Get the current ANSI color sequence at the point a string ends

lastcolor.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

# String color helpers
class ::String
  ESCAPE_REGEX = /(?<=\[)(?:(?:(?:[349]|10)[0-9]|[0-9])?;?)+(?=m)/.freeze

  # Get the calculated ANSI color at the end of the string
  #
  # If you want to inject a colored string into another
  # string on the console, you want the color to return to
  # what it was before the inserted string when the inserted
  # string ends. This gets you that previous color.
  #
  # @return     ANSI escape sequence to match previous color
  #
  def last_color_code
    m = scan(ESCAPE_REGEX)

    em = ['0']
    fg, bg, rgbf, rgbb = nil

    m.each do |c|
      case c
      when '0'
        em = ['0']
        fg, bg, rgbf, rgbb = nil
      when /^[34]8/
        case c
        when /^3/
          fg = nil
          rgbf = c
        when /^4/
          bg = nil
          rgbb = c
        end
      else
        c.split(/;/).each do |i|
          x = i.to_i
          if x <= 9
            em << x
          elsif x >= 30 && x <= 39
            rgbf = nil
            fg = x
          elsif x >= 40 && x <= 49
            rgbb = nil
            bg = x
          elsif x >= 90 && x <= 97
            rgbf = nil
            fg = x
          elsif x >= 100 && x <= 107
            rgbb = nil
            bg = x
          end
        end
      end
    end

    escape = "\e[#{em.join(';')}m"
    escape += "\e[#{rgbb}m" if rgbb
    escape += "\e[#{rgbf}m" if rgbf
    escape + "\e[#{[fg, bg].delete_if(&:nil?).join(';')}m"
  end
end

str = "\e[1;31;47mHe\e[38;2;144;10;38mllo\e[4mThereHowAre"

puts "#{str} \e[0m\e[0;31;43m\e[7mHIGHLIGHT #{str.last_color_code}RESET"