How do I validate a zip code in a form?

Hi

I need to validate the length and only digits in a zip code field.

As far as I can see it’s only possible to validate email and phone as default in Unbounce?

<script type="text/javascript"\>  
var d = window.module.lp.form.data;  
/\* Change the main error message title \*/  
jQuery(document).ready(function() {  
jQuery('#'+d.errorContainerId).find('h3').text(  
"Please correct the following errors:"  
);  
});  
/\* Change the individual error messages \*/  
d.validationMessages = {  
"customer_full_name": {  
required: "Type your name"
},  
"customer_full_address": {  
required: "Type your address"
},  
"customer_email": {  
required: "Type your email",  
email: "Please type a valid email"
},
"customer_zipcode": {  
required: "Type your zip code",
},
"customer_phone": {  
required: "Type your phone number",
phone: "Type a valid phone number"
},
};  
</script>

Found a solution:

<script>
lp.jQuery(function($) {
  var ruleID = 'zipcode';
  var field = 'customer_zipcode';
  var message = 'Indtast kun postnummeret';
  var rules = module.lp.form.data.validationRules[field];
  $.validator.addMethod(ruleID, function(value, field) {
        var valid = /^(?:[0-9]{4})?$/.test(value);
        return valid || (!rules.required && !value);
        }, message);
rules[ruleID] = true;
});
</script>