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bug#34641: rx: (or ...) order unpredictable
From: |
Noam Postavsky |
Subject: |
bug#34641: rx: (or ...) order unpredictable |
Date: |
Sun, 24 Feb 2019 21:37:56 -0500 |
On Sun, 24 Feb 2019 at 13:41, Mattias Engdegård <mattiase@acm.org> wrote:
>
> 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.
I don't see the problem, isn't "ab?" semantically equivalent to
"ab\\|a" (and "a\\|ab")?
> (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.
This sounds familiar, though I can't locate a report for it.