title: Convert cell array of cells into cell array of strings in MATLAB
body: <p>Using regexp with tokens on cell array of strings I've got cell array of cells. Here is simplified example:</p>
  
  <pre><code>S = {'string 1';'string 2';'string 3'};
  res = regexp(S,'(\d)','tokens')
  res = 
  
      {1x1 cell}
      {1x1 cell}
      {1x1 cell}
  res{2}{1}
  ans = 
      '2'
  </code></pre>
  
  <p>I know I have only one match per cell string in S. How I can convert this output into cell arrays of strings in a vectorized form?</p>