word-sub.rb

def parse(str)
  matches = str.scan(/\([^|]+\|[^)]+\)/)
  pairs = matches.map { |match| match[1...-1].split("|") }
  (pairs.count - 1)
    .times
    .to_a
    .reduce(pairs[0]) { |acc, i| acc.product(pairs[i + 1]) }
    .map { |substition_array| substition_array.flatten }
    .map do |subs|
      matches
        .zip(subs)
        .reduce(str) do |acc, (match, sub)|
          acc.gsub(match, sub)
        end
    end
end

input = "(I|you) (want|need) to (buy|purchase) (this|that)?"
puts parse(input)