import re


def mmyy_valid_date(date_str):
    """
    Check if a date string is a valid date. Expected format is mmyy.
    :param date_str: a date string
    :return: date_str or None
    """
    check = re.match("^(0[1-9]|1[0-2])\/?([0-9]{2})$", date_str)
    if check:
        return date_str
    else:
        return None