public static function escape($string) {
    $escaped_chars = str_split(self::ESCAPED_CHARACTERS);
    $escaped_chars_replace = array_map(function($char) {
      return '\\' . $char;
    }, $escaped_chars);
    $string = str_replace($escaped_chars, $escaped_chars_replace, $string);
    // Ignored whitespace.
    return preg_replace('/\s+/u', ' ', $string);
  }