3 files changed, 1 insertions, 8 deletions
diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index c0dddd5b14..9e6b77e2f3 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,10 +1,5 @@
 ## Rails 4.0.0 (unreleased) ##
 
-*   Fixed a bug in `ActiveSupport::Inflector#underscore` where acroynms are
-    incorrectly parsed as camelCases.
-
-    *Fred Wu*
-
 *   Fix deletion of empty directories in `ActiveSupport::Cache::FileStore`.
 
     *Charles Jones*
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 648508d68e..39648727fd 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -91,7 +91,7 @@ module ActiveSupport
       word = camel_cased_word.to_s.dup
       word.gsub!('::', '/')
       word.gsub!(/(?:([A-Za-z\d])|^)(#{inflections.acronym_regex})(?=\b|[^a-z])/) { "#{$1}#{$1 && '_'}#{$2.downcase}" }
-      word.gsub!(/(?!#{inflections.acronym_regex})\b([A-Z\d]+)([A-Z][a-z])/,'\1_\2')
+      word.gsub!(/([A-Z\d]+)([A-Z][a-z])/,'\1_\2')
       word.gsub!(/([a-z\d])([A-Z])/,'\1_\2')
       word.tr!("-", "_")
       word.downcase!
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index 2b74055e68..4806ce07f6 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -167,13 +167,11 @@ class InflectorTest < ActiveSupport::TestCase
   def test_underscore_acronym_sequence
     ActiveSupport::Inflector.inflections do |inflect|
       inflect.acronym("API")
-      inflect.acronym("APIs")
       inflect.acronym("JSON")
       inflect.acronym("HTML")
     end
 
     assert_equal("json_html_api", ActiveSupport::Inflector.underscore("JSONHTMLAPI"))
-    assert_equal("namespaced/apis", ActiveSupport::Inflector.underscore("Namespaced::APIs"))
   end
 
   def test_underscore