title: How to replace "@" post tag with regex in PHP?
body: <pre><code>aaaaa @username bbbb cc ddddddd
  </code></pre>
  
  <p>İ want get this tag and replace or preg_replace to html.</p>
  
  <p>like this,</p>
  
  <pre><code>aaaaa &lt;a href="/username/"&gt;@username&lt;/a&gt; bbbb cc ddddddd
  </code></pre>