public function singleTaxonomy($taxonomyName, $emptyValue = 'none')
    {
        $taxonomyValues = $this->config->get('taxonomy/' . $taxonomyName . '/options');

        // If by accident, someone uses a "tags" taxonomy.
        if (empty($taxonomyValues)) {
            return '[a-z0-9-_]+';
        }
        $taxonomyValues = array_keys($taxonomyValues);
        $requirements = implode('|', $taxonomyValues);

        if ($emptyValue !== null) {
            $requirements .= '|' . $emptyValue;
        }

        return $requirements;
    }