Re: Possible problem with looking-back function

["Andreas Röhler" <andreas.roehler@online.de>]

Am 19.08.2010 11:02, schrieb Stephen J. Turnbull:
> Andreas Röhler writes:
>
>   >  that surprises me.  Check must be done against the string already
>   >  found.  So workload should grow lineary resp. slightly ascending.
>
> That's incorrect.  In regexp matching abstractly defined, there is no
> "string already matched."  In general backtracking must be done to get
> a correct POSIX match, and it's potentially very expensive.  It would
> be sometimes possible (as in this case) to identify a non-backtracking
> algorithm as you suggest, but that would mean that different regexps
> would be treated differently, or that some regexps would be
> ridiculously expensive.
>
>    

Hi,

just for the curious:
Coming upon the matter, as in markup languages point may be inside the 
start tag.
Solved it whith function below

--begstr is the startup-tag
Sometimes begstr is used inside beg-end function quoted: (regexp-quote 
begstr),
therefor the startup tag is delivered in both forms here, making 
looking-at working
- not sure, if its really necessary btw--


(defun ar-leave-begstr-backward (begstr unquoted-beg)
   (let* ((stringcount (length unquoted-beg))
          (collected (char-to-string (char-after)))
          (indx (string-match (regexp-quote collected) unquoted-beg)))
     (while (and indx (not (ignore-errors (looking-at begstr)))(< 1 
stringcount))
       (forward-char -1)
       (setq collected (concat (char-to-string (char-after)) collected))
       (setq indx (string-match (regexp-quote collected) unquoted-beg))
       (setq stringcount (1- stringcount)))))

See beg-end.el

at

https://code.launchpad.net/s-x-emacs-werkstatt/