title: Removing spaces and anything that is not alphanumeric
body: <p>I'm trying to remove everything that is not alphanumeric, or is a space with _:</p>
  
  <pre><code>$filename = preg_replace("([^a-zA-Z0-9]|^\s)", "_", $filename);
  </code></pre>
  
  <p>What am I doing wrong here, it doesn't seem to work. I've tried several regex combinations...(and I'm generally not very bright).</p>