title: Remove numbers at the beginning and end of a string
body: <p>I've got the following vector:</p>
  
  <pre><code>words &lt;- c("5lang","kasverschil2","b2b")
  </code></pre>
  
  <p>I want to remove <code>"5"</code> in <code>"5lang"</code> and <code>"2"</code> in <code>"kasverschil2"</code>. But I do NOT want to remove <code>"2"</code> in <code>"b2b"</code>.</p>