code: protected function isIdentifier(&$length, &$content)
      {
          static $identifierDelimiters = [
              '"' => '"',
              '[' => ']',
              '`' => '`',
          ];
  
          if (!isset($identifierDelimiters[$this->substring(1)])) {
              return false;
          }
  
          $delimiter = $identifierDelimiters[$this->substring(1)];
          $offset = $this->offset;
          while (true) {
              $offset = $this->indexAfter($delimiter, $offset + 1);
              if ($delimiter === ']' || $this->substring(1, true, $offset) !== $delimiter) {
                  break;
              }
          }
          $length = $offset - $this->offset;
          $content = $this->substring($length - 2, true, $this->offset + 1);
          if ($delimiter !== ']') {
              $content = strtr($content, ["$delimiter$delimiter" => $delimiter]);
          }
  
          return true;
      }
docs: {@inheritdoc}