Markdown reader: treat 4 or more * or _ in a row as literal text.

(Instead of trying to parse as strong or emph, which leads to
exponential performance problems.)

git-svn-id: https://pandoc.googlecode.com/svn/trunk@1634 788f1e2b-df1e-0410-8736-df70ead52e1b

src/Text/Pandoc/Readers/Markdown.hs

@@ -824,6 +824,7 @@ inlineParsers = [ str
                 , endline
                 , code
                 , charRef
+                , (fourOrMore '*' <|> fourOrMore '_')
                 , strong
                 , emph
                 , note
@@ -921,6 +922,12 @@ mathInline = try $ do
   notFollowedBy digit
   return $ intercalate " " words'
 
+-- to avoid performance problems, treat 4 or more _ or * in a row as a literal
+-- rather than attempting to parse for emph/strong
+fourOrMore :: Char -> GenParser Char st Inline
+fourOrMore c = try $ count 4 (char c) >> many (char c) >>= \s ->
+                       return (Str $ replicate 4 c ++ s)
+
 emph :: GenParser Char ParserState Inline
 emph = ((enclosed (char '*') (notFollowedBy' strong >> char '*') inline) <|>
         (enclosed (char '_') (notFollowedBy' strong >> char '_' >>