require "table2"

local function expand_range(s)
    local list = {}
    local items = s:split(",")
    for items as item do
        local count = item:split(""):count(|c| -> c == "-")
        if count == 0 or (count == 1 and item[1] == "-") then
            list:insert(tonumber(item))
        else
            local items2 = item:split("-")
            local first, last
            if count == 1 then
                first = tonumber(items2[1])
                last  = tonumber(items2[2])
            elseif count == 2 then
                first = tonumber(items2[2]) * -1
                last  = tonumber(items2[3])
            else
                first = tonumber(items2[2]) * -1
                last  = tonumber(items2[4]) * -1
            end
            for i = first, last do list:insert(i) end
        end
    end
    return list
end

local s = "-6,-3--1,3-5,7-11,14,15,17-20"
print(expand_range(s):concat(", "))