title: Use JavaScript to allow only specific characters in HTML input
body: <p>I have written some JavaScript and jQuery code that accepts only numeric input in a textbox.
  But this is not enough; I need to limit the input to certain numbers.</p>
  
  <p>This textbox needs to deal with SSN numbers (Swedish SSN), and it has to start with 19 or 20. I want to force it to start with these numbers, but I can't manage to limit it to these.</p>
  
  <pre><code>        $('input.SSNTB').keydown(function (event) {
            var maxNrOfChars = 12;
            var ssnNr = $('input.SSNTB').val();          
          if (event.keyCode == 46 || event.keyCode == 8 || event.keyCode == 9 || event.keyCode == 27 ||
          // Allow: Ctrl+A
          (event.keyCode == 65 &amp;&amp; event.ctrlKey === true) ||
          // Allow: home, end, left, right
          (event.keyCode &gt;= 35 &amp;&amp; event.keyCode &lt;= 39)) {
              // let it happen, don't do anything               
              return;
          }
          else {              
              // Ensure that it is a number and stop the keypress
              if (((event.keyCode &lt; 48 || event.keyCode &gt; 57) &amp;&amp; (event.keyCode &lt; 96 || event.keyCode &gt; 105))) {
                  console.log("if-1");
                  event.preventDefault();
              }
              if (event.shiftKey == true) {
                  console.log("if-3");
                  event.preventDefault();
              }
              //rules to make sure the textbox starts with correct number                
              if (event.keyCode != 49 || event.keyCode != 50 || event.keyCode != 97 || event.keyCode != 98) {
                  console.log("if-4, Keycode:" + event.keyCode);
                  event.preventDefault();
              }
          }
      });
  </code></pre>
  
  <p>The last if-case is executed to for testing this it is. It is executed as planed but it wont limit the input chars as its built for.</p>
  
  <p>any tips or ideas?</p>