Using a field token with caps a contextual argument does not pass to child view

Using a custom entity, I had field names such as voteId or firstName. In creating a child view using a contextual argument and trying to pass the argument from the parent view with a token such as :
{{ raw_fields.voteId }}

the value would not get passed. I traced it down to views_field_view\src\Plugin\views\field\View.php::getTokenArgument():

  protected function  getTokenArgument($token) {
    // Trim whitespace and remove the brackets around the token.
    preg_match('{{\s?(?<type>[a-z_]+)\.(?<id>[a-z_]+)\s?}}', $token, $match);

    return [
      'type' => $match['type'],
      'id' => $match['id'],
    ];
  }

The regex match doesn't allow for caps.