[ruby-core:117426] [Ruby master Misc#20406] Question about Regexp encoding negotiation

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


> By default, a regexp with only US-ASCII characters has US-ASCII encoding:

I was wondering what kind of check is used for that and it seems to be chec=
king the Regexp source when building it (makes sense):
```
$ ruby -e 'p /a/.encoding'
#<Encoding:US-ASCII>
$ ruby -e 'p /a#{}b/.encoding'
#<Encoding:US-ASCII>
$ ruby -e 'p /a#{"c"}b/.encoding'
#<Encoding:US-ASCII>
$ ruby -e 'p /a#{"=E9"}b/.encoding'
#<Encoding:UTF-8>
```

----------------------------------------
Misc #20406: Question about Regexp encoding negotiation
https://bugs.ruby-lang.org/issues/20406#change-107801

* Author: andrykonchin (Andrew Konchin)
* Status: Open
----------------------------------------
I am wondering what are the rules to calculate Regexp literal encoding in c=
ase 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 modi=
fiers.
> //n ...
> //u ...
> //e ...
> //s ...

Looking at the following examples I would assume that these rules are follo=
wed 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 `U=
S-ASCII`. So I am wondering what rule is applied in this case.



--=20
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