Re: Compiled regexp?

[Stefan Monnier]

>   ;; Concat in defconst
>   (defconst my-pattern (concat "^" "xyz"))
>   (re-search-forward my-pattern ...)
-vs-
>   ;; Concat in re-search-forward
>   (defconst my-partial-pattern "xyz")
>   (re-search-forward (concat "^" my-partial-pattern) ...)
[...]
> Can anyone confirm this is the case?

The second will incur the additional cost of the `concat' at each
iteration, obviously, but other than that the current Emacs code will
not take advantage of the fact that the same string is passed in the
first code, whereas a new string is use in the second.

I don't forsee the Emacs code changing such that it's significantly more
efficient to reuse the exact same string rather than a copy of it.

If we ever try to do something to avoid the cost of re-compiling
a regexp, I'd expect that we'd provide a way to have explicit access to
compiled regexps (with a `regexp-compile' function, the result of which
would be accepted by re-search-forward as an alternative to a string).


        Stefan "Who generally prefers the first form"