lesser_grapheme_cluster_counter.rb

#! /usr/bin/env ruby

#load 'unicode.rb' # ActiveSupport::Multibyte::Unicode.unpack_graphemes

def lesser_graphemes_counter(str)
  codepoints = str.unpack("U*")  
  variation_selectors = codepoints.count { |c| (0xFE00..0xFE0F).include?(c) }
  skintone_modifiers = codepoints.count { |c| (0x1F3FB..0x1F3FF).include?(c) }
  regional_indicators = codepoints.count { |c| (0x1F1E6..0x1F1FF).include?(c) }
  zero_width_joiners = codepoints.count(0x200D)
  enclosing_keycaps = codepoints.count(0x20E3)
  codepoints.count - variation_selectors - skintone_modifiers - (regional_indicators / 2) - (zero_width_joiners * 2) - enclosing_keycaps
end

puts "RUBY_VERSION = #{RUBY_VERSION}"

[
  [0x31,    0xFE0F,  0x20E3],                             # Keycap Digit One 1️⃣
  [0x1F44F, 0x1F3FD],                                     # Clapping Hands Sign, Type-4 👏🏽
  [0x1F468, 0x200D,  0x1F468, 0x200D,  0x1F466],          # Family: Man, Man, Boy 👨‍👨‍👦
  [0x1F1EF, 0x1F1F5],                                     # 🇯🇵
  [0x1F1EF, 0x1F1F5, 0x1F1EF, 0x1F1F5, 0x1F1EF, 0x1F1F5], # 🇯🇵🇯🇵🇯🇵
].map { |e| e.pack("U*") }.map do |emoji|
  puts "\n"
  puts "\"#{emoji}\".length = #{emoji.length}"
  puts "\"#{emoji}\".codepoints.count = #{emoji.codepoints.count}"
  puts "\"#{emoji}\".scan(/\\X/).count = #{emoji.scan(/\X/).count}"
#  puts "ActiveSupport::Multibyte::Unicode.unpack_graphemes(\"#{emoji}\").count = #{ActiveSupport::Multibyte::Unicode.unpack_graphemes(emoji).count}"
  puts "lesser_graphemes_counter(\"#{emoji}\") = #{lesser_graphemes_counter(emoji)}"
end

For anyone else coming across this, from Ruby 2.5.x or later this is a lot simpler with str.grapheme_clusters.length:

puts "RUBY_VERSION = #{RUBY_VERSION}"

[
  [0x31,    0xFE0F,  0x20E3],                             # Keycap Digit One 1️⃣
  [0x1F44F, 0x1F3FD],                                     # Clapping Hands Sign, Type-4 👏🏽
  [0x1F468, 0x200D,  0x1F468, 0x200D,  0x1F466],          # Family: Man, Man, Boy 👨‍👨‍👦
  [0x1F1EF, 0x1F1F5],                                     # 🇯🇵
  [0x1F1EF, 0x1F1F5, 0x1F1EF, 0x1F1F5, 0x1F1EF, 0x1F1F5], # 🇯🇵🇯🇵🇯🇵
].map { |e| e.pack("U*") }.map do |emoji|
  puts "\n"
  puts "\"#{emoji}\".length = #{emoji.length}"
  puts "\"#{emoji}\".codepoints.count = #{emoji.codepoints.count}"
  puts "\"#{emoji}\".scan(/\\X/).count = #{emoji.scan(/\X/).count}"
  puts "\"#{emoji}\".grapheme_clusters.length = #{emoji.grapheme_clusters.length}"
end