How to allow tab in Javascript function

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Javascript

Hi

I have a JavaScript function to allow number ,backspace which I use on Key Press Event. This is not allowing tab key to work. Please help

function allownumbers(e) {
          var key = window.event ? e.keyCode : e.which;
          var keychar = String.fromCharCode(key);
          var reg = new RegExp("[0-9.]")
          if (key == 8) {
              keychar = String.fromCharCode(key);
          }
          if (event.keyCode == 9) {
              //TAB is pressed keyCode = 9 for TAB key
              return
          }




          if (key == 13) {
              key = 8;
              keychar = String.fromCharCode(key);
          }
          return reg.test(keychar);
          
      }

Thanks.