md_to_link.rb

line = "[yahoo.co.jp Yahoo]"

def markup_to_html(line)
	arr = line.split(/(\[|\])/)
	i = 0
	while arr[i] != nil
		item = arr[i]
		if arr[i] == "[" && arr[i+2] == "]"
			if arr[i+1].index(/\s/)
				url = arr[i+1].split(/\s/)[0]
				str = arr[i+1].split(/\s/)[1]
				arr[i+1] = "<a href=\"#{url}\">#{str}</a>"
				arr.delete_at(i)
				arr.delete_at(i+1)
			else
				arr[i+1] = "<a href=\"#{arr[i+1]}\">#{arr[i+1]}</a>"
				arr.delete_at(i)
				arr.delete_at(i+1)
			end
		end
		i += 1
	end
	return arr.join
end

print markup_to_html(line)