Reproducing ruby/ruby#21709

Reproduction script

require "json"

result = { ruby_version: RUBY_VERSION }

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

begin
  re = /#{prefix}#{suffix}/
  result[:regexp_built] = true
  result[:regexp_raised] = nil
rescue => e
  result[:regexp_built] = false
  result[:regexp_raised] = e.class.name
end

begin
  s = "#{prefix}#{suffix}"
  result[:string_built] = true
  result[:string_encoding] = s.encoding.name
  result[:string_raised] = nil
rescue => e
  result[:string_built] = false
  result[:string_encoding] = nil
  result[:string_raised] = e.class.name
end

JSON.dump(result)