private function convertRelativePaths($css, $fullPathName)
  {
    // Note: URLs like url(test/test(1).jpg) i.e. URL with ( or ) in name, or not supported.

    // The pcre.backtrack_limit option can trigger a NULL return, with no errors. To prevent we reach this limit we
    // split the CSS into an array of lines.
    $lines = explode("\n", $css);

    $lines = preg_replace_callback('/(url\([\'"]?)(([^()]|(?R))+)([\'"]?\))/i',
      function ($matches) use ($fullPathName) {
        return $matches[1].Url::combine($this->getPathInResources($fullPathName), $matches[2]).$matches[4];
      },
                                   $lines);

    if ($lines===null)
    {
      $this->logError("Converting relative paths failed for '%s'", $fullPathName);
    }

    return implode("\n", $lines);
  }