title: regex match square brackets once
body: <p>There is a text file with the following info:</p>
  
  <pre><code>[[parent]]
  
  [son]
  
  [daughter]
  </code></pre>
  
  <p>How to get only [son] and [daughter]?</p>
  
  <pre><code>$0 ~ /\[([a-z])*\]/        ???
  </code></pre>