title: How to optimize three consecutive str.replace() calls?
body: <pre><code>expr='9subtract5equal4'
  
  expr = expr.replace('subtract', '-')
  expr = expr.replace('plus', '+')
  expr = expr.replace('equal', '==')
  </code></pre>
  
  <p>I feel the last three lines code are very ugly, so I've tried to optimize using <code>map</code> and other functional programming functions. But I did't find a good way to achieve that. Any suggestions?</p>