Javascript validation for numeric or null

I have a page with 3 text boxes T1,T2,T3
I want to force the user to enter numeric values or a null

The following function works for the numeric part but I need to also accept if null

  function allnumeric(inputtxt)
   {
      var numbers = /^[0-9]+$/;
      if(inputtxt.value.match(numbers))
      {
      alert('Your new number has been accepted');
      document.Form_Edit.T1.focus();
      return true;
      }
      else
      {
      alert('Please input numeric characters only');
      document.Form_Edit.T1.focus();
        window.history.back()
      return false;
      }  
}