Re: Compiled regexp?

[Andreas Schwab <schwab@linux-m68k.org>]

Bastien <bzg@gnu.org> writes:

> After Christopher submitted a patch for org-mode, Carsten and him
> discussed the difference between these two patterns:
>
>   ;; Concat in defconst
>   (defconst my-pattern (concat "^" "xyz"))
>   (re-search-forward my-pattern ...)
>
>   ;; Concat in re-search-forward
>   (defconst my-partial-pattern "xyz")
>   (re-search-forward (concat "^" my-partial-pattern) ...)
>
> Both Carsten and I thought there was some optimization done
> by Emacs so that the first pattern is more efficient than the
> second one.  (concat "^" "xyz") would be "cached", not eval'ed
> each time you search for my-pattern.

The first pattern evaluates (concat "^" "xyz") once when my-pattern is
defined.  The second case evalutes (concat "^" my-partial-pattern) each
time the containing sexp is evaluated.  Other than that there is no
difference wrt. re-search-forward.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."