code: static function findError($path)
      {
          Checker::argStringNonEmpty("path", $path);
  
          $matchResult = preg_match('%^(?:
                    [\x09\x0A\x0D\x20-\x7E]            # ASCII
                  | [\xC2-\xDF][\x80-\xBF]             # non-overlong 2-byte
                  | \xE0[\xA0-\xBF][\x80-\xBD]         # excluding overlongs, FFFE, and FFFF
                  | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}  # straight 3-byte
                  | \xED[\x80-\x9F][\x80-\xBF]         # excluding surrogates
                  )*$%xs', $path);
  
          if ($matchResult !== 1) {
              return "must be valid UTF-8; BMP only, no surrogates, no U+FFFE or U+FFFF";
          }
  
          if ($path[0] !== "/") return "must start with \"/\"";
          $l = strlen($path);
          if ($l === 1) return null;  // Special case for "/"
  
          if ($path[$l-1] === "/") return "must not end with \"/\"";
  
          // TODO: More checks.
  
          return null;
      }
docs: If the given path is a valid Dropbox path, return `null`,
  otherwise return an English string error message describing what is wrong with the path.
  
  @param string $path
  The path you want to check for validity.
  
  @return string|null
  If the path was valid, return `null`.  Otherwise, returns
  an English string describing the problem.