title: Received comma while excluding letter using reqex
body: <p>How can I exclude letters from input <code>rpn("2 3 + 7  d -");</code> ?</p>
  
  <p>I've tried with this:</p>
  
  <pre><code>function rpn(input) {
  
      var str = input.split(/[a-z]/);
      return arr;
  };
  </code></pre>
  
  <p>but then I receive <code>2 3 + 7 , -</code>, so comma instead of <code>d</code>
  I then tried to remove spaces with <code>var str = input.split(/[a-z]\s+/);</code>, but this didn't work at all.</p>
  
  <p>I'm looking for a result like that: <code>2,3,+,7,-</code>, so I can push it to an array and work with it there.</p>