diff --git a/casbin/core_enforcer.py b/casbin/core_enforcer.py
index b0dff85..2b79c48 100644
--- a/casbin/core_enforcer.py
+++ b/casbin/core_enforcer.py
@@ -539,6 +539,6 @@ def configure_logging(logging_config=None):
     def _get_expression(expr, functions=None):
         expr = expr.replace("&&", "and")
         expr = expr.replace("||", "or")
-        expr = expr.replace("!", "not")
+        # expr = expr.replace("!", "not")
 
         return SimpleEval(expr, functions)