JSFiddle - React, Tailwind, and code Playground
by zlojnaxa
HTML
<input type="text" id="inp">
<div id="warning"></div>JavaScript
var inp = document.getElementById('inp'), warning = document.getElementById('warning');
inp.onkeypress = (event) => !/[a-z\d]/i.test(event.key) ? (warning.innerHTML = 'Запрещенные символы', false) : (warning.innerHTML = '', true);