Ruby / Oniguruma case-insensitive non-word quirk

output.log

$ ruby -v re.rb 
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux]
non-word:
  /\W/    => []
  /\W/i   => []
  /[\W]/  => []
  /[\W]/i => ["fF", "Ss"]

not word/leading word:
  /^\w/    => ["f"]
  /^\w/i   => ["f"]
  /[^\w]/  => []
  /[^\w]/i => []

word:
  /\w/    => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /\w/i   => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /[\w]/  => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /[\w]/i => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]

not non-word/leading non-word:
  /^\W/    => []
  /^\W/i   => []
  /[^\W]/  => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /[^\W]/i => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]

output2.log

$ jruby -v re.rb 
jruby 9.0.4.0 (2.2.2) 2015-11-12 b9fb7aa OpenJDK 64-Bit Server VM 24.91-b01 on 1.7.0_91-b02 +jit [linux-amd64]
non-word:
  /\W/    => []
  /\W/i   => []
  /[\W]/  => []
  /[\W]/i => ["fF", "S", "s", "k"]

not word/leading word:
  /^\w/    => ["f"]
  /^\w/i   => ["f"]
  /[^\w]/  => []
  /[^\w]/i => []

word:
  /\w/    => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /\w/i   => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /[\w]/  => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /[\w]/i => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]

not non-word/leading non-word:
  /^\W/    => []
  /^\W/i   => []
  /[^\W]/  => ["f", "F", "f", "G", "S", "s", "o", "E", "k"]
  /[^\W]/i => ["f", "F", "f", "G", "o", "E"]

re.rb

$string = 'fFfGSsoEk'.freeze

def demo re, desc
  puts "#{desc}:"
  print "  /#{re}/    => "; p $string.scan(/#{re}/)
  print "  /#{re}/i   => "; p $string.scan(/#{re}/i)
  print "  /[#{re}]/  => "; p $string.scan(/[#{re}]/)
  print "  /[#{re}]/i => "; p $string.scan(/[#{re}]/i)
  puts ''
end

demo('\W', 'non-word')
demo('^\w', 'not word/leading word')

demo('\w', 'word')
demo('^\W', 'not non-word/leading non-word')