example.rb

    a = "A\s*(?<A>.+?(?=\nB\s*))"
    b = "[\n]+B\s*(?<B>.+?(?=\nC\s*))"
    c = "[\n]+C\s*(?<C>.+?(?=\nD\s*))"
    d = "[\n]+D(?<D>(?=[^?]+^E).+?(?=\nE\s+)|.+?(?=\n))"
    e = "((?=[\n]+E\s+)[\n]+E\s+(?<E>(.+?))\n)?"
    all = "#{a}#{b}#{c}#{d}#{e}"
    nothing = "?<nothing>A\s+"
    result = []

    output.scan(/(?<nr>^\d{1,2})\.\s+(?<content>.+?(?=A\s+))((?=#{all})(#{all})|(#{nothing}))/m) do 
      result << $~
    end