title: sed with mathematics operation
body: <p>I have a text file with following structure</p>
  
  <pre><code>    &lt;h2&gt;Time 455&lt;/h2&gt;
  Random texts may contain numbers
      &lt;h2&gt;Time 460&lt;/h2&gt;
  Random texts may contain numbers
      &lt;h2&gt;Time 483&lt;/h2&gt;
  Random texts with numbers
      &lt;h2&gt;Time 555&lt;/h2&gt;
  Random texts with numbers
      &lt;h2&gt;Time 550&lt;/h2&gt;
  Random texts may contain numbers
  </code></pre>
  
  <p>I want to deduct (for example) 5 from number from each of the line containing h2 time </p>
  
  <p>So the final output will be something like</p>
  
  <pre><code>    &lt;h2&gt;Time 450&lt;/h2&gt;
  Random texts may contain numbers
      &lt;h2&gt;Time 455&lt;/h2&gt;
  Random texts may contain numbers
      &lt;h2&gt;Time 478&lt;/h2&gt;
  Random texts with numbers
      &lt;h2&gt;Time 550&lt;/h2&gt;
  Random texts with numbers
      &lt;h2&gt;Time 545&lt;/h2&gt;
  Random texts may contain numbers
  </code></pre>
  
  <p>Note: Im using Mac terminal</p>