diff --git a/jsonschema/_utils.py b/jsonschema/_utils.py
index 3177caea..b880e8f6 100644
--- a/jsonschema/_utils.py
+++ b/jsonschema/_utils.py
@@ -82,12 +82,13 @@ def find_additional_properties(instance, schema):
     Assumes ``instance`` is dict-like already.
     """
     properties = schema.get("properties", {})
-    patterns = "|".join(schema.get("patternProperties", {}))
+    patterns = schema.get("patternProperties", {})
     for property in instance:
-        if property not in properties:
-            if patterns and re.search(patterns, property):
-                continue
-            yield property
+        if property in properties:
+            continue
+        if any(re.search(pattern, property) for pattern in patterns):
+            continue
+        yield property
 
 
 def extras_msg(extras):
diff --git a/jsonschema/tests/test_utils.py b/jsonschema/tests/test_utils.py
index f716a36a..387c4dbd 100644
--- a/jsonschema/tests/test_utils.py
+++ b/jsonschema/tests/test_utils.py
@@ -2,7 +2,7 @@
 from math import nan
 from unittest import TestCase
 
-from jsonschema._utils import equal, uniq
+from jsonschema._utils import equal, find_additional_properties, uniq
 
 
 class Unhashable:
@@ -223,3 +223,21 @@ def test_unhashable_inside_mapping_falls_back(self):
         self.assertTrue(
             uniq([{"k": Unhashable(1)}, {"k": Unhashable(2)}]),
         )
+
+
+class TestFindAdditionalProperties(TestCase):
+    def test_empty_pattern_matches_all_properties(self):
+        # An empty-string pattern is a valid regex that matches every property,
+        # so patternProperties covers everything and nothing is additional.
+        schema = {"patternProperties": {"": {"type": "integer"}}}
+        instance = {"foo": 1, "bar": 2}
+        self.assertEqual(
+            list(find_additional_properties(instance, schema)), [],
+        )
+
+    def test_no_pattern_properties_leaves_all_additional(self):
+        schema = {"properties": {"foo": {}}}
+        instance = {"foo": 1, "bar": 2}
+        self.assertEqual(
+            list(find_additional_properties(instance, schema)), ["bar"],
+        )