Generic substitution with Procs

content.rb

class Content
  MATCHERS = [{:regex => /\[Contact\]/i, :proc => Proc.new{|options| options[:contact].name}},
              {:regex => /\[Site\]/i, :proc => Proc.new{|options| options[:site].name}}]

  attr_accessor :body

  def prepare_form_letter(contact, site)
    parsed = self.body
    MATCHERS.each do |matcher|
      parsed.gsub!(matcher[:regex], matcher[:proc].call({:contact => contact, :site => site}))
    end
    parsed
  end
end

class Contact
  attr_accessor :name
end

class Site
  attr_accessor :name
end

### Runtime ###
c = Contact.new
c.name = "Hello"

s = Site.new
s.name = "World!"

con = Content.new
con.body = "Dear [Contact],..." #All of the form letter above

con.prepare_form_letter(c,s) # Outputs form letter, with names replaced in the content.