Credit card CVV validation to 4 digits only
http://jsfiddle.net/nadeemmnn2007/C4Y99/52/
<input class="numeric" type="text" name="usrname" />
$('.numeric').keypress(function(e) {
var verified = (e.which == 8 || e.which == undefined || e.which == 0) ? null : String.fromCharCode(e.which).match(/[^0-9]/);debugger;
if (verified || e.delegateTarget.value.length>3 || e.ctrlKey ==true) { if(e.which!=8 ){e.preventDefault();}}
}).on('paste',function(e){ e.preventDefault();});