title: extract words with specific character sequence
body: <p>I have a list of strings. I only want to extract the words within each string that have a specific character sequence.</p>
  
  <p>For example </p>
  
  <pre><code>l1=["grad madd have", "ddim middle left"]
  </code></pre>
  
  <p>I want all the words that have sequence "dd"</p>
  
  <p>so I would like to get</p>
  
  <pre><code>[["madd"], ["ddim", "middle"]]
  </code></pre>
  
  <p>I've been trying patterns of the form</p>
  
  <pre><code>[re.findall(r'(\b.*?dd.*\s+)',word) for word in l1] 
  </code></pre>
  
  <p>but have had little success </p>