<script>

function cc(s){

        if(/[^0-9\.]/.test(s)) return "invalid value";

        s=s.replace(/^(\d*)$/,"$1.");

        s=(s+"00").replace(/(\d*\.\d\d)\d*/,"$1");

        s=s.replace(".",",");

        var re=/(\d)(\d{3},)/;

        while(re.test(s))

                s=s.replace(re,"$1,$2");

        s=s.replace(/,(\d\d)$/,".$1");

        return "￥" + s.replace(/^\./,"0.")

        }

</script>

<input onchange="this.value=cc(this.value)">