ruby on rails - Route constraint fatal

Question

I tried writing my routes in these 3 ways but both of them result into a fatal error. I don't understand why. It looks like it has to do with the constraint because as soon as I remove the regex there are no errors.

resources :posts do
  collection do
    get "level/:level", action: "level", constraints: { level: /^[0-3]$|^blank$/ }
  end
end

get "posts/level/:level", to: "posts#level", constraints: { level: /^[0-3]$|^blank$/ }

resources :posts do
  collection do
    constraints(level: /^[0-3]$|^blank$/) do
      get "level/:level", action: "level"
    end
  end
end

Failure/Error: require File.expand_path("../config/environment", __dir__)

FrozenError:
  can't modify frozen #<Class:#<Array:0x0000000005853ce8>

Answer 1

I've changed the regex to /[0-3]|blank/ and now it works:

resources :posts do
  collection do
    get "videos"
    get "level/:level", action: "level", constraints: { level: /[0-3]|blank/ }
  end
end