```ruby
require 'benchmark/ips'
require 'active_support/all'

ActiveSupport::Inflector.inflections do |inflect|
    inflect.acronym('RESTful')
end

module ActiveSupport
  module Inflector
    def underscore2(camel_cased_word)
      return camel_cased_word unless /[A-Z-]|::/.match?(camel_cased_word)
      word = camel_cased_word.to_s.gsub("::", "/")
      word.gsub!(inflections.acronyms_underscore_regex) { "#{$1 && '_' }#{$2.downcase}" }
      word.gsub!(/([A-Z\d]+)([A-Z][a-z])|([a-z\d])([A-Z])/) do
        if first_match = $1
          first_match << '_' << $2
        else
          $3 << '_' << $4
        end
      end
      word.tr!("-", "_")
      word.downcase!
      word
    end
  end
end

%w(RESTfulController Foo FooBar Foo::BarBaz FOOBar::EggSpam).each do |str|
  puts "== Comparing with #{str.inspect} (#{RUBY_VERSION}) =="
  unless ActiveSupport::Inflector.underscore(str) == ActiveSupport::Inflector.underscore2(str)
    raise "#{ActiveSupport::Inflector.underscore2(str)} != #{ActiveSupport::Inflector.underscore(str)}"
  end

  Benchmark.ips do |x|
    x.report('underscore') { ActiveSupport::Inflector.underscore(str) }
    x.report('underscore2') { ActiveSupport::Inflector.underscore2(str) }
    x.compare!
  end
  puts
end
```

```
== Comparing with "RESTfulController" (2.7.2) ==
Warming up --------------------------------------
          underscore    16.527k i/100ms
         underscore2    16.118k i/100ms
Calculating -------------------------------------
          underscore    165.223k (± 0.9%) i/s -    826.350k in   5.001882s
         underscore2    163.251k (± 0.9%) i/s -    822.018k in   5.035736s

Comparison:
          underscore:   165222.7 i/s
         underscore2:   163250.9 i/s - same-ish: difference falls within error

== Comparing with "Foo" (2.7.2) ==
Warming up --------------------------------------
          underscore    66.965k i/100ms
         underscore2    72.961k i/100ms
Calculating -------------------------------------
          underscore    669.221k (± 0.8%) i/s -      3.348M in   5.003557s
         underscore2    729.815k (± 1.1%) i/s -      3.721M in   5.099188s

Comparison:
         underscore2:   729815.0 i/s
          underscore:   669221.3 i/s - 1.09x  (± 0.00) slower

== Comparing with "FooBar" (2.7.2) ==
Warming up --------------------------------------
          underscore    26.869k i/100ms
         underscore2    26.446k i/100ms
Calculating -------------------------------------
          underscore    268.071k (± 0.7%) i/s -      1.343M in   5.011813s
         underscore2    264.400k (± 0.6%) i/s -      1.322M in   5.001317s

Comparison:
          underscore:   268071.4 i/s
         underscore2:   264400.1 i/s - 1.01x  (± 0.00) slower

== Comparing with "Foo::BarBaz" (2.7.2) ==
Warming up --------------------------------------
          underscore    20.611k i/100ms
         underscore2    20.609k i/100ms
Calculating -------------------------------------
          underscore    205.671k (± 1.3%) i/s -      1.031M in   5.011478s
         underscore2    204.304k (± 1.2%) i/s -      1.030M in   5.044446s

Comparison:
          underscore:   205670.8 i/s
         underscore2:   204304.1 i/s - same-ish: difference falls within error

== Comparing with "FOOBar::EggSpam" (2.7.2) ==
Warming up --------------------------------------
          underscore    15.348k i/100ms
         underscore2    18.932k i/100ms
Calculating -------------------------------------
          underscore    154.686k (± 0.9%) i/s -    782.748k in   5.060656s
         underscore2    188.706k (± 0.8%) i/s -    946.600k in   5.016580s

Comparison:
         underscore2:   188705.6 i/s
          underscore:   154685.7 i/s - 1.22x  (± 0.00) slower
```