title: Return part of string of every cell in pd.Dataframe
body: <p>There is a pd.DataFrame<br />
  df:</p>
  <pre><code>  |         crit         |  acc_num |
  --|----------------------|----------|
  0 | 050_11_20 0021351-05 | 27045980 |   
  1 | 051_09_20 0021351-05 | 27145990 |   
  2 | 058_77_21 0021351-05 | 27245910 |   
  3 | 075_85_21 0021351-05 | 27445920 |   
  </code></pre>
  <p>All data is str.<br />
  <strong>How can I return lines 2,3 which has '21' on position [8,9] in column 'crit'?</strong></p>