Bug #21558: Regexp.union with multiple regexps returns a non-shareable object (should be shareable if possible)

• Status: Feedback
• Assignee: -
• Target version: -
• ruby -v: ruby 3.5.0dev (2025-08-30T18:24:25Z master 5c7dfe85a1) +PRISM [arm64-darwin24]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[niku (niku _)]

Regexp.union returns a non-shareable object when given multiple Regex arguments. Is this expected behavior?
It would be preferable if the resulting Regexp were shareable, since single-argument Regexp.union does produce a shareable object.

irb(main):001> a = /a/
=> /a/
irb(main):002> b = Regexp.union(a)
=> /a/
irb(main):003> c = Regexp.union(a, a)
=> /(?-mix:a)|(?-mix:a)/
irb(main):004> Ractor.shareable?(a)
=> true
irb(main):005> Ractor.shareable?(b)
=> true
irb(main):006> Ractor.shareable?(c)
=> false

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Feedback

Regexp.union creates a new instance, except for a single Regexp argument case.
And it is impossible to revert a Ractor-shareable object to non-Ractor-shareable.
So it feels natural that the new instance is not Ractor-shareable as well as String.new and others, to me.

[nobu (Nobuyoshi Nakada)]

Another example is Range.
All instances of Range are automatically frozen so they are Ractor-shareable.
Personally, I think freezing all Regexp instances also may be nice, but that's a different story than Regexp.union.

[niku (niku _)]

Regexp.union creates a new instance, except for a single Regexp argument case.

Ah, that makes perfect sense now. Thank you for the detailed explanation.

irb(main):001> a = /a/
=> /a/
irb(main):002> a.__id__
=> 7232
irb(main):003> b = Regexp.union(a)
=> /a/
irb(main):004> b.__id__
=> 7232
irb(main):005> c = Regexp.union(a, a)
=> /(?-mix:a)|(?-mix:a)/
irb(main):006> c.__id__
=> 17912

Personally, I think freezing all Regexp instances also may be nice, but that's a different story than Regexp.union.

I agree with your opinion. I'll create a new issue.

[niku (niku _)]

I've found an issue that already talked about it https://bugs.ruby-lang.org/issues/17256.

[znz (Kazuhiro NISHIYAMA)]

• Related to Feature #17256: Freeze all Regexp objects added

[Eregon (Benoit Daloze)]

• Related to Feature #8948: Frozen regex added

[Eregon (Benoit Daloze)]

There is also #8948. I didn't get time to try it, but anyone is welcome to try and find if there are any failures in CI/the test suites and if so how to fix them.