Validating array inputs using jquery validation plugin

You have two problems:

1. You aren’t selecting the form element properly.

   You’re looking for a form with id “voucherform” and your markup does not contain one. You probably want to change your selector to:

   $("form[name="voucherform"]").validate({ ... });
   

2. Brackets ([]) are not valid in JavaScript identifiers. This means that your script is not being parsed correctly. To resolve this, just wrap those fields that have brackets in them in quotes, i.e. 'reg_number[]' instead of reg_number[].

Tweaked validate code:

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")
        }
    }
});

Example: http://jsfiddle.net/hfrhs/