Check if given value is a valid URL string.

    :param value: a value to test
    :returns: True if the value is valid

def is_valid_url(value):
    """Check if given value is a valid URL string.

    :param value: a value to test
    :returns: True if the value is valid
    """
    match = URL_REGEX.match(value)
    host_str = urlparse(value).hostname
    return match and is_valid_host(host_str)