Subject: Re: bug#34641: rx: (or ...) order unpredictable

From: Noam Postavsky <[email-masked]>
Date: Sun, 24 Feb 2019 21:37:56 -0500

On Sun, 24 Feb 2019 at 13:41, Mattias Engdegård <[email-masked]> 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.