[ruby-core:117441] [Ruby master Misc#20406] Question about Regexp encoding negotiation

["Eregon (Benoit Daloze) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20406 has been updated by Eregon (Benoit Daloze).


Indeed, on a similar topic I wonder how much encoding negotiation at Regexp creation time matters.
Because there is another encoding negotiation between the regexp and the string being matched which happens when matching.
Maybe the Regexp encoding should e.g. always be US-ASCII if there are only 7-bit characters in the Regexp source,
or maybe always UTF-8 in that case since it's most likely a regexp will be matched against UTF-8 strings,
this illustrates the Regexp encoding doesn't really matter for the 7-bit source case.

Or maybe Regexp literals should just always use the source encoding, that would make things a lot simpler and closer to string literals.
And the `/nesu` flag would just override the source encoding (and maybe be eventually deprecated, but probably not worth it if their semantics are clear).

I'm not sure what's the point of `Regexp#fixed_encoding?` either, it seems regardless of it a Regexp can be matched with strings of different but compatible encodings (the docs about this in `ri Regexp` are incorrect).

----------------------------------------
Misc #20406: Question about Regexp encoding negotiation
https://bugs.ruby-lang.org/issues/20406#change-107821

* Author: andrykonchin (Andrew Konchin)
* Status: Open
----------------------------------------
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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