Re: [Orgmode] proposal: defconst/defcustom org-tags-regexp

[Carsten Dominik]

On Jul 19, 2007, at 15:08, Eddward DeVilla wrote:

> On 7/18/07, Carsten Dominik <address@hidden> wrote:
> > Do you know a solution for this problem?
>
> Does emacs let you manually compile a regular expression?  If so, it
> might be possible to recompile REs when ever they change.

This is not the issue.  Yes, Emacs compiles regular expressions
whenever necessary.  However, consider the following loop:

   (while (re-search-forward "^aaa" nil t)
     (do-something)

In this case, Emacs will compile the regexp once and then use
it potentially many times.

However, in the following case:

   (while (re-search-forward (concat "^" some-variable) nil t)
      (do-something)

The regular expression wil be re-compiled all the time.
In this case you can work around it by doing:

   (let ((re (concat "^" some-variable)))
     (while (re-search-forward re nil t)
       (do-something...))

But if the loop is calling functions, and the regular expression
is being built and used in those functions, this might slow down
things.

Now, I am not sure how slow actually, I am basically following the
arguments in Jeffrey E. F. Friedl's excellent book
"Mastering Regular Expressions", but I am not sure how large the
effect will be in the end.  I remember that in perl this was 
significant,
so I am assuming it is in Emacs.

I would like to factor out more regexps into variables, and may do
so with time - but this does not have priority.

- Carsten



--
Carsten Dominik
Sterrenkundig Instituut "Anton Pannekoek"
Universiteit van Amsterdam
Kruislaan 403
NL-1098SJ Amsterdam
phone: +31 20 525 7477