diff --git a/activesupport/lib/active_support/inflector.rb b/activesupport/lib/active_support/inflector.rb
index 8fca3e36da..9d3976da20 100644
--- a/activesupport/lib/active_support/inflector.rb
+++ b/activesupport/lib/active_support/inflector.rb
@@ -146,7 +146,7 @@ module Inflector
   #   "active_record/errors".camelize(:lower) #=> "activeRecord::Errors"
   def camelize(lower_case_and_underscored_word, first_letter_in_uppercase = true)
     if first_letter_in_uppercase
-      lower_case_and_underscored_word.to_s.gsub(/\/(.?)/) { "::" + $1.upcase }.gsub(/(^|_)(.)/) { $2.upcase }
+      lower_case_and_underscored_word.to_s.gsub(/\/(.?)/) { "::#{$1.upcase}" }.gsub(/(?:^|_)(.)/) { $1.upcase }
     else
       lower_case_and_underscored_word.first + camelize(lower_case_and_underscored_word)[1..-1]
     end