diff --git a/lib/redcloth3.rb b/lib/redcloth3.rb
index d720af9d8..7c9df0727 100644
--- a/lib/redcloth3.rb
+++ b/lib/redcloth3.rb
@@ -1078,7 +1078,7 @@ class RedCloth3 < String
                         line = "<redpre##{ @pre_list.length }>"
                         first.match(/<#{ OFFTAGS }([^>]*)>/)
                         tag = $1
-                        $2.to_s.match(/(class\=\S+)/i)
+                        $2.to_s.match(/(class\=("[^"]+"|'[^']+'))/i)
                         tag << " #{$1}" if $1
                         @pre_list << "<#{ tag }>#{ aftertag }"
                     end
diff --git a/test/unit/helpers/application_helper_test.rb b/test/unit/helpers/application_helper_test.rb
index 02b15fbf5..6327442c6 100644
--- a/test/unit/helpers/application_helper_test.rb
+++ b/test/unit/helpers/application_helper_test.rb
@@ -275,6 +275,9 @@ RAW
       "<!-- opening comment" => "<p>&lt;!-- opening comment</p>",
       # remove attributes except class
       "<pre class='foo'>some text</pre>" => "<pre class='foo'>some text</pre>",
+      '<pre class="foo">some text</pre>' => '<pre class="foo">some text</pre>',
+      "<pre class='foo bar'>some text</pre>" => "<pre class='foo bar'>some text</pre>",
+      '<pre class="foo bar">some text</pre>' => '<pre class="foo bar">some text</pre>',
       "<pre onmouseover='alert(1)'>some text</pre>" => "<pre>some text</pre>",
     }
     to_test.each { |text, result| assert_equal result, textilizable(text) }