import re


def validate_mac_addr(macstring):
    """Ensure that string is a valid MAC address in standard format."""
    regexstr = "[0-9a-f]{2}:[0-9a-f]{2}:[0-9a-f]{2}:[0-9a-f]{2}:[0-9a-f]{2}:[0-9a-f]{2}$"
    return re.match(regexstr, macstring.lower())