AlVar,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
function my_keyupA(str) {
document.getElementById('div1').innerHTML = str.replace(/a/g, "A")
}
</script>
</head>
<body>
<input type="text" name="x_charA" value="" id="x_charA" oninput="my_keyupA(this.value)"><br>
<table class=t1>
<tr>
<td><div class=d1 id="div1"></div>
</td>
</tr>
</table>
</body>
</html>