title: Add string after a specified string
body: <p>Say I have the following HTML string</p>
  
  <pre><code>&lt;head&gt;
  
  &lt;/head&gt;
  
  &lt;body&gt;
  &lt;img src="stickman.gif" width="24" height="39" alt="Stickman"&gt;
  &lt;a href="http://www.w3schools.com"&gt;W3Schools&lt;/a&gt;
  &lt;/body&gt; 
  </code></pre>
  
  <p>I want to add a string in between the <code>&lt;head&gt;</code> tags. So the final HTML string become</p>
  
  <pre><code>&lt;head&gt;
  &lt;base href="http://www.w3schools.com/images/"&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
  &lt;img src="stickman.gif" width="24" height="39" alt="Stickman"&gt;
  &lt;a href="http://www.w3schools.com"&gt;W3Schools&lt;/a&gt;
  &lt;/body&gt; 
  </code></pre>
  
  <p>So I have to search for the <strong>first</strong> occurrence of the <code>&lt;head&gt;</code> string then insert <code>&lt;base href="http://www.w3schools.com/images/"&gt;</code> right after.</p>
  
  <p>How do I do this in C#. </p>