diff --git a/src/main/py/read_math_function.py b/src/main/py/read_math_function.py
index 7a0d803..370b39b 100644
--- a/src/main/py/read_math_function.py
+++ b/src/main/py/read_math_function.py
@@ -6,3 +6,9 @@ def read_constant(type, value):
 
 def is_string(value):
     return isinstance(value, str)
+
+
+def is_allowed_string(type, value):
+    allowed = "+-*/()^"
+    result = all(character.isdigit() or character in allowed for character in value)
+    return result
\ No newline at end of file
diff --git a/src/test/py/test_read_math_function.py b/src/test/py/test_read_math_function.py
index c30d2ad..f8e8bf2 100644
--- a/src/test/py/test_read_math_function.py
+++ b/src/test/py/test_read_math_function.py
@@ -11,6 +11,8 @@ class read_function(unittest.TestCase):
         self.assertEqual(is_string("42*e^x"), True)
     def test_constant_isstring_false(self):
         self.assertEqual(is_string(42), False)
+    def test_constant_isallowed_string_true(self):
+        self.assertEqual(is_allowed_string("c", "42^(2)+1-3*4"), True)
 
 if __name__ == '__main__':
     unittest.main()