title: Regex match even number of letters
body: <p>I need to match an expression in Python with regular expressions that only matches even number of letter occurrences. For example:</p>
  
  <pre>
  AAA        # no match
  AA         # match
  fsfaAAasdf # match
  sAfA       # match
  sdAAewAsA  # match
  AeAiA      # no match
  </pre>
  
  <p>An even number of As SHOULD match.</p>