Re: parens matching not matching all matching parens

[Greg Hill <ghill@synergymicro.com>]

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At 9:19 PM +0200 9/16/04, Arjan Bos wrote:
>Hi all,
>
>I'm currently developing yet another rich text format writer. And as 
>you might know, RTF is using curly braces a lot. Alas, every now and 
>then, a normal parentheses pops up between a set of matching {}. 
>Both the parens matching colouring and the forward-sexp / 
>backward-sexp can't handle this. How can I (help to) solve this?
>
>An rtf snippet is included here:
>
>{\rtf1\mac\ansicpg10000\uc1
>{\*\pnseclvl4\pnlcltr\pnstart1\pnindent720\pnhang{\pntxta )}}}
>
>This is a complete piece of (non-sensical, but correct) rtf. The 
>first `{' matches the last `}'. Only C-M-f jumps from the first `{' 
>to the one-to-last `}'.
>
>I'm wondering if this is a bug or not.
>
>TIA,
>
>Arjan

Arjan,

Personally, I would call it a bug in the 'scan-sexps built-in 
function, which is called by 'forward-sexp, which is normally bound 
to "C-M-f".  For some reason 'scan-sexps seems to treat a ")" as 
matching a "{".  You might want to submit a bug report, but there is 
no guanentee the maintainers of Emacs will not call that odd behavior 
a "feature."

On the other hand, if you just want to get on with life, you can try 
putting something like the following code in your .emacs file.  The 
'defun defines a function that scans forward for the first '(', '[', 
'<' or '{' following point, whatever it see first, then scans for a 
matching closing character, ignoring any other characters.  The 
'(setq forward-sexp-function... effectively replaces the guts of 
'forward-sexp with the new function, so you will not have to rebind 
your keystroke.  It works in the backward direction as well ("C-M-b").

(defun forward-pexp (&optional arg)
   (interactive "p")
   (or arg (setq arg 1))
   (let (open close next notstrc notstro notstre depth)
     (catch 'done
       (cond ((> arg 0)
	     (skip-chars-forward "^([{<")
	     (setq open (char-after))
	     (cond ((eq open ?\()
		    (setq close ?\)))
		   ((eq open ?\[)
		    (setq close ?\]))
		   ((eq open ?\{)
		    (setq close ?\}))
		   ((eq open ?\<)
		    (setq close ?\>))
		   (t
		    (throw 'done nil) ) )
	     (setq notstro (concat "^" (char-to-string open))
		   notstre (concat notstro (char-to-string close)) )
	     (while (and (> arg 0) (not (eobp)))
	       (skip-chars-forward notstro)
	       (forward-char 1)
	       (setq depth 1)
	       (while (and (> depth 0) (not (eobp)))
		 (skip-chars-forward notstre)
		 (setq next (char-after))
		 (cond ((eq next open)
			(setq depth (1+ depth)) )
		       ((eq next close)
			(setq depth (1- depth)) )
		       (t
			(throw 'done nil) ) )
		 (forward-char 1) )
	       (setq arg (1- arg) ) ) )
	    ((< arg 0)
	     (skip-chars-backward "^)]}>")
	     (setq close (char-before))
	     (cond ((eq close ?\))
		    (setq open ?\())
		   ((eq close ?\])
		    (setq open ?\[))
		   ((eq close ?\})
		    (setq open ?\{))
		   ((eq close ?\>)
		    (setq open ?\<))
		   (t
		    (throw 'done nil) ) )
	     (setq notstrc (concat "^" (char-to-string close))
		   notstre (concat notstrc (char-to-string open)) )
	     (while (and (< arg 0) (not (bobp)))
	       (skip-chars-backward notstrc)
	       (forward-char -1)
	       (setq depth 1)
	       (while (and (> depth 0) (not (bobp)))
		 (skip-chars-backward notstre)
		 (setq next (char-before))
		 (cond ((eq next close)
			(setq depth (1+ depth)) )
		       ((eq next open)
			(setq depth (1- depth)) )
		       (t
			(throw 'done nil) ) )
		 (forward-char -1) )
	       (setq arg (1+ arg)) ) )	) ) ))

(setq forward-sexp-function 'forward-pexp)

I should warn you, by the way, that I just hacked this code together 
in a few minutes after work tonight, and have not tested it 
exhuastively.  Let me know if you have any questions or problems.

--Greg

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