In the Forms Business Process Library, there is a 

• CSS, JavaScript, and Formula Examples

Inside here there is validation on an email field using Javascript, that if the two fields do not match then remove the Submit Button, you may be able to adapt this code to help you.

$(document).ready(function () {

  $('.Submit').hide(); 

  

  $('.email, .confirm').on('blur input', function () {



        if ($('.email input').val() != $('.confirm input').val()) {

           $('.Submit').hide();

            if ($('.warningText').length > 0 || $('.confirm input').val() == '') { return; }

            else {

                $('<p class="warningText">The email addresses you entered do not match.</p>').insertAfter('.confirm');

            }

        }

        else

            $('.warningText').remove();

        $('.Submit').show();



    });

});