How do I validate one field's length based on another field's selection?

Question

I am creating a form for our offices where I need to check one field for length of a serial number based on the answer to a previous field's selection.  Depending on the product selection, the serial number has two different lengths.  Both fields are marked as required, but I want to make sure that we receive the full serial number for our product.  Previous forms were done on paper and instructions were for the last 4 digits only which now we want the full serial number.  One product has a 12 digit number and the other is 6.
I'm guessing that I need a validation script in the serial number field, but I am not a JavaScript coder and haven't found the right thing when searching.  Any help would be great!
Thanks!

Answer 1

Never trust your users to follow what you expect them to do... That often leads to mistakes and problems later on.
I would reset the s/n field when a selection is made in the drop-down. To do so set the drop-down field to commit the selected value immediately and add code as its custom validation script:
this.getField("Serial Number").value = "";
To validate the s/n field you can use something like this as that field's custom validation script (of course, you'll need to adjust the field names and values to match the actual ones in your file):
event.rc = true;
if (event.value) {
     var selection = this.getField("Dropdown1").valueAsString;
     if (selection=="1" && event.value.length!=6) {
          app.alert("Error! The s/n must be 6 digits long.");
          event.rc = false;
     if (selection=="2" && event.value.length!=12) {
          app.alert("Error! The s/n must be 12 digits long.");
          event.rc = false;