[ruby-core:102613] [Ruby master Bug#16842] `inspect` prints the UTF-8 character U+0085 (NEXT LINE) verbatim even though it is not printable

Issue #16842 has been updated by naruse (Yui NARUSE).


Why U+0085 is categorized as `Print` in Ruby is historically Oniguruma trea=
ts as that.
https://moriyoshi.hatenablog.com/entry/20090307/1236410006

I'm neutral about the change, but I want the change should have detailed co=
mment or link to this ticket.

----------------------------------------
Bug #16842: `inspect` prints the UTF-8 character U+0085 (NEXT LINE) verbati=
m even though it is not printable
https://bugs.ruby-lang.org/issues/16842#change-90601

* Author: sawa (Tsuyoshi Sawada)
* Status: Assigned
* Priority: Normal
* Assignee: duerst (Martin D=FCrst)
* ruby -v: ruby 2.8.0dev (2020-05-09T13:24:57Z master 889b0fe46f) [x86_64-l=
inux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
The UTF-8 character U+0085 (NEXT LINE) is not printable, but `inspect` prin=
ts the character verbatim (within double quotation):

```ruby
0x85.chr(Encoding::UTF_8).match?(/\p{print}/) # =3D> false
0x85.chr(Encoding::UTF_8).inspect
#=3D> "\"
\""
```

My understanding is that non-printable characters are not printed verbatim =
with `inspect`:

```ruby
"\n".match?(/\p{print}/) # =3D> false
"\n".inspect #=3D> "\"\\n\""
```

while printable characters are:

```ruby
"a".match?(/\p{print}/) # =3D> true
"a".inspect # =3D> "\"a\""
```

I ran the following script, and found that U+0085 is the only character wit=
hin the range U+0000 to U+FFFF that behaves like this.

```ruby
def verbatim?(char)
  !char.inspect.start_with?(%r{\"\\[a-z]})
end

def printable?(char)
  char.match?(/\p{print}/)
end

(0x0000..0xffff).each do |i|
  begin
    char =3D i.chr(Encoding::UTF_8)
  rescue RangeError
    next
  end
  puts '%#x' % i unless verbatim?(char) =3D=3D printable?(char)
end
```



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