JSFiddle - React, Tailwind, and code Playground

HTML

<span>www.mysite.com/users/</span>
<input id="short_url" type="text" />

CSS

.invalid{
    background-color:red;
}

JavaScript

(function(){

      var shortUrlRegexp = /^[a-zA-Z]{1}$/;

      $("#short_url").on("keyup", function(){

          var text = $(this).val();

          var isMatch = shortUrlRegexp.test(text);

          $(this).toggleClass("invalid", !isMatch);

      });
    
}());