diff --git a/re_assert.py b/re_assert.py
index 840401e..8818da1 100644
--- a/re_assert.py
+++ b/re_assert.py
@@ -1,8 +1,13 @@
+import sys
+
 from typing import Any
 from typing import Optional
 from typing import Pattern
 
-import regex
+if sys.implementation.name == "cpython":
+    import regex
+else:
+    import re as regex
 
 
 class Matches:  # TODO: Generic[AnyStr] (binary pattern support)
@@ -12,6 +17,9 @@ class Matches:  # TODO: Generic[AnyStr] (binary pattern support)
         self._type = type(pattern)
 
     def _fail_message(self, fail: str) -> str:
+        if sys.implementation.name != "cpython":
+            return "regex failed to match"
+
         # binary search to find the longest substring match
         pos, bound = 0, len(fail)
         while pos < bound:
diff --git a/setup.cfg b/setup.cfg
index 46303ca..74cf999 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -20,7 +20,7 @@ classifiers =
 [options]
 py_modules = re_assert
 install_requires =
-    regex
+    regex; python_implementation=="CPython"
 python_requires = >=3.6.1
 
 [bdist_wheel]