Fix to_snake conversion

Fixes a case for `to_snake` conversion for sequence of uppercase letters followed by a lowercase letter

pydantic/alias_generators.py

@@ -39,6 +39,12 @@ def to_snake(camel: str) -> str:
     Returns:
         The converted string in snake_case.
     """
-    snake = re.sub(r'([a-zA-Z])([0-9])', lambda m: f'{m.group(1)}_{m.group(2)}', camel)
-    snake = re.sub(r'([a-z0-9])([A-Z])', lambda m: f'{m.group(1)}_{m.group(2)}', snake)
+    # Handle the sequence of uppercase letters followed by a lowercase letter
+    snake = re.sub(r'([A-Z]+)([A-Z][a-z])', lambda m: f'{m.group(1)}_{m.group(2)}', camel)
+    # Insert an underscore between a lowercase letter and an uppercase letter
+    snake = re.sub(r'([a-z])([A-Z])', lambda m: f'{m.group(1)}_{m.group(2)}', snake)
+    # Insert an underscore between a digit and an uppercase letter
+    snake = re.sub(r'([0-9])([A-Z])', lambda m: f'{m.group(1)}_{m.group(2)}', snake)
+    # Insert an underscore between a lowercase letter and a digit
+    snake = re.sub(r'([a-z])([0-9])', lambda m: f'{m.group(1)}_{m.group(2)}', snake)
     return snake.lower()

tests/test_utils.py

@@ -529,6 +529,7 @@ def test_snake2camel(value: str, result: str) -> None:
         ('Camel2Snake', 'camel_2_snake'),
         ('_CamelToSnake', '_camel_to_snake'),
         ('CamelToSnake_', 'camel_to_snake_'),
+        ('CAMELToSnake', 'camel_to_snake'),
         ('__CamelToSnake__', '__camel_to_snake__'),
         ('Camel2', 'camel_2'),
         ('Camel2_', 'camel_2_'),