Tryit Editor v1.4

<html>
<head>
<script type="text/javascript">
function checkLen(x,y)
{
if (y.length==x.maxLength)
  {
  var next=x.tabIndex;
  if (next<document.getElementById("myForm").length)
    {
    document.getElementById("myForm").elements[next].focus();
    }
  }
}
</script>
</head>

<body>
<p>This script automatically jumps to the next field when a field's maxlength has been reached:</p>

<form id="myForm">
<input size="3" tabindex="1" maxlength="3" onkeyup="checkLen(this,this.value)">
<input size="2" tabindex="2" maxlength="2" onkeyup="checkLen(this,this.value)">
<input size="3" tabindex="3" maxlength="3" onkeyup="checkLen(this,this.value)">
</form>
</body>

</html>

Your Result:

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