.)\\2"); // Noncompliant, group 2 isn't defined at all
    Pattern.compile("(.)|\\1"); // Noncompliant, group 1 and the back reference are in different branches
    Pattern.compile("(?<x>.)|\\k<x>"); // Noncompliant, group x and the back reference are in different branches

### Compliant solution

    Pattern.compile("(.)\\1");
    Pattern.compile("(?<x>.)\\k<x>");