2 files changed, 18 insertions, 6 deletions
diff --git a/activesupport/lib/active_support/inflector/inflections.rb b/activesupport/lib/active_support/inflector/inflections.rb
index a639eddf10..6cfa8bb5aa 100644
--- a/activesupport/lib/active_support/inflector/inflections.rb
+++ b/activesupport/lib/active_support/inflector/inflections.rb
@@ -66,17 +66,21 @@ module ActiveSupport
         @__instance__[locale] ||= new
       end
 
-      attr_reader :plurals, :singulars, :uncountables, :humans, :acronyms, :acronym_regex
+      attr_reader :plurals, :singulars, :uncountables, :humans,
+                  :acronyms, :acronym_regex
+      attr_reader :acronyms_camelize_regex, :acronyms_underscore_regex # :nodoc:
 
       def initialize
-        @plurals, @singulars, @uncountables, @humans, @acronyms, @acronym_regex = [], [], Uncountables.new, [], {}, /(?=a)b/
+        @plurals, @singulars, @uncountables, @humans, @acronyms = [], [], Uncountables.new, [], {}
+        define_acronym_regex_patterns
       end
 
       # Private, for the test suite.
       def initialize_dup(orig) # :nodoc:
-        %w(plurals singulars uncountables humans acronyms acronym_regex).each do |scope|
+        %w(plurals singulars uncountables humans acronyms).each do |scope|
           instance_variable_set("@#{scope}", orig.send(scope).dup)
         end
+        define_acronym_regex_patterns
       end
 
       # Specifies a new acronym. An acronym must be specified as it will appear
@@ -130,7 +134,7 @@ module ActiveSupport
       #   camelize 'mcdonald'   # => 'McDonald'
       def acronym(word)
         @acronyms[word.downcase] = word
-        @acronym_regex = /#{@acronyms.values.join("|")}/
+        define_acronym_regex_patterns
       end
 
       # Specifies a new pluralization rule and its replacement. The rule can
@@ -225,6 +229,14 @@ module ActiveSupport
           instance_variable_set "@#{scope}", []
         end
       end
+
+      private
+
+        def define_acronym_regex_patterns
+          @acronym_regex             = @acronyms.empty? ? /(?=a)b/ : /#{@acronyms.values.join("|")}/
+          @acronyms_camelize_regex   = /^(?:#{@acronym_regex}(?=\b|[A-Z_])|\w)/
+          @acronyms_underscore_regex = /(?:(?<=([A-Za-z\d]))|\b)(#{@acronym_regex})(?=\b|[^a-z])/
+        end
     end
 
     # Yields a singleton instance of Inflector::Inflections so you can specify
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 72de42b1c3..43f21c03c7 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -71,7 +71,7 @@ module ActiveSupport
       if uppercase_first_letter
         string = string.sub(/^[a-z\d]*/) { |match| inflections.acronyms[match] || match.capitalize }
       else
-        string = string.sub(/^(?:#{inflections.acronym_regex}(?=\b|[A-Z_])|\w)/) { |match| match.downcase }
+        string = string.sub(inflections.acronyms_camelize_regex) { |match| match.downcase }
       end
       string.gsub!(/(?:_|(\/))([a-z\d]*)/i) { "#{$1}#{inflections.acronyms[$2] || $2.capitalize}" }
       string.gsub!("/".freeze, "::".freeze)
@@ -92,7 +92,7 @@ module ActiveSupport
     def underscore(camel_cased_word)
       return camel_cased_word unless /[A-Z-]|::/.match?(camel_cased_word)
       word = camel_cased_word.to_s.gsub("::".freeze, "/".freeze)
-      word.gsub!(/(?:(?<=([A-Za-z\d]))|\b)(#{inflections.acronym_regex})(?=\b|[^a-z])/) { "#{$1 && '_'.freeze }#{$2.downcase}" }
+      word.gsub!(inflections.acronyms_underscore_regex) { "#{$1 && '_'.freeze }#{$2.downcase}" }
       word.gsub!(/([A-Z\d]+)([A-Z][a-z])/, '\1_\2'.freeze)
       word.gsub!(/([a-z\d])([A-Z])/, '\1_\2'.freeze)
       word.tr!("-".freeze, "_".freeze)