title: Can't act with string's methods on a matched group
body: <p>I'm exercising a standard regexp:</p>
  
  <pre><code>Bill Gates → GATES, Bill
  </code></pre>
  
  <p>So I do:</p>
  
  <pre><code>In [21]: re.sub("([^ ]+) (.+)", r"\2".upper() + r", \1", "Bill Gates")
  Out[21]: 'Gates, Bill'
  </code></pre>
  
  <p>Why it doesn't work? How does one applies string methods to a matched string?</p>