Re: A function to take the regexp-matched subsring directly

[Stefan Monnier <monnier@iro.umontreal.ca>]

> +;;;###autoload
> +(defun regexp-match (regexp string &optional n)
> +  "Return the N -th matched substring for REGEXP in STRING.
> +N defaults to 0 (the whole match).
> +
> +This function does not change the match data."
> +  (declare (pure t) (side-effect-free t))
> +  (let ((n (or n 0)))
> +    (save-match-data
> +      (when (string-match regexp string)
> +        (match-string n string)))))

`save-match-data` is costly and extremely rarely needed.
So I'd much rather not save it here.

> +  (save-match-data
> +    (when (string-match regexp string)
> +      (let ((match-index (1- (/ (length (match-data)) 2)))
> +            matches)
> +        (while (<= 0 match-index)
> +          (push (match-string match-index string) matches)
> +          (setq match-index (1- match-index)))
> +        matches))))

I suspect it'd be more efficient to iterate directly on the `match-data` rather
than on an integer (which suffers from an O(N²) complexity).


        Stefan