解决方法
<input id="ssn"/>
<script type="text/javascript">
$('#ssn').keyup(function() {
var val = this.value.replace(/\D/g,'');
var newVal = '';
while (val.length > 3) {
newVal += val.substr(0,3) + '-';
val = val.substr(3);
}
newVal += val;
this.value = newVal;
});
</script>
