Jekyll filter for summary text between two <!-- CUT --> tags

_pluginsummarize.rb

module Jekyll
  module Filters
    def summarize(str, splitstr = /<!-- CUT -->\s(.*)\s<!-- CUT -->/)
      
        splitstr.match(str) do |m|
            return m[1]
        end

    end
  end
end

summarize.rb

module Jekyll
  module Filters
    def summarize(str, splitstr = /\s*<div id="extended">/)
      str.split(splitstr)[0]
    end
  end
end