Ruby sample code to highlight the difference among Japanese-character conversion methods

japanese_char_conversion.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

# Sample code for my answer:
# https://stackoverflow.com/questions/58829740/how-do-i-filter-out-invisible-characters-without-affecting-japanese-character-se/58863231#58863231

require 'nkf'
begin
  require 'iconv'  # https://rubygems.org/gems/iconv/
rescue LoadError
  warn "WARNING: Gem 'iconv' is not installed, hence is ignored"
end

orig = "b2◇〒α()あ相〜\u3000＿８Ｄ━●★】$£€Ç♡㌔③\u2028ﾊﾝｶｸ"
print "Orig:"; p orig
print "Enc: "; p orig.encode('ISO-2022-JP', undef: :replace, replace: '').encode('UTF-8')
print "NKF: "; p NKF.nkf('-w -E', NKF.nkf('-e', orig))

output = ''
Iconv.open('iso-2022-jp-2', 'utf-8') do |cd|
  cd.discard_ilseq=true
  # Note: cd.transliterate='?'  raises Iconv::IllegalSequence
  output = cd.iconv orig << cd.iconv(nil)
  ## The original way described in the iconv Gem website
  # orig.each_char { |s| output << cd.iconv(s) }
  # output << cd.iconv(nil)
end
s2 = Iconv.conv('utf-8', 'iso-2022-jp-2', output)
print "Icon:"; p s2

## Results (as of Ruby 2.6.5):
#    Orig:"b2◇〒α()あ相〜　＿８Ｄ━●★】$£€Ç♡㌔③\u2028ﾊﾝｶｸ"
#    Enc: "b2◇〒α()あ相〜　＿８Ｄ━●★】$£"
#    NKF: "b2◇〒α()あ相〜　＿８Ｄ━●★】$£㌔③ハンカク"
#    Icon:"b2◇〒α()あ相〜　＿８Ｄ━●★】$£€Ç♡③ﾊﾝｶｸ"


### For developers:

# p output               # Sanity check.
# p s2.encoding          # Sanity check.
# p s2.valid_encoding?   # Sanity check.
# p Iconv.ctlmethods     # List of the available instance methods of iconv Gem

## Some say this would work, but it seemed not as of Ruby 2.6.5.
# p Iconv.iconv('iso-2022-jp-2', 'utf-8//IGNORE', orig)
# p Iconv.conv('iso-2022-jp-2', 'utf-8//TRANSLIT', orig)