[ruby-core:115084] [Ruby master Bug#19932] Regexp o modifier freeze interpolation between method calls

18 Oct 2023

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

I'm not sure I was clear.

It's not that clear reading the documentation that the "freeze" will continue to live outside the local scope. All examples are always outside a method definition. As for local variables I would expect `myregexp = /#{myvar}/o` to be re-evaluated between several method calls. By reading the doc I understood the regexp object will be frozen in a local context (e.g. in a loop) but if it's I put that in a method I understood the regexp object will be frozen locally during the method call but that it will be dynamically re-evaluated in another method call then re-frozen for that new call duration.

If that's still the expected behavior, it could be nice then to introduce another modifier that will unfroze the regexp object when leaving the method call so that a use case like in the PoC would work. Because there are many use case where you want a regexp object to be frozen for example because you have a loop with 1M items where the regexp will be called o neach item but you still want to be able to call the method again with different arguments.

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Bug #19932: Regexp o modifier freeze interpolation between method calls
https://bugs.ruby-lang.org/issues/19932#change-104956

* Author: noraj-acceis (Alexandre ZANNI)
* Status: Closed
* 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
----------------------------------------
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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