The following example is from the Cucumber wiki Show archive.org snapshot :

Given a blog post named "Random" with Markdown body
  """
  Some Title, Eh?
  ==============
  Here is the first paragraph of my blog post. Lorem ipsum dolor sit amet,
  consectetur adipiscing elit.
  """

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That multi-line string will be given as the last block argument in your step definitions:

Given /^a blog post named "([^\"]*)" with Markdown body$/ do |title, markdown|
  Post.create!(:title => title, :body => markdown)
end