From: "jpcamara (JP Camara) via ruby-core" <ruby-core@...>
Date: 2025-08-03T12:52:35+00:00
Subject: [ruby-core:122901] [Ruby Bug#21529] Deprecate the /o modifier and warn against using it

Issue #21529 has been updated by jpcamara (JP Camara).


Byroot brought to my attention that my example doesn���t make a lot of sense because it doesn���t interpolate anything. 

I���m hard pressed to find an example to compare with dynamic interpolation, since I think that the core issue with /o is that dynamic interpolation doesn���t work the way anyone would ever expect. 

But here���s an example that precompiles a regex, which I think is the only comparison that is apples to apples, at its core (no pun intended)

```ruby

require "benchmark"

def letters
  "A-Za-z"
end

words = %w[the quick brown fox jumped over the lazy dog]
PRECOMPILED = /\A[#{letters}]+\z/.freeze

Benchmark.bm do |bm|
  bm.report("without /o:") do
    words.each do |word|
      word.match(PRECOMPILED)
    end
  end

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

The performance is the same again, but at least the example is slightly more relevant.

----------------------------------------
Bug #21529: Deprecate the /o modifier and warn against using it
https://bugs.ruby-lang.org/issues/21529#change-114204

* Author: jpcamara (JP Camara)
* Status: Open
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
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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