How can I restrict the user to enter "MM/YYYY" (example 10/1999) format only in a text box.

function checkDateFormat(dateValue)
{
  dates = dateValue.split("/");
  if(dates.length!=2)
    return "1";
  if(dates[0].length!=2 || dates[1].length!=4)
    return "1";
  if(isNan(dates[0]) || isNan(dates[1])) 
    return "2";
  return "0";
}
function validateThis(obj)
{
  if(checkDateFormat(obj.value)=="1") 
  {
    alert('Please enter the date in the format MM/YYYY');
    return false;
  }
  if(checkDateFormat(obj.value)=="2") 
  {
    alert('Please enter a valid date');
    return false;
  }
  if(checkDateFormat(obj.value)=="0")
    return true;
}

<input type=text onblur="if(!validateThis(this)) { this.focus(); }">