[ruby-core:117568] [Ruby master Misc#20434] Deprecate regular expression modifiers

["kddnewton (Kevin Newton) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20434 has been reported by kddnewton (Kevin Newton).

----------------------------------------
Misc #20434: Deprecate regular expression modifiers
https://bugs.ruby-lang.org/issues/20434

* Author: kddnewton (Kevin Newton)
* Status: Open
----------------------------------------
This is a follow-up to @duerst's comment here: https://bugs.ruby-lang.org/issues/20406#note-6.

As noted in the other issue, there are many encodings that factor in to how a regular expression operates. This includes:

* The encoding of the file
* The encoding of the string parts within the regular expression
* The regular expression encoding modifiers
* The encoding of the string being matched

At the time the modifiers were introduced, I believe the modifiers may have been the only (??) encoding that factored in here. At this point, however, they can lead to quite a bit of confusion, as noted in the other ticket.

I would like to propose to deprecate the regular expression encoding modifiers. Instead, we could suggest in a warning to instead create a regular expression with an encoded string. For example, when we find:

```ruby
/\x81\x40/s
```

we would instead suggest:

```ruby
::Regexp.new(::String.new("\x81\x40", encoding: "Windows-31J"))
```

or equivalent. As a migration path, we could do the following:

1. Emit a warning to change to the suggested expression
2. Change the compiler to compile to the suggested expression when those flags are found
3. Remove support for the flags

Step 2 may be unnecessary depending on how long of a timeline we would like to provide. To be clear, I'm not advocating for any particular timeline, and would be fine with this being multiple years/versions to give plenty of time for people to migrate. But I do think this would be a good change to eliminate confusion about the interaction between the four different encodings at play.



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