After we see a key, we must look ahead to determine our next action. Cases:

[] follows the key. Then the value must be an array.
= follows the key. (A value comes next)
& or the end of string follows the key. Then the key is a flag.
otherwise, a hash follows the key.

Source Code

# File action_controller/request.rb, line 643
def post_key_check(key)
  if scan(/\[\]/) # a[b][] indicates that b is an array
    container(key, Array)
    nil
  elsif check(/\[[^\]]/) # a[b] indicates that a is a hash
    container(key, Hash)
    nil
  else # End of key? We do nothing.
    key
  end
end