template.rb

class Template
  def initialize(content)
    @content = content
  end

  def render(context)
    context = Hashie::Mash.new(context)
    
    @content.gsub(/(%{([^}]+)})/) do
      context[Regexp.last_match[2]]
    end
  end
end

@context is a Hash of some description, so you can do

Template.new("This is %{thing} and he is %{other_thing}").render(
  thing: "Adam",
  other_thing: "Awesome"
)

so String#gsub replaces things with other things, it takes strings or regular expressions, if you pass 2 parameters, it'll replace every match of the first parameter, with the second parameter. If you pass it 1 parameter and a block, it'll execute the block for every match, and use the return value of the block as what to replace with.

the block takes 1 parameter, which is the whole match (e.g. "%{blah}") but I wanted the third match, which you can't access directly as the list of matches from the regular expression isn't passed to the block (for some reason). So you can use the perl syntax $0 $1 $2 to get the matches, or use Regexp.last_match[x] which looks a bit nicer.

Thanks

Talking of the regex stuff, you can visualise regular expressions with rubular

http://www.rubular.com/r/YLwi7xPkZP

so you see each match matches 2 things, the whole text (%{blah}) and the just key (blah) which is what I wanted.