diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 361571db98..35be823ea1 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Fixed a problem where String#truncate_words would get stuck with a complex
+    string.
+
+    *Henrik Nygren*
+
 *   Fixed a roundtrip problem with AS::SafeBuffer where primitive-like strings
     will be dumped as primitives:
 
diff --git a/activesupport/lib/active_support/core_ext/string/filters.rb b/activesupport/lib/active_support/core_ext/string/filters.rb
index 096292dc58..b88976eab2 100644
--- a/activesupport/lib/active_support/core_ext/string/filters.rb
+++ b/activesupport/lib/active_support/core_ext/string/filters.rb
@@ -93,7 +93,7 @@ class String
   def truncate_words(words_count, options = {})
     sep = options[:separator] || /\s+/
     sep = Regexp.escape(sep.to_s) unless Regexp === sep
-    if self =~ /\A((?:.+?#{sep}){#{words_count - 1}}.+?)#{sep}.*/m
+    if self =~ /\A((?>.+?#{sep}){#{words_count - 1}}.+?)#{sep}.*/m
       $1 + (options[:omission] || '...')
     else
       dup
diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index 24037c665a..cb24147ab3 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -249,6 +249,15 @@ class StringInflectionsTest < ActiveSupport::TestCase
     assert_equal "Hello<br>Big<br>World!", "Hello<br>Big<br>World!".truncate_words(3, :omission => "[...]", :separator => '<br>')
   end
 
+  def test_truncate_words_with_complex_string
+    Timeout.timeout(10) do
+      complex_string = "aa aa aaa aa aaa aaa aaa aa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaaa aaaaa aaaaa aaaaaa aa aa aa aaa aa  aaa aa aa aa aa a aaa aaa \n a aaa <<s"
+      assert_equal complex_string.truncate_words(80), complex_string
+    end
+  rescue Timeout::Error
+    assert false
+  end
+
   def test_truncate_multibyte
     assert_equal "\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 ...".force_encoding(Encoding::UTF_8),
       "\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 \354\225\204\353\235\274\353\246\254\354\230\244".force_encoding(Encoding::UTF_8).truncate(10)