Example of a simple templating language written in Ruby.

template.rb

class VariableLang

  def initialize(path)
    @path = path
    @content = File.read(path)
  end

  def render(locals = {})
    locals.inject(@content.dup) do |buffer, vars|
      key = vars.first
      value = vars.last
      buffer.gsub(/@#{key}/, value)
    end
  end
end

template = VariableLang.new('template.var')
content = template.render(
  to: 'Josh',
  service: 'Sassy',
  from: 'Sassy Team'
)

puts content

template.var

Hello @to,

Thanks for signing up to @service! You'll love it here.

Cheers,
@from

And if you wanted to validate all variables have been replaced:

class VariableLang
  MissingVariableError = Class.new(Exception)

  def initialize(path, options = {})
    @path = path
    @content = File.read(path)
    @options = options
  end

  def render(locals = {})
    locals.inject(@content.dup) do |buffer, vars|
      buffer.gsub(/@#{vars[0]}/, vars[1])
    end.tap do |content|
      if @options[:raise_on_missing]
        unless (vars = fetch_missing_variables(content)).empty?
          raise(MissingVariableError, "Variables are present in the template that have not been replaced: #{vars.join(', ')}.")
        end
      end
    end
  end

private

  def fetch_missing_variables(content)
    matches = content.scan(/@\w+/)
    matches ? matches.to_a : []
  end
end