Preserve extensions when replacing reserved filenames

pathvalidate/_filename.py

@@ -86,7 +86,7 @@ def sanitize(self, value: PathType, replacement_text: str = "") -> PathType:
                 replacement_word = self._reserved_name_handler(e)
                 if e.reserved_name != replacement_word:
                     sanitized_filename = re.sub(
-                        re.escape(e.reserved_name), replacement_word, sanitized_filename
+                        re.escape(e.reserved_name), replacement_word, sanitized_filename, count=1
                     )
             elif e.reason == ErrorReason.INVALID_CHARACTER and self._is_windows(
                 include_universal=True

test/test_reserved_name_extensions.py

@@ -0,0 +1,34 @@
+import pytest
+
+from pathvalidate import is_valid_filename, sanitize_filename, sanitize_filepath
+from pathvalidate.handler import ReservedNameHandler
+
+
+@pytest.mark.parametrize("platform", ["windows", "universal"])
+@pytest.mark.parametrize("filename", ["con.icon", "CON.CON.txt", "aux.aux", "COM1.COM1"])
+@pytest.mark.parametrize(
+    ["handler", "prefix", "suffix"],
+    [
+        [ReservedNameHandler.add_leading_underscore, "_", ""],
+        [ReservedNameHandler.add_trailing_underscore, "", "_"],
+    ],
+)
+def test_reserved_name_handler_preserves_extension(filename, platform, handler, prefix, suffix):
+    stem, extension = filename.split(".", 1)
+    expected = f"{prefix}{stem}{suffix}.{extension}"
+
+    result = sanitize_filename(filename, platform=platform, reserved_name_handler=handler)
+
+    assert result == expected
+    assert is_valid_filename(result, platform=platform)
+
+
+def test_additional_reserved_name_preserves_extension():
+    assert (
+        sanitize_filename("foo.foo", platform="linux", additional_reserved_names=["foo"])
+        == "foo_.foo"
+    )
+
+
+def test_filepath_preserves_extension():
+    assert sanitize_filepath("folder/con.icon", platform="windows") == r"folder\con_.icon"