diff --git a/activesupport/CHANGELOG b/activesupport/CHANGELOG
index 15974628bbb4d..e265345b356a1 100644
--- a/activesupport/CHANGELOG
+++ b/activesupport/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Fix unicode JSON regexp for Onigurama compatibility.  #6494 [whitley]
+
 * update XmlSimple to 1.0.10. Closes #6532. [nicksieger]
 
 * Update dependencies to allow constants to be defined alongside their siblings. A common case for this is AR model classes with STI; user.rb might define User, Administrator and Guest for example. [Nicholas Seckar]
diff --git a/activesupport/lib/active_support/json/encoders/core.rb b/activesupport/lib/active_support/json/encoders/core.rb
index 483471ee3ea18..d8571be9c8692 100644
--- a/activesupport/lib/active_support/json/encoders/core.rb
+++ b/activesupport/lib/active_support/json/encoders/core.rb
@@ -32,7 +32,7 @@ module Encoders #:nodoc:
           ESCAPED_CHARS[s]
         }.gsub(/([\xC0-\xDF][\x80-\xBF]|
                  [\xE0-\xEF][\x80-\xBF]{2}|
-                 [\xF0-\xF7][\x80-\xBF]{3})+/ux) { |s|
+                 [\xF0-\xF7][\x80-\xBF]{3})+/nx) { |s|
           s.unpack("U*").pack("n*").unpack("H*")[0].gsub(/.{4}/, '\\\\u\&')
         } + '"'
       end