Invert all hex colors (e.g. #fa74b3) as a unix pipe

invert-hex-colors.lua

#!/usr/bin/lua

-- Pattern matching a hex color, e.g. #fa74b3
local hex_color = '#' .. string.rep('[0-9a-f]', 6)

-- Invert a hex color
function invert_hex_color(color)
  -- Drop the initial '#'
  color = string.sub(color, 2)
  local inverted_color = '#'

  -- Fetch the different components and invert them
  for i = 0, 2 do
    -- Fetch the substring
    local component = string.sub(color, 1 + i * 2, 2 + i * 2)

    -- Convert to numerical
    component = tonumber('0x' .. component)

    -- Invert
    component = 0xff - component
    
    -- Append
    inverted_color = inverted_color .. string.format('%.2x', component)
  end

  return inverted_color
end

-- Apply on the entire input
for line in io.lines() do
  local line = line.gsub(line, hex_color, invert_hex_color)
  print(line)
end