CamelCase to lowercase_with_underscored

import re


def camelcase_to_lowercase_with_underscores(name):
    """
    >>> camelcase_to_lowercase_with_underscores('CamelCase')
    'camel_case'
    >>> camelcase_to_lowercase_with_underscores('CamelCamelCase')
    'camel_camel_case'
    >>> camelcase_to_lowercase_with_underscores('Camel2Camel2Case')
    'camel2_camel2_case'
    >>> camelcase_to_lowercase_with_underscores('getHTTPResponseCode')
    'get_http_response_code'
    >>> camelcase_to_lowercase_with_underscores('get2HTTPResponseCode')
    'get2_http_response_code'
    >>> camelcase_to_lowercase_with_underscores('HTTPResponseCode')
    'http_response_code'
    >>> camelcase_to_lowercase_with_underscores('HTTPResponseCodeXYZ')
    'http_response_code_xyz'
    """

    s1 = re.sub('(.)([A-Z][a-z]+)', r'\1_\2', name)
    return re.sub('([a-z0-9])([A-Z])', r'\1_\2', s1).lower()