IE8-如何获取input光标所在位置（selectionstart）

<input type="text" id="txt" value="1234567" />
<input type="button" value="获取text光标位置" onclick="alert(getCaret(document.getElementById('txt')))" />
<script>
    function getCaret(el) {
        if (el.selectionStart) {//IE9-或者标准浏览器
            return el.selectionStart;
        } else if (document.selection) {//IE8-或者支持selection对象的浏览器
            el.focus();
            var r = document.selection.createRange();
            if (r == null) {
                return 0;
            }
            var re = el.createTextRange(), rc = re.duplicate();
            re.moveToBookmark(r.getBookmark());
            rc.setEndPoint('EndToStart', re);
            return rc.text.length;
        }
        return 0;
    }
</script>

function getCursortPosition (ctrl) {
    var CaretPos = 0;   // IE Support
    if (document.selection) {
    ctrl.focus ();
        var Sel = document.selection.createRange ();
        Sel.moveStart ('character', -ctrl.value.length);
        CaretPos = Sel.text.length;
    }
    // Firefox support
    else if (ctrl.selectionStart || ctrl.selectionStart == '0')
        CaretPos = ctrl.selectionStart;
    return (CaretPos);
}

function setCaretPosition(ctrl, pos){
    if(ctrl.setSelectionRange)
    {
        ctrl.focus();
        ctrl.setSelectionRange(pos,pos);
    }
    else if (ctrl.createTextRange) {
        var range = ctrl.createTextRange();
        range.collapse(true);
        range.moveEnd('character', pos);
        range.moveStart('character', pos);
        range.select();
    }
}