[ruby-core:115082] [Ruby master Bug#19932] Regexp o modifier freeze interpolation between method calls

18 Oct 2023

      Issue #19932 has been reported by noraj-acceis (Alexandre ZANNI).

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Bug #19932: Regexp o modifier freeze interpolation between method calls
https://bugs.ruby-lang.org/issues/19932

* Author: noraj-acceis (Alexandre ZANNI)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
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Taken the following PoC:

```ruby
def poc(regexp)
  hs = [
    'azerty',
    'azertyui',
    'azertyuiop'
  ]
  out = []
  hs.each do |h|
    out << h if /#{regexp}/.match?('azerty')
  end
  out
end

p poc('az')
p poc('wxc')
```

I have the following output:

```ruby
["azerty", "azertyui", "azertyuiop"]
[]
```

Now because the regexp never change inside the method once set I could add the `o` modifier so that `#{}` interpolation is computed only once.

So the PoC becomes:

```ruby
def poc(regexp)
  hs = [
    'azerty',
    'azertyui',
    'azertyuiop'
  ]
  out = []
  hs.each do |h|
    out << h if /#{regexp}/o.match?('azerty')
  end
  out
end

p poc('az')
p poc('wxc')
```

Output:

```
["azerty", "azertyui", "azertyuiop"]
["azerty", "azertyui", "azertyuiop"]
```

So each future call to the method will be equals to the result of the first call because the regexp is "frozen".

I was expecting the regexp to be "frozen" only for the current call in an effort of performance but instead it's "frozen" for future calls too.

Another example of usage of `o` modifier here: https://learnbyexample.github.io/Ruby_Regexp/modifiers.html#o-modifier

So is that a side effect or is that the expected behavior?

-- 
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