JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    	<form id = "myForm" action="/test" method="get" >
    	<p>
    	<label> Zip Code
    		<input type = "text" id = "zip"  />
    	</label>
    	<input type = "submit" name = "submit" />
    	</p>
    	</form>
    </body>

JavaScript

document.getElementById("myForm").onsubmit = validateForm;
    	
    	function zipcheck(sZip)
    	{
    		var postalRegex = /^d{5}(-\d{4})?$/;
    		return postalRegex.test(sZip);
    	}
      
    	function validateForm()
    	{
    		if (zipcheck(document.getElementById("zip").value)){
    			return true;
    		}else
    		{
    			alert(document.getElementById("zip").value + " is not a valid zip code");
    			return false;
    		}
    	}