def validate_business(form, field):
    """Valiates a PayPal business string.

    It can either be an email address or a paypal business account ID.
    """
    if not is_valid_mail(field.data, multi=False) and not re.match(r'^[a-zA-Z0-9]{13}$', field.data):
        raise ValidationError(_('Invalid email address / paypal ID'))