Markdown reader: remove restriction on identifiers...

so they no longer need to begin with a letter. Closes #7920.

src/Text/Pandoc/Readers/Markdown.hs

@@ -635,7 +635,7 @@ identifier = do
 identifierAttr :: PandocMonad m => MarkdownParser m (Attr -> Attr)
 identifierAttr = try $ do
   char '#'
-  result <- identifier
+  result <- T.pack <$> many1 (alphaNum <|> oneOf "-_:.") -- see #7920
   return $ \(_,cs,kvs) -> (result,cs,kvs)
 
 classAttr :: PandocMonad m => MarkdownParser m (Attr -> Attr)

test/command/7920.md

@@ -0,0 +1,20 @@
+```
+% pandoc -t native
+[hi]{#123}
+^D
+[ Para [ Span ( "123" , [] , [] ) [ Str "hi" ] ] ]
+```
+
+```
+% pandoc -t markdown
+[hi]{#123}
+^D
+[hi]{#123}
+```
+
+```
+% pandoc -t markdown
+# Hi {#123}
+^D
+# Hi {#123}
+```