== 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

[source,python]
----
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

[source,python]
----
import re
pattern1 = re.compile(r"(.)\1")
pattern2 = re.compile(r"(?P<x>.)(?P=x)")
----

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

include::../message.adoc[]

include::../highlighting.adoc[]

'''
== Comments And Links
(visible only on this page)

include::../comments-and-links.adoc[]

endif::env-github,rspecator-view[]