GNU bug report logs - #34641
rx: (or ...) order unpredictable

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#34641: closed (rx: (or ...) order unpredictable)
Date: Sat, 02 Mar 2019 14:38:02 +0000

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Your message dated Sat, 2 Mar 2019 15:37:26 +0100
with message-id <CB3D3586-6548-4673-85AF-064A93F7E142 <at> acm.org>
and subject line Re: bug#34641: rx: (or ...) order unpredictable
has caused the debbugs.gnu.org bug report #34641,
regarding rx: (or ...) order unpredictable
to be marked as done.

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help-debbugs <at> gnu.org.)


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From: Mattias Engdegård <mattiase <at> acm.org>
To: bug-gnu-emacs <at> gnu.org
Subject: rx: (or ...) order unpredictable
Date: Sun, 24 Feb 2019 19:40:33 +0100

The rx (or ...) construct sometimes reorders its subexpressions, which makes its semantics unpredictable. For example,

(rx (or "ab" "a") (or "a" "ab"))
=>
"\\(?:ab?\\)\\(?:ab?\\)"

The user reasonably expects (or e1 e2) to translate to E1\|E2, where ei translates to Ei, or a semantic equivalent. Not having this control makes rx useless or dangerous for many purposes.

The reason for the reordering is the use of regex-opt behind the scenes. Whether rx is the place to do this kind of optimisation is a matter of opinion; mine is that it belongs in the regexp engine, together with other, more aggressive optimisations (DFA, native-code generation, etc) could be performed as well.

We could determine whether any string is a prefix of another. If not, regexp-opt should be safe to call. Alternatively, this check could be done in regexp-opt (activated by a flag). That would be my preferred short-term solution.

(Speaking of regexp-opt, it has another bug that does not affect rx: it returns the empty string if given an empty list of strings. The correct return value is a regexp that never matches anything. Fix it, document it, or turn it into an error?)

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From: Mattias Engdegård <mattiase <at> acm.org>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 34641-done <at> debbugs.gnu.org
Subject: Re: bug#34641: rx: (or ...) order unpredictable
Date: Sat, 2 Mar 2019 15:37:26 +0100

2 mars 2019 kl. 15.23 skrev Eli Zaretskii <eliz <at> gnu.org>:
> 
> LGTM, thanks.

Thank you, pushed.

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