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>");
