crossword-8.x-1.x-dev/tests/modules/crossword_tests/crossword_tests.routing.yml

crossword_tests.formatter:
  path: '/crossword-tests/{id}/{formatter}/{redacted}'
  defaults:
    _controller: '\Drupal\crossword_tests\Controller\CrosswordTestsController::build'
    redacted: 'nope'
  requirements:
    _permission: 'access content'
    id: \d+ #digit
    view_mode: \w+ #word