title: Pandas DataFrame get substrings from column
body: <p>I have a column named "KL" with for example:</p>
  
  <pre><code>sem_0405M4209F2057_1.000
  sem_A_0103M5836F4798_1.000
  </code></pre>
  
  <p>Now I want to extract the four digits after "M" and the four digits after "F". But with <code>df["KL"].str.extract</code> I can't get it to work.</p>
  
  <p>Locations of M and F vary, thus just using the slice <code>[9:13]</code> won't work for the complete column.</p>