How to remove/replace with an exception?
<p>For example the file listed in:</p>

<blockquote>
  <p>ASDFFFFFASDGH</p>
  
  <p>HSHGDSFFFFHDHSA</p>
</blockquote>

<p>How can I remove all the words except "F"?</p>

<p>I want my result to be:</p>

<blockquote>
  <p>FFFFF</p>
  
  <p>FFFF</p>
</blockquote>
