title: Python: Intersection of full string from list with partial string
body: <p>Let's say I have a string and a list of strings:</p>
  
  <pre><code>a = 'ABCDEFG'
  
  b = ['ABC', 'QRS', 'AHQ']
  </code></pre>
  
  <p>How can I pull out the string in list b that matches up perfectly with a section of the string a? So the would return would be something like <code>['ABC']</code></p>
  
  <p>The most important issue is that I have tens of millions of strings, so that time efficiency is essential.</p>