[ruby-core:123945] [Ruby Bug#21709] Regexp interpolation is inconsistent with String interpolation

28 Nov 2025

      Issue #21709 has been updated by thyresias (Thierry Lambert).

Ok.
Here is the code that shows the inconsistency Regexp/String for interpolation, from your examples:
```ruby
# inconsistent Regexp/String interpolation behavior

prefix = '\p{In_Arabic}'
suffix = '\p{In_Arabic}'.encode('US-ASCII')

begin
  re = /#{prefix}#{suffix}/
rescue => ex
  puts "fail: #{ex.message} (#{ex.class})"
  # fail: encoding mismatch in dynamic regexp : US-ASCII and UTF-8 (RegexpError)
end

s = "#{prefix}#{suffix}"
re = /#{s}/
puts "ok: #{s.inspect} (#{s.encoding}) -> #{re.inspect} (#{re.encoding})"
# ok: "\\p{In_Arabic}\\p{In_Arabic}" (UTF-8) -> /\p{In_Arabic}\p{In_Arabic}/ (UTF-8)

begin
  re = /#{suffix}/
rescue => ex
  puts "fail: #{ex.message} (#{ex.class})"
# fail: invalid character property name {In_Arabic}: /\p{In_Arabic}/ (RegexpError)
end

s = "#{suffix}"
re = /#{s}/
puts "ok: #{s.inspect} (#{s.encoding}) -> #{re.inspect} (#{re.encoding})"
# ok: "\\p{In_Arabic}" (UTF-8) -> /\p{In_Arabic}/ (UTF-8)
```

----------------------------------------
Bug #21709: Regexp interpolation is inconsistent with String interpolation
https://bugs.ruby-lang.org/issues/21709#change-115347

* Author: thyresias (Thierry Lambert)
* Status: Open
* ruby -v: ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x64-mingw-ucrt]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
```ruby
%w(foo être).each do |s|
  puts "string: #{s.inspect} -> #{s.encoding}"
  puts "escaped: #{Regexp.escape(s).inspect} -> #{Regexp.escape(s).encoding}"
end
```
Output:
```
string: "foo" -> UTF-8
escaped: "foo" -> US-ASCII
string: "être" -> UTF-8
escaped: "être" -> UTF-8
```
The result should always match the encoding of the argument.

-- 
https://bugs.ruby-lang.org/