bug#30568: 27.0.50; `rx' doesn't create optimal regex for (group (or ...))

bug#30568: 27.0.50; `rx' doesn't create optimal regex for (group (or ...))

[p.stephani2]

emacs -Q -batch --eval=3D'(progn (princ (rx (group (or "aaa" "bbb")))) (ter=
pri))'
=3D=3D> \(\(?:aaa\|bbb\)\)

This should generate \(aaa\|bbb\) instead.  Of course, these regexes are
equivalent, but the second one is easier to read (and maybe faster).


In GNU Emacs 27.0.50 (build 16, x86_64-pc-linux-gnu, GTK+ Version 3.22.24)
  of 2018-02-21 built on localhost
Repository revision: d599dce1353ce59d134fcff21cde02c70025253d
Windowing system distributor 'The X.Org Foundation', version 11.0.11903000
System Description: Debian GNU/Linux buster/sid

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

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Memory information:
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bug#30568: 27.0.50; `rx' doesn't create optimal regex for (group (or ...))

[Mattias Engdegård]

> (rx (group (or "aaa" "bbb")))
> ==> \(\(?:aaa\|bbb\)\)
>
> This should generate \(aaa\|bbb\) instead.  Of course, these regexes are
> equivalent, but the second one is easier to read (and maybe faster).

This remains unchanged, I'm afraid, despite rx being completely rewritten. Not that it matters much: brackets do not generate any regexp bytecode, thus matching performance isn't affected once the regexp has been compiled. When the brackets are required, there is no waste:

(rx (+ (or "aaa" "bbb"))) 
=> "\\(?:aaa\\|bbb\\)+"

Still, it's a bit untidy, and I like that you reported it. We could add a special value for the PAREN argument to regexp-opt to prevent bracketing altogether, I suppose. It isn't immediately on my to-do list, however.