JSFiddle - React, Tailwind, and code Playground

HTML

<form name="myform">
<input type="text" name="myinput" size=15>
<input type="button" onclick="checkpostal()" value="check">
</form>

JavaScript

window.checkpostal = function(){
    var re5digit=/^\d{5}$/ //regular expression defining a 5 digit number
    if (document.myform.myinput.value.search(re5digit)==-1) {
        //if match failed
        alert("Please enter a valid 5 digit number inside form")
    }
}