[ruby-core:122909] [Ruby Bug#21529] Deprecate the /o modifier and warn against using it

Issue #21529 has been updated by luke-gru (Luke Gruber).


This is also racy with ractors, and with multiple ractors is not guaranteed to run only once.

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Bug #21529: Deprecate the /o modifier and warn against using it
https://bugs.ruby-lang.org/issues/21529#change-114213

* Author: jpcamara (JP Camara)
* Status: Open
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
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I recently ran into a bug in some code because it was using the /o modifier as an optimization, not realizing it created a permanent, immutable value after the first time it gets evaluated. I dug into how the modifier works in CRuby and the history of it here: https://jpcamara.com/2025/08/02/the-o-in-ruby-regex.html.

The feature seems like a total footgun with almost no upside. If I run a benchmark between a local regex, and a regex cached by /o, there is no real difference.

```rb
require "benchmark"

def letters
  "A-Za-z"
end

words = %w[the quick brown fox jumped over the lazy dog]

Benchmark.bm do |bm|
  bm.report("without /o:") do
    regex = /\A[A-Za-z]+\z/
    words.each do |word|
      word.match(regex)
    end
  end

  bm.report("with /o:   ") do
    words.each do |word|
      word.match(/\A[#{letters}]+\z/o)
    end
  end
end
```

Most of the time I found that "without /o" actually came out ahead.

```
                 user     system      total        real
without /o:  0.000019   0.000003   0.000022 (  0.000014)
with /o:     0.000020   0.000001   0.000021 (  0.000020)
```

I'd like to deprecate the feature and update the docs to warn against using it. I'd be happy to submit a PR doing that.

Thanks!



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