title: PHP preg_replace causing preg_last_error()
body: <p>Not being great at regex I am trying to fix some code which errors on several preg_replace functions eg</p>
  
  <pre><code>$string = preg_replace('#(&lt;[^&gt;]+[\x00-\x20\"\'\/])(on|xmlns)[^&gt;]*&gt;#iUu',"$1&gt;",$string);
  </code></pre>
  
  <p>The error value is 4 if thats any use.
  What would be helpful is a regex tester which shows what string format will pass the regexpression; no online regex testers do.
  Because the above function is returning an error, the return string is null.
  If anyone can hint what I need to change or look into appreciate.</p>