Corrected regex serialization for OpenAPI. (#7389)

* replace \Z by \z in regex

* fix test cases for Django >= 3.0

* fix isort

* Added comment for why `\z`.

Co-authored-by: Carlton Gibson <carlton@noumenal.es>

rest_framework/schemas/openapi.py

@@ -554,7 +554,9 @@ class AutoSchema(ViewInspector):
             if isinstance(v, URLValidator):
                 schema['format'] = 'uri'
             if isinstance(v, RegexValidator):
-                schema['pattern'] = v.regex.pattern
+                # In Python, the token \Z does what \z does in other engines.
+                # https://stackoverflow.com/questions/53283160
+                schema['pattern'] = v.regex.pattern.replace('\\Z', '\\z')
             elif isinstance(v, MaxLengthValidator):
                 attr_name = 'maxLength'
                 if isinstance(field, serializers.ListField):

tests/schemas/test_openapi.py

@@ -855,6 +855,7 @@ class TestOperationIntrospection(TestCase):
         assert properties['url']['type'] == 'string'
         assert properties['url']['nullable'] is True
         assert properties['url']['default'] == 'http://www.example.com'
+        assert '\\Z' not in properties['url']['pattern']
 
         assert properties['uuid']['type'] == 'string'
         assert properties['uuid']['format'] == 'uuid'