code: private function readDigits()
      {
          [$char, $code] = $this->readChar();
  
          if ($code >= 48 && $code <= 57) { // 0 - 9
              $value = '';
  
              do {
                  $value        .= $char;
                  [$char, $code] = $this->moveStringCursor(1, 1)->readChar();
              } while ($code >= 48 && $code <= 57); // 0 - 9
  
              return $value;
          }
  
          if ($this->position > $this->source->length - 1) {
              $code = null;
          }
  
          throw new SyntaxError(
              $this->source,
              $this->position,
              'Invalid number, expected digit but got: ' . Utils::printCharCode($code)
          );
      }
docs: Returns string with all digits + changes current string cursor position to point to the first char after digits