For my current project I needed a custom validator method for the jQuery validator plug-in. I wanted to validate that a user has chosen a value from a combo box where the value is of type Guid.
  First I implemented a JavaScript function which tests whether a given value represent a valid Guid but not an empty Guid (An empty Guid has the following form: 00000000-0000-0000-0000-000000000000). To achieve this result I use regular expressions
          var isValidGuid = function(value) {
      var validGuid = /^(\{|\()?[0-9a-fA-F]{8}-([0-9a-fA-F]{4}-){3}[0-9a-fA-F]{12}(\}|\))?$/;
      var emptyGuid = /^(\{|\()?0{8}-(0{4}-){3}0{12}(\}|\))?$/;
      return validGuid.test(value) && !emptyGuid.test(value);
    }
   
 
The above function returns true for a valid Guid and false for every thing else.
Now I have to add this validation method to the validator plug-in as follows
  
    $.validator.addMethod("isValidGuid", function(value, element) {
      return isValidGuid(value);
    }, 'Please select a valid and non empty Guid value.');
   
 
Finally I can use this custom validation method on any form element by assigning it the class isValidGuid, e.g.
  
    <select id="title" class="isValidGuid" title="Please select a title!">
      <option value="00000000-0000-0000-0000-000000000000" selected="selected">(select)</option>
      <option value="33a1eb15-cdbc-4c85-be01-dcb4f393c0a5">Engineer</option>
      <option value="43b5d0f7-4915-41f1-b3b9-d6335299cc22">Physicist</option>
      <option value="d80322f2-bb76-447c-a6ac-77f145bac70d">Technician</option>
    </select>          
   
 
when applied we have the following outcome
