Strips any national prefix (such as 0, 1) present in a number.

    Arguments:
    number -- The normalized telephone number that we wish to strip any
              national dialing prefix from
    metadata -- The metadata for the region that we think this number
              is from.

    Returns a 3-tuple of
     - The carrier code extracted if it is present, otherwise an empty string.
     - The number with the prefix stripped.
     - Boolean indicating if a national prefix or carrier code (or both) could be extracted.

def _maybe_strip_national_prefix_carrier_code(number, metadata):
    """Strips any national prefix (such as 0, 1) present in a number.

    Arguments:
    number -- The normalized telephone number that we wish to strip any
              national dialing prefix from
    metadata -- The metadata for the region that we think this number
              is from.

    Returns a 3-tuple of
     - The carrier code extracted if it is present, otherwise an empty string.
     - The number with the prefix stripped.
     - Boolean indicating if a national prefix or carrier code (or both) could be extracted.
     """
    carrier_code = U_EMPTY_STRING
    possible_national_prefix = metadata.national_prefix_for_parsing
    if (len(number) == 0 or
        possible_national_prefix is None or
        len(possible_national_prefix) == 0):
        # Early return for numbers of zero length.
        return (U_EMPTY_STRING, number, False)

    # Attempt to parse the first digits as a national prefix.
    prefix_pattern = re.compile(possible_national_prefix)
    prefix_match = prefix_pattern.match(number)
    if prefix_match:
        general_desc = metadata.general_desc
        # Check if the original number is viable.
        is_viable_original_number = _match_national_number(number, general_desc, False)
        # prefix_match.groups() == () implies nothing was captured by the
        # capturing groups in possible_national_prefix; therefore, no
        # transformation is necessary, and we just remove the national prefix.
        num_groups = len(prefix_match.groups())
        transform_rule = metadata.national_prefix_transform_rule
        if (transform_rule is None or
            len(transform_rule) == 0 or
            prefix_match.groups()[num_groups - 1] is None):
            # If the original number was viable, and the resultant number is not, we return.
            # Check that the resultant number is viable. If not, return.
            national_number_match = _match_national_number(number[prefix_match.end():], general_desc, False)
            if (is_viable_original_number and not national_number_match):
                return (U_EMPTY_STRING, number, False)

            if (num_groups > 0 and
                prefix_match.groups(num_groups) is not None):
                carrier_code = prefix_match.group(1)
            return (carrier_code, number[prefix_match.end():], True)
        else:
            # Check that the resultant number is still viable. If not,
            # return. Check this by copying the number and making the
            # transformation on the copy first.
            transformed_number = re.sub(prefix_pattern, transform_rule, number, count=1)
            national_number_match = _match_national_number(transformed_number, general_desc, False)
            if (is_viable_original_number and not national_number_match):
                return ("", number, False)
            if num_groups > 1:
                carrier_code = prefix_match.group(1)
            return (carrier_code, transformed_number, True)
    else:
        return (carrier_code, number, False)