JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.aspnetcdn.com/ajax/jquery.validate/1.9/jquery.validate.min.js"></script>
<form name="voucherform" action="something" method="post">
  <input type="text" name="reg_number[]" value="" />
  <input type="text" name="reg_number[]" value="" />
  <input type="text" name="reg_number[]" value="" />
  <input type="submit" name="submit" value="submit" />
</form>

<div class="errormsg"></div>

JavaScript

$(document).ready(function() {

    // validate signup form on keyup and submit
    var validator = $("form[name='voucherform']").validate({
        showErrors: function(errorMap, errorList) {
            $(".errormsg").html($.map(errorList, function(el) {
                return el.message;
            }).join(", "));
        },
        wrapper: "span",
        rules: {
            'reg_number[]': {
                required: true,
                minlength: 2,
                remote: {
                    url: '<?php echo base_url();?>sales/invoice_check',
                    async: false,
                    type: 'post'
                }

            }
        },
        messages: {
            'reg_number[]': {
                required: "Enter Reg Number",
                minlength: jQuery.format("Enter at least {0} characters"),
                remote: jQuery.format("{0} is already in use")
            }
        }
    });
});