private function parseSingle( $key, $value, $string, $index = null )
    {
        if ( isset( $index ) ) {
            $string = str_replace( $this->l_delim . '%index%' . $this->r_delim, $index, $string );
        }
        return str_replace( $this->l_delim . $key . $this->r_delim, $value, $string );
    }