[ruby-core:117595] [Ruby master Misc#20434] Deprecate encoding-releated regular expression modifiers

Issue #20434 has been updated by byroot (Jean Boussier).


`/\x81\x40/.force_encoding("Windows-31J")` wouldn't work because `String#fo=
rce_encoding` mutates the string, and Regexp literals are immutable.

Similarly `String#encode` doesn't just change the string encoding attribute=
, but convert the bytes to the new encoding. So I'd expect `/\3000/.encode(=
"Windows-31J")` to fail with:

```ruby
\x81" on UTF-8 (Encoding::InvalidByteSequenceError)
```

So I think the String API to mirror would be `String.new(encoding:)`

   - `Regexp.new(/\x81\x40/, encoding: Encoding::WINDOWS_31J)`
   - `Regexp.new("\x81\x40", encoding: Encoding::WINDOWS_31J)`

But if we want an instance method, I think something like:

`/\x81\x40/.encoded(Encoding::WINDOWS_31J)`, which by the way would also be=
 useful on `String`, e.g., this is common:


```ruby
# frozen_string_literal: true
THING =3D "f=E9e".dup.force_encoding(Encoding::ISO8859_1)
```

So it could become:

```ruby
# frozen_string_literal: true
THING =3D "f=E9e".encoded(Encoding::ISO8859_1)
```


----------------------------------------
Misc #20434: Deprecate encoding-releated regular expression modifiers
https://bugs.ruby-lang.org/issues/20434#change-107997

* Author: kddnewton (Kevin Newton)
* Status: Open
----------------------------------------
This is a follow-up to @duerst's comment here: https://bugs.ruby-lang.org/i=
ssues/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 modifi=
ers. Instead, we could suggest in a warning to instead create a regular exp=
ression 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 fl=
ags 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, a=
nd 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 e=
liminate confusion about the interaction between the four different encodin=
gs at play.



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