<p>I'm reading all the files in a single directory and I want to filter on JPG,JPEG,GIF and PNG.</p>

<p>Both capital and small letters. Those are the only files to be accepted.</p>

<p>I am currently using this:</p>

<pre><code>$testPics = takeFiles($picsDir, "([^\s]+(?=\.(jpg|JPG|jpeg|JPEG|png|PNG|gif|GIF))\.\2)");
</code></pre>

<p>and the function takeFiles looks like this:</p>

<pre><code>function takerFiles($dir, $rex="") {
    $dir .= "/";
    $files = array();
    $dp = opendir($dir);
    while ($file = readdir($dp)) {
      if ($file == '.')  continue;
      if ($file == '..') continue;
      if (is_dir($file)) continue;
      if ($rex!="" &amp;&amp; !preg_match($rex, $file)) continue;
      $files[] = $file;
    }
    closedir($dp);
    return $files;
  }
</code></pre>

<p>And it always returns nothing. So something must be wrong with my regex code.</p>

I think something is wrong with your regex. Try testing regexes here first:
https://www.regexpal.com/
I think this one might work for you:
```
/^.*\.(jpg|jpeg|png|gif)$/i
```
Note the /i at the end - this is the "case insensitive" flag, saves you having to type out all permutations :)