title: Replace number in a string by bracketed number Python
body: <p>I have a string like this:</p>
  
  <pre><code>s = k0+k1+k1k2+k2k3+1+12
  </code></pre>
  
  <p>I want to convert this, such that every number, which follows a letter (<code>k</code> here) becomes surrounded by square  brackets:</p>
  
  <pre><code>k[0]+k[1]+k[1]k[2]+k[2]k[3]+1+12
  </code></pre>
  
  <p>What is a good way to do that?</p>
  
  <p>What I tried: Use <code>replace()</code> function 4 times (but it cannot handle numbers not followed by letters).</p>