JSFiddle - React, Tailwind, and code Playground

HTML

<form name="cdp_form" id="cdp_form">
<input type ="text" id="univer_number" maxlength="7"    size="25"   name="univer_number" onkeyup="validateForm()" />
</form>

CSS

span { background:yellow; }

JavaScript

//form validation
function validateForm()
{
var x=document.forms["cdp_form"]["univer_number"].value
if (x==null || x=="")
  {
  alert("University number (URN) field must be filled in");
  cdp_form.univer_number.focus();
  return false;
  }
else if (is_valid = !/^[0-9]+$/.test(x))
    {
    alert("University number (URN) field must have numeric characters");
    cdp_form.univer_number.focus();
    return false;
  }
}