title: How can I match \n\r with preg_replace in php
body: <p>I want to remove the empty paragraphs in html:</p>
  
  <pre><code>&lt;p&gt;
  &lt;/p&gt;
  </code></pre>
  
  <p>use <code>/&lt;p&gt;\n\r&lt;\/p&gt;/</code> can't match this
  while use <code>/&lt;p&gt;.{0,2}&lt;\/p&gt;/</code> works, why?</p>