[ruby-core:123159] [Ruby Bug#21558] Regexp.union with multiple regexps returns a non-shareable object (should be shareable if possible)

Issue #21558 has been updated by 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.

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Bug #21558: Regexp.union with multiple regexps returns a non-shareable object (should be shareable if possible)
https://bugs.ruby-lang.org/issues/21558#change-114495

* Author: niku (niku _)
* Status: Feedback
* 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
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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
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
```





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