diff --git a/lib/jekyll/converters/markdown.rb b/lib/jekyll/converters/markdown.rb
index 7633ccf7..e3c89934 100644
--- a/lib/jekyll/converters/markdown.rb
+++ b/lib/jekyll/converters/markdown.rb
@@ -91,7 +91,7 @@ module Jekyll
 
       private
       def custom_class_allowed?(parser_name)
-        parser_name !~ /[^A-Za-z0-9]/ && self.class.constants.include?(
+        parser_name !~ /[^A-Za-z0-9_]/ && self.class.constants.include?(
           parser_name.to_sym
         )
       end