title: Change first digit in a string to '#' using regex in python
body: <pre><code>import re    
  sentence = "the cat is called 303worldDomination"    
  print re.sub(r'\d', r'#', sentence)
  </code></pre>
  
  <p>However, this substitutes all the digits in the string with '#'</p>
  
  <p>I only want the first digit in the string to be substituted by '#'</p>