title: Using a negation in the sed command in UNIX
body: <p>I am new to shell, started learning it few days ago and I encountered a problem. I want to use the sed command in order to filter the files. 
  First, I want to delete all files from the list that don't contain the word zad
  So I do this : </p>
  
  <pre><code>ls | sed '/[^{zad}]/d'
  </code></pre>
  
  <p>I don't get why does this remove all the files from the list? Does it remove all files that do not contain z, a or d in their names?</p>