Question

How do I do a live search only once the user has typed in 4 numbers into the input?

It basically needs to contact the php page using $ajax which I already have.

I thought it might be an input.length == 4 but I tried it and failed.

Answer 1

You can use regular expressions to check if the value of the input consists exactly of four digits.

$("#test").keyup(function() {
  if(/^\d{4}$/.test($(this).val())) {
    alert('please call $.ajax here');
  }
});

where #test is:

<input type="text" id="test" />

HERE is the code.