[ruby-core:117437] [Ruby master Misc#20406] Question about Regexp encoding negotiation

[duerst via ruby-core <ruby-core@ml.ruby-lang.org>]

Issue #20406 has been updated by duerst (Martin Dürst).


This is a more general comment, but my impression is that the encoding flags on regular expressions may be outdated. They exist since before Ruby introduced encoding information for Strings,... in Ruby 1.9. It may be time now to look into how/when they can be deprecated.

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Misc #20406: Question about Regexp encoding negotiation
https://bugs.ruby-lang.org/issues/20406#change-107813

* 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.


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.



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