FriendlyId::SlugGenerator#wildcard must escape underscores.

lib/friendly_id/slug_generator.rb

@@ -65,7 +65,10 @@ def separator
     end
 
     def wildcard
-      "#{normalized}#{separator}%"
+      # Underscores (matching a single character) and percent signs (matching
+      # any number of characters) need to be escaped
+      # (While this seems like an excessive number of backslashes, it is correct)
+      "#{normalized}#{separator}".gsub(/[_%]/, '\\\\\&') + '%'
     end
   end
 end

test/slugged_test.rb

@@ -10,6 +10,15 @@ class Article < ActiveRecord::Base
   friendly_id :name, :use => :slugged
 end
 
+class Novelist < ActiveRecord::Base
+  extend FriendlyId
+  friendly_id :name, :use => :slugged, :sequence_separator => '_'
+
+  def normalize_friendly_id(string)
+    super.gsub("-", "_")
+  end
+end
+
 class SluggedTest < MiniTest::Unit::TestCase
 
   include FriendlyId::Test
@@ -112,6 +121,14 @@ def model_class
     end
   end
 
+  test "should correctly sequence slugs with underscores" do
+    transaction do
+      record1 = Novelist.create! :name => 'wordsfail, buildings tumble'
+      record2 = Novelist.create! :name => 'word fail'
+      assert_equal 'word_fail', record2.slug
+    end
+  end
+
 end
 
 class SlugSeparatorTest < MiniTest::Unit::TestCase