private function extractPlaceholders(): void
  {
    $unknown = [];

    preg_match_all('(@[A-Za-z0-9\_\.]+(\%type)?@)', $this->routineSourceCode, $matches);
    if (!empty($matches[0]))
    {
      foreach ($matches[0] as $placeholder)
      {
        if (isset($this->replacePairs[strtoupper($placeholder)]))
        {
          $this->replace[$placeholder] = $this->replacePairs[strtoupper($placeholder)];
        }
        else
        {
          $unknown[] = $placeholder;
        }
      }
    }

    $this->logUnknownPlaceholders($unknown);
  }