title: RegEx: Remove non-letters UTF-8 Safe, Quickly
body: <p>I'm trying to remove everything except valid letters (from any language) in PHP. I've been using this:</p>
  
  <pre><code>$content=preg_replace('/[^\pL\p{Zs}]/u', '', $content);
  </code></pre>
  
  <p>But it's painfully slow. Takes about 30x longer than:</p>
  
  <pre><code>$content=preg_replace('/[^a-z\s]/', '', $content);
  </code></pre>
  
  <p>I'm dealing with large amounts of data, so it really isn't feasible to use a slow method.</p>
  
  <p>Is there a faster way of doing this?</p>