Empty comments field or

Question

The easiest way to achieve this is to use jQuery on the client side to handle this validation. There are a number of ways you can achieve this, though one of the easiest would be to use a ready made solution of which one can be found HERE

You’ll need to download the plugin file and include it within your header.php file like so;

<script type="text/javascript" src="https://wordpress.stackexchange.com/questions/54150/<?php bloginfo("template_directory');?>/js/charCount.js"/></script>

Directly beneath the above line you will then place the following;

<script type="text/javascript">
     jQuery(document).ready(function(){
         //custom usage
         $("#txtTweet").charCount({
             allowed: 500,
             minChar: 15,
             warning: 20,
             counterText: 'Characters left: ',
             disableControl: '#btnSubmit',
             isDisable: true // is the control disable or not
         });
 });
 </script>

Make sure that the above snippets of code are placed within your;

<head>

    // usual head stuff here

    <?php wp_head();?>

    // include the charCount.js

    // include the custom usage snippet

</head>

The reason you include your files after wp_head is that this function will usually output your jQuery.js file which needs to be declared before your charCount.js and custom code to work correctly.

That link again is HERE – for which you can see a working demonstration.