Болат,
a не русская? тогда
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<textarea id="text2" name=""></textarea>
<div id="div1"></div>
<script>
window.addEventListener('DOMContentLoaded', function() {
var textarea = document.querySelector('#text2'),
div = document.querySelector('#div1');
textarea.addEventListener('input', function() {
div.innerHTML = textarea.value.replace(/a/gim,'A');
});
});
</script>
</body>
</html>