JQuery :: Validation Plugin - Optional Field For User Entry

In a registration form middleName field is optional. When the users enters his/her middleName, then it should validate that field. I have used the following code to achieve the above scenario, but it is not working.

$("#middleName").rules("add", {checkName: true, required: false, messages: {checkName: "Please enter a valid middle name"} });
jQuery.validator.addMethod("checkName",
function(value, element) {
var regExp = new RegExp(/^[a-zA-Z0-9s|,|.|-|']+$/);
return regExp.test(value);
},
"Please enter a valid name."
);
How to achieve the above scenario.