diff --git a/src/latexify/transformers/identifier_replacer.py b/src/latexify/transformers/identifier_replacer.py
index 430e50e..2c3320c 100644
--- a/src/latexify/transformers/identifier_replacer.py
+++ b/src/latexify/transformers/identifier_replacer.py
@@ -3,9 +3,9 @@
 from __future__ import annotations
 
 import ast
-import re
+import keyword
 import sys
-from typing import ClassVar, cast
+from typing import cast
 
 
 class IdentifierReplacer(ast.NodeTransformer):
@@ -24,8 +24,6 @@ def x(y):
             return z
     """
 
-    _IDENTIFIER_PATTERN: ClassVar[re.Pattern] = re.compile(r"^[A-Za-z_][A-Za-z0-9_]*$")
-
     def __init__(self, mapping: dict[str, str]):
         """Initializer.
 
@@ -38,9 +36,9 @@ def __init__(self, mapping: dict[str, str]):
         self._mapping = mapping
 
         for k, v in self._mapping.items():
-            if not self._IDENTIFIER_PATTERN.match(k):
+            if not str.isidentifier(k) or keyword.iskeyword(k):
                 raise ValueError(f"'{k}' is not an identifier name.")
-            if not self._IDENTIFIER_PATTERN.match(v):
+            if not str.isidentifier(v) or keyword.iskeyword(v):
                 raise ValueError(f"'{v}' is not an identifier name.")
 
     def _replace_args(self, args: list[ast.arg]) -> list[ast.arg]:
diff --git a/src/latexify/transformers/identifier_replacer_test.py b/src/latexify/transformers/identifier_replacer_test.py
index 75c16e0..def9052 100644
--- a/src/latexify/transformers/identifier_replacer_test.py
+++ b/src/latexify/transformers/identifier_replacer_test.py
@@ -15,6 +15,8 @@ def test_invalid_mapping() -> None:
         IdentifierReplacer({"123": "foo"})
     with pytest.raises(ValueError, match=r"'456' is not an identifier name."):
         IdentifierReplacer({"foo": "456"})
+    with pytest.raises(ValueError, match=r"'def' is not an identifier name."):
+        IdentifierReplacer({"foo": "def"})
 
 
 def test_name_replaced() -> None: