JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.aspnetcdn.com/ajax/jquery.validate/1.17.0/jquery.validate.js"></script>
<form id="myFormId">
    <input type='text' name='myField' />
    <br />
    <input type="submit" value="Submit" />
</form>

JavaScript

$(function() {

    var myValidator = $("#myFormId").validate({
        rules: {
            myField: {
                myCustomRule: true
            }
        }
    });

    $.validator.addMethod("myCustomRule", function(value, element) {
        return this.optional(element) || value === "foo" || /^\d+$/.test(value);
    }, "myMessage");

});