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Re: A function to take the regexp-matched subsring directly
From: |
Philip Kaludercic |
Subject: |
Re: A function to take the regexp-matched subsring directly |
Date: |
Sun, 30 Oct 2022 17:29:21 +0000 |
Stefan Monnier <monnier@iro.umontreal.ca> writes:
>> +;;;###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.
What makes it so expensive? The implementation appears to be trivial.
> So I'd much rather not save it here.
If the function is supposed to be side-effect-free, then it ought not to
sometimes replace the match data and not touch it when optimised away.
>> + (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