From: "Mattias Engdegård" <mattiase@acm.org>
To: 34641@debbugs.gnu.org
Subject: bug#34641: rx: (or ...) order unpredictable
Date: Sun, 24 Feb 2019 19:40:33 +0100 [thread overview]
Message-ID: <836B8DC2-9358-40AC-83AF-7C4D960D9A53@acm.org> (raw)
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?)
next reply other threads:[~2019-02-24 18:40 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-24 18:40 Mattias Engdegård [this message]
2019-02-24 19:06 ` bug#34641: rx: (or ...) order unpredictable Eli Zaretskii
2019-02-24 21:18 ` Mattias Engdegård
2019-02-24 22:44 ` Basil L. Contovounesios
2019-02-25 14:26 ` Mattias Engdegård
2019-03-02 12:33 ` Eli Zaretskii
2019-03-02 14:05 ` Mattias Engdegård
2019-03-02 14:08 ` Mattias Engdegård
2019-03-02 14:23 ` Eli Zaretskii
2019-03-02 14:37 ` Mattias Engdegård
2019-03-02 23:48 ` Phil Sainty
2019-03-03 8:54 ` Mattias Engdegård
2019-03-07 9:00 ` Phil Sainty
2019-02-25 2:37 ` Noam Postavsky
2019-02-25 9:56 ` Mattias Engdegård
2019-02-25 14:43 ` Noam Postavsky
2019-02-25 14:48 ` Mattias Engdegård
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