title: Extracting the nth character from a vector of strings
body: <p>I have a row vector which looks like this:</p>
  
  <pre><code>1   118_o_m_n_a.jpg
  2   122_m_f_h_a.jpg
  3   152_y_f_n_a.jpg
  4   160_y_m_h_a.jpg
  5   083_o_m_a_a.jpg
  </code></pre>
  
  <p>how do I extract all entries in which, for instance, the 9th element (letter) is "n" (or "h", or "a" in this case)?</p>