diff --git a/casbin/core_enforcer.py b/casbin/core_enforcer.py
index b0dff852..21e97646 100644
--- a/casbin/core_enforcer.py
+++ b/casbin/core_enforcer.py
@@ -14,6 +14,7 @@
 
 import copy
 import logging
+import re
 
 from casbin.effect import Effector, get_effector, effect_to_bool
 from casbin.model import Model, FunctionMap
@@ -539,6 +540,6 @@ 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 = re.sub(r"!(?!=)", "not ", expr)
 
         return SimpleEval(expr, functions)