pukiwiki2md.rb

class Pukiwiki2md
  STRONG_REG = /''(.+?)''/
  LINK_REG = /\[\[(.+?):(.+?)\]\]/

  def convert_strong!(line)
    line.gsub!(STRONG_REG, "**\\1**")
    line
  end

  def convert_link!(line)
    line.gsub!(LINK_REG, "[\\1](\\2)")
    line
  end

  def convert_decorator(line)
    [:convert_strong!, :convert_link!].each do |converter|
      send(converter, line)
    end
    line
  end

end

pukiwiki2md_test.rb

require 'minitest/autorun'
require './pukiwiki2md'

class Pukiwiki2mdTest < Minitest::Test
  def setup
    @p2m = Pukiwiki2md.new
  end

  def test_convert_strong
    actual = @p2m.convert_decorator("aaaa''bbbb''cccc")
    assert_equal "aaaa**bbbb**cccc", actual
  end

  def test_convert_strong_double
    actual = @p2m.convert_decorator("aaaa''bbbb''cccc''ddddd''eeeee")
    assert_equal "aaaa**bbbb**cccc**ddddd**eeeee", actual
  end

  def test_convert_strong_and_link
    actual = @p2m.convert_decorator("aaa''bbbb''cccc[[hoge:https://hoge]]ddd")
    assert_equal "aaa**bbbb**cccc[hoge](https://hoge)ddd", actual
  end
end