How to write java script for accept only decimal no which is not > 20?

function checkDec(el) {
 var ex = /^[0-9]+\.?[0-9]*$/;
 if (ex.test(el.value) == false) {
 el.value = el.value.substring(0, el.value.length - 1);
}

if (el.value.length == 2) {
 if (txtValue.value > 20) {
   alert("Value must be less than 20.");
 } else {
   txtValue.value = el.value + "."; 
  }
}

if (el.value.split(".").length == 2) {
 if(el.value.split(".")[1].length>2){
   //Put your logic for after decimal points
 }
} else {
 //Put your logic for after decimal points
 }
}

<input type="text" id="txtValue" onkeyup="checkDec(this);" />

 var val = document.getElementById("txtInput").value;
 var re = "^\d{1,5}(?:\.\d{1,2})?$";
 if (! re.test(val)) 
  {
       //Regular expression failed

  } 

^[\d,]+(\.\d{1,2})?$

^\d{1,9}(\.\d{1,2})?%?$