Re: match-string debugging problem

[Stephen Berman <Stephen.Berman@gmx.net>]

On 10 Mar 2005 18:06:52 +0100 Pascal Bourguignon <spam@mouse-potato.com> wrote:

> Stephen Berman <Stephen.Berman@gmx.net> writes:
>
[...]
>>     (let ((mystring-list ()))
>>       (while (re-search-forward
>> 	      (concat "^" (regexp-quote mystring1) "\\(.+\\)"
>> 		      (regexp-quote mystring2) "$")
>> 	      (point-max) t)
>> 	(setq mystring-list (append (list (match-string 1)) mystring-list)))
>>       (insert "\n")
>>       (setq mystring-list (reverse mystring-list))
>>       (dolist (elt mystring-list)
>> 	(insert elt " ")))))
>> 
>> After evalling this code and typing `M-x mystring-list', buffer *test*
>> consists of these lines:
>> 
>> +++++ test1 ~~~~~
>> +++++ test2 ~~~~~
>> +++++ test3 ~~~~~
>> +++++ test4 ~~~~~
>> +++++ test5 ~~~~~
>> test1 test2 test3 test4 test5 
>> 
>> The last line indicates that match-string correctly matches the
>> strings that build mystring-list.  But when I step through the code
>> with edebug, match-string always returns nil and a wrong-type-argument
>> error is raised at the insert (since nil is not char-or-string-p).
>> (Edebug isn't the problem: evalling first the regexp search code in
>> *test* and then (match-string 1) also returns nil.)  Because of this
>> I'm having a hard time debugging other code that uses match-string.
>> Can someone explain what's going on?
>
> re-search-forward uses global state (buffer, matched range, etc) as
> match-data, to communicate with match-string.  When you're debugging,
> this global state is switched or modified.  One could consider it a
> bug in the debugger.
>
> See: match-data
>      save-match-data   
>      save-excursion
>      save-buffer

How do you know about this state difference?  I couldn't find any
mention of it in the documentation of the functions you mention, nor
elsewhere in the Elisp manual (CVS version).  I glanced through the
comments in search.c but didn't see anything obviously (to me)
relevant.  If it's defined in the C source code I'd appreciate a
pointer, even though I probably wouldn't be able to understand it at
present.

In any case, what I did find is that I can in fact track match-string
by explicitly entering the ordinary Lisp debugger at that point.  So
putting `(debug)' before `(match-string 1)' in the above code stops
execution there and stepping into the function by pressing `d' returns
the matched string.  So if there's a bug, then it's only in Edebug.  I
couldn't find any relevant discussion in the Edebug section of the
manual.  So at the very least, the Elisp documentation could be more
explicit on this issue.

Steve Berman