camel_to_snake.py

import re

def camel_to_snake(input_string):
    """
    Convert camelCase or PascalCase strings to snake_case.
    
    Args:
        input_string (str): The input string in camelCase or PascalCase
        
    Returns:
        str: The converted string in snake_case
    """
    # First regex: Insert underscore between lowercase and uppercase letters
    # Handles cases like 'camelCase' -> 'camel_Case'
    lowercase_to_uppercase = re.sub(r'([a-z])([A-Z])', r'\1_\2', input_string)
    
    # Second regex: Insert underscore between uppercase letter and uppercase+lowercase
    # Handles cases like 'XMLHttpRequest' -> 'XML_Http_Request'
    acronym_boundaries = re.sub(r'([A-Z])([A-Z][a-z])', r'\1_\2', lowercase_to_uppercase)
    
    # Convert to lowercase
    return acronym_boundaries.lower()