<input maxlength='5' placeholder="MM/YY" type="text" onkeyup="formatString(event);" onblur="check(event)">

function check(e){
  var input = e.target.value.split('/');
  var date=new Date();
  var inputDate= new Date(2000+ +input[1],input[0]);
  console.log(input,inputDate.toString())
  if(inputDate<date) alert('OOOps!');
}

function formatString(e) {
  
  var inputChar = String.fromCharCode(event.keyCode);
  var code = event.keyCode;
  var allowedKeys = [8];
  if (allowedKeys.indexOf(code) !== -1) {
    return;
  }

  event.target.value = event.target.value.replace(
    /^([1-9]\/|[2-9])$/g, '0$1/' // 3 > 03/
  ).replace(
    /^(0[1-9]|1[0-2])$/g, '$1/' // 11 > 11/
  ).replace(
    /^([0-1])([3-9])$/g, '0$1/$2' // 13 > 01/3
  ).replace(
    /^(0?[1-9]|1[0-2])([0-9]{2})$/g, '$1/$2' // 141 > 01/41
  ).replace(
    /^([0]+)\/|[0]+$/g, '0' // 0/ > 0 and 00 > 0
  ).replace(
    /[^\d\/]|^[\/]*$/g, '' // To allow only digits and `/`
  ).replace(
    /\/\//g, '/' // Prevent entering more than 1 `/`
  );
}