how to validate date in dd/mmm/yyyy format

this is simple validation in date format

function a(){
var i=document.getElementById("t1").value;

if(abc(i)){
alert("valid")}
else
{
alert("not valid");
}
}
function abc(output){
var i=output;
var reg= new RegExp("^([0][1-9]|[1-2][0-9]|[3][0-1])/(jan|JAN|Jan|feb|Feb|FEB|mar|Mar|MAR|apr|Apr|APR|may|May|MAY|May|Jun|jun|JUN|jul|JUL|Aug|aug|AUG|Sep|sep|SEP|Oct|oct|OCT|nov|Nov|NOV|dec|DEC|Dec)/([1-9][0-9][0-9][0-9])");
//document.write(i);
return reg.test(i);
//if(i=="")
//alert("blank");
}
 </script>