Use string lengths instead of regexp to extract path

The regexp was introduced in 186ac4cdaa,
and looks cosmetic. While they should be functionally identical in
theory, in practice, case insensitive (but preserving) filesystems can
give results that are differently-cased from the pattern we supplied.

I don't know how to force the filesystem to do the surprising thing,
even when running in an environment that _could_, so no new test.

actionpack/lib/action_controller/metal/helpers.rb

@@ -100,8 +100,7 @@ module ActionController
       #   # => ["application", "chart", "rubygems"]
       def all_helpers_from_path(path)
         helpers = Array(path).flat_map do |_path|
-          extract = /^#{Regexp.quote(_path.to_s)}\/?(.*)_helper.rb$/
-          names = Dir["#{_path}/**/*_helper.rb"].map { |file| file.sub(extract, '\1'.freeze) }
+          names = Dir["#{_path}/**/*_helper.rb"].map { |file| file[_path.to_s.size + 1..-"_helper.rb".size - 1] }
           names.sort!
         end
         helpers.uniq!