title: sed delete lines not containing specific string
body: <p>I'm new to <code>sed</code> and I have the following question. In this example:</p>
  
  <pre><code>some text here
  blah blah 123
  another new line
  some other text as well
  another line
  </code></pre>
  
  <p>I want to delete all lines except those that contain either string 'text' <s>and</s> or string 'blah', so my output file looks like this:</p>
  
  <pre><code>some text here
  blah blah 123
  some other text as well
  </code></pre>
  
  <p>Any hints how this can be done using <code>sed</code>?</p>