import re


def is_valid_mac_address(mac):
    """Validates the format of a MAC address.

    Args:
        mac (str): the MAC address to validate

    Returns:
        bool: True if valid, False otherwise

    Examples:
        >>> valid_mac('invalid_mac')
        False

        >>> valid_mac('28:cf:e9:18:ca:01')
        True
    """
    regex = "[0-9a-f]{2}([-:])[0-9a-f]{2}(\\1[0-9a-f]{2}){4}$"

    return re.match(regex, mac.lower())