<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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'))