It's possible to pass a regular expression as a rule pattern in the rule definition. The following example shows you how to do it:

rule /\.html$/ => '.md' do |t|
  sh "pandoc -s #{t.source} -o #{t.name}"
end

We used the new method in the preceding example, source, which is called on the task. Its name explicitly says what it does—it returns the name of the source. In our case, this is the Markdown file.