Bug #21529
openDeprecate the /o modifier and warn against using it
Description
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.
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!
Updated by jpcamara (JP Camara) 1 day ago
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)
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.
Updated by byroot (Jean Boussier) 1 day ago
My point was that /o
is only "optimized" compared to the same interpolation but uncached, so:
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
words.each do |word|
word.match(/\A[#{letters}]+\z/)
end
end
bm.report("with /o: ") do
words.each do |word|
word.match(/\A[#{letters}]+\z/o)
end
end
end
But yes, in the overwhelming majority of cases, you are much better to explicitly only performance the interpolation once and store the resulting regexp in a constant.
/o
is definitely a footgun, but also a very rare construct. In my opinion improving the documentation is more than welcome, but I'm not convinced deprecating is worth it, as I assume the problems come from people seeing it in the docs and misunderstanding the docs.
Updated by jpcamara (JP Camara) 1 day ago
Yea I hear ya. So should I just submit a PR with my suggestions for the docs and close this?
Updated by byroot (Jean Boussier) about 8 hours ago
So should I just submit a PR with my suggestions for the docs
Sure. I think updating the code snippets, and suggesting to use a constant could go a long way.