diff --git a/Lib/lib2to3/tests/test_main.py b/Lib/lib2to3/tests/test_main.py
index 225c15de188..86cb438e358 100644
--- a/Lib/lib2to3/tests/test_main.py
+++ b/Lib/lib2to3/tests/test_main.py
@@ -94,10 +94,11 @@ class TestMain(unittest.TestCase):
             self.assertIn("Writing converted %s to %s" % (
                     os.path.join(self.py2_src_dir, name),
                     os.path.join(self.py3_dest_dir, name+suffix)), stderr)
+        sep = re.escape(os.sep)
         self.assertRegex(
-                stderr, r"No changes to .*/__init__\.py".replace("/", os.sep))
+                stderr, r"No changes to .*/__init__\.py".replace("/", sep))
         self.assertNotRegex(
-                stderr, r"No changes to .*/trivial\.py".replace("/", os.sep))
+                stderr, r"No changes to .*/trivial\.py".replace("/", sep))
 
     def test_filename_changing_on_output_two_files(self):
         """2to3 two files in one directory with a new output dir."""