[ruby-core:117431] [Ruby master Misc#20407] Question about applying encoding modifier to an interpolated Regexp

["andrykonchin (Andrew Konchin) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20407 has been reported by andrykonchin (Andrew Konchin).

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Misc #20407: Question about applying encoding modifier to an interpolated Regexp
https://bugs.ruby-lang.org/issues/20407

* Author: andrykonchin (Andrew Konchin)
* Status: Open
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I am wondering how Regexp encoding modifiers (u, s, e, n) interfere in encoding negotiation of interpolated Regexp literal.

Examples #1

```ruby
# encoding: us-ascii

# Unicode: Ф - U+0424
# windows-1251: Ф - 0xD4

# without encoding modifier
puts /a #{ "\xd4".force_encoding("windows-1251") } c/.encoding    # Windows-1251
puts /a #{ "b".encode("windows-1251") } c/.encoding               # US-ASCII
puts /a #{ "\u0424".force_encoding("UTF-8") } c/.encoding         # UTF-8
puts /a #{ "\xc2\xa1".b } c/.encoding                             # ASCII-8BIT

# with encoding modifier
puts /a #{ "\xd4".force_encoding("windows-1251") } c/e.encoding   # Windows-1251
puts /a #{ "b".encode("windows-1251") } c/e.encoding              # EUC-JP
puts /a #{ "\u0424".force_encoding("UTF-8") } c/e.encoding        # UTF-8
puts /a #{ "\xc2\xa1".b } c/e.encoding                            # ASCII-8BIT

# string literals concatenation
puts ("a" + "\xd4".force_encoding("windows-1251") + "c").encoding # Windows-1251
puts ("a" + "b".encode("windows-1251") + "c").encoding            # US-ASCII
puts ("a" + "\u0424".force_encoding("UTF-8") + "c").encoding      # UTF-8
puts ("a" + "\xc2\xa1".b + "c").encoding                          # ASCII-8BIT
```

Example #2

```ruby
# encoding: utf-8

# windows-1251: Ф - 0xD4
# unicode: Ф - U+0424

# without encoding modifier
puts /a #{ "\xd4".force_encoding("windows-1251") } c/.encoding    # Windows-1251
puts /a #{ "b".encode("windows-1251") } c/.encoding               # US-ASCII
puts /a #{ "\u0424".force_encoding("UTF-8") } c/.encoding         # UTF-8
puts /a #{ "\xc2\xa1".b } c/.encoding                             # ASCII-8BIT

# with encoding modifier
puts /a #{ "\xd4".force_encoding("windows-1251") } c/e.encoding   # Windows-1251
puts /a #{ "b".encode("windows-1251") } c/e.encoding              # EUC-JP
puts /a #{ "\u0424".force_encoding("UTF-8") } c/e.encoding        # UTF-8
puts /a #{ "\xc2\xa1".b } c/e.encoding                            # ASCII-8BIT

# string literals concatenation
puts ("a" + "\xd4".force_encoding("windows-1251") + "c").encoding # Windows-1251
puts ("a" + "b".encode("windows-1251") + "c").encoding            # UTF-8
puts ("a" + "\u0424".force_encoding("UTF-8") + "c").encoding      # UTF-8
puts ("a" + "\xc2\xa1".b + "c").encoding                          # ASCII-8BIT
```

In the examples above the `e` modifier changes Regexp's encoding only in one case when Regexp's encoding would be `US-ASCII` without the modifier:

```ruby
# encoding: us-ascii

puts /a #{ "b".encode("windows-1251") } c/.encoding                        # US-ASCII
puts /a #{ "b".encode("windows-1251") } c/e.encoding                       # EUC-JP
```

```ruby
# encoding: utf-8

puts /a #{ "b".encode("windows-1251") } c/.encoding                        # US-ASCII
puts /a #{ "b".encode("windows-1251") } c/e.encoding                       # EUC-JP
```

And the `e` modifier doesn't change Regexp's final encoding in all the other cases either Regexp's encoding without modifier is a file source encoding or `ASCII-8BIT`.

Looking at the following example:

```ruby
# encoding: us-ascii

# without modifier
p /\xc2\xa1 #{ "a" }\xc2\xa1/.encoding                                 # ASCII-8BIT
p /a #{ "\xc2\xa1".force_encoding("EUC-JP") } b/.encoding              # EUC-JP
p /a #{ "\xc2\xa1".b } b/.encoding                                     # ASCII-8BIT

# with modifier
p /\xc2\xa1 #{ "a" }\xc2\xa1/e.encoding                                # EUC-JP
p /a #{ "\xc2\xa1".force_encoding("EUC-JP") } b/e.encoding             # EUC-JP
p /a #{ "\xc2\xa1".b } b/e.encoding                                    # ASCII-8BIT
```

we can notice that the `e` modifier change `ASCII-8BIT` to `EUC-JP` in the first case but doesn't in the third one. So I assume that the `e` modifier could be applied to the Regexp fragments (`\xc2\xa1` and `\xc2\xa1`) before encoding negotiation and not to the whole result after negotiation.

Could you please clarify how it works?



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