[devote]

<script type="text/javascript" src="http://devote.github.io/files/letjs/let.js"></script>
<script type="text/javascript">
  letJS.setHandler('data-action', function(rules) {
    if (rules.expectedValue.length == +rules.rule) {
      document.getElementById('debug').innerHTML = rules.rule + ' чисел вбил(а) ты';
    } else {
      document.getElementById('debug').innerHTML = '';
    }
  });
</script>
<input type="text" data-action="13" data-let-input="/[0-9]+/" />
<div id="debug"></div>