protected function updateBltVersionConstant($tag) {
    // Change version constant in Blt.php.
    $this->taskReplaceInFile($this->bltRoot . '/src/Robo/Blt.php')
      // Test group:
      // @codingStandardsIgnoreStart
      // const VERSION = '9.x-dev';
      // const VERSION = '9.0.x-dev';
      // const VERSION = '9.0.0-alpha1';
      // const VERSION = '9.0.0-beta2';
      // const VERSION = '9.0.0';
      // @codingStandardsIgnoreEnd
      ->regex('/(const VERSION = \')[0-9]{1,2}\.[0-9x]{1,2}(\.[0-9x](-(alpha|beta|rc|dev)[0-9]{0,2})?|-dev?)(\';)/')
      ->to('${1}' . $tag . '${5}')
      ->run();
  }