title: Adding leading zero with regular expression
body: <p>say I have a string like</p>
  
  <pre><code>'1 - hello.mp3'
  '22 - hellox.mp3'
  '223 - hellox.mp3'
  'hellox.mp3'
  </code></pre>
  
  <p>I hope to output to be</p>
  
  <pre><code>'001 - hello.mp3'
  '022 - hellox.mp3'
  '223 - hellox.mp3'
  'hellox.mp3'
  </code></pre>
  
  <p>that is if the starting is number, appending 0 to make it three digits.</p>
  
  <p>Is there a way to achieve using regex in python?</p>