Javascript Check User Input

Hi Everyone

I have attached a javascript function that is called onkeyup which should the user enter any characters that are not numbers it will replace this with an empty space.

What I would like to do now is to validate an expiry date entered in the format mm/yy.  So for example the first,second, fourth and fifth character must be a digit.  The third character must be the / character.  If the user enters anything expect this format then I want to replace what they have typed with an empty space like before.

Any help greatly appreciated thanks

function numInputOnly(ob) 
{
  var invalidChars = /[^0-9]/gi
  
if(invalidChars.test(ob.value)) 
{
  ob.value = ob.value.replace(invalidChars,"");
}


}

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