<p>StackExchange <a href="http://meta.stackexchange.com/questions/160100/a-new-search-engine-for-stack-exchange">replaced the search engine a few months ago</a>. Among the many changes are how string and substring searches are handled.</p>

<blockquote>
  <ul>
  <li>Quoted phrases are <strong><em>exact</em></strong> matches except for case-sensitivity, for example, [you can search for code][4] or <a href="http://stackoverflow.com/search?q=%22%2a=%22">symbols</a>.</li>
  <li>If not in a quoted phrase, words will now be stemmed (<a href="http://meta.stackoverflow.com/search?q=stem%20test">example</a>)</li>
  </ul>
</blockquote>

<p>As @Iain said earlier, quoting matters.</p>