title: Preg match filenames from html content
body: <p>This expression find filenames in html content which contains digits, letters and underscores.</p>
  
  <pre><code>preg_match_all('/(\w+\.\w{2,4})/', $content);
  </code></pre>
  
  <p>But do not find filename like file-name.txt. How can I change expression to find filenames with dash and other legal characters?</p>