[ruby-core:117408] [Ruby master Misc#20406] Question about Regexp encoding negotiation

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

Issue #20406 has been reported by andrykonchin (Andrew Konchin).

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Misc #20406: Question about Regexp encoding negotiation
https://bugs.ruby-lang.org/issues/20406

* Author: andrykonchin (Andrew Konchin)
* Status: Open
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I am wondering what are the rules to calculate Regexp literal encoding in case an encoding modifier is specified.

1.

From the documentstion:

> By default, a regexp with only US-ASCII characters has US-ASCII encoding:
> ...
> A regular expression containing non-US-ASCII characters is assumed to use the source encoding. This can be overridden with one of the following modifiers.
> //n ...
> //u ...
> //e ...
> //s ...

Looking at the following examples I would assume that these rules are followed except one case:

```ruby
 p /\xc2\xa1/e     .encoding # EUC-JP
 p /#{ }\xc2\xa1/e .encoding # EUC-JP

 p /a/e            .encoding # EUC-JP
 p /a #{} a/e      .encoding # EUC-JP
 p /#{} a/e        .encoding # US-ASCII
```

The last Regexp `/#{} a/e` is supposed to have `EUC-JP` encoding but has `US-ASCII`. So I am wondering what rule is applied in this case.

2.

In case of interpolated Regexp with encoding modifier I suppose there is no any encoding negotiation as far as if a Regexp fragment's encoding doesn't match a fixed encoding a SyntaxError is raised. Is this assumption correct?

```ruby
/#{ "фв" } a/e
# regexp encoding option 'e' differs from source encoding 'UTF-8' (SyntaxError)
```



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