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 and will fail with an re.error exception

Noncompliant code example

import re
pattern1 = re.compile(r"\1(.)") # Noncompliant, group 1 is defined after the back reference
pattern2 = re.compile(r"(.)\2") # Noncompliant, group 2 isn't defined at all
pattern3 = re.compile(r"(.)|\1") # Noncompliant, group 1 and the back reference are in different branches
pattern4 = re.compile(r"(?P<x>.)|(?P=x)") # Noncompliant, group x and the back reference are in different branches

Compliant solution

import re
pattern1 = re.compile(r"(.)\1")
pattern2 = re.compile(r"(?P<x>.)(?P=x)")