template = 'Oceania has always been at war with %s.'
template % 'Eurasia'

template % 'Eastasia'

'To 2 decimal places: %.2f' % Math::PI

'Zero-padded: %.5d' % Math::PI

require 'erb'
template = ERB.new %q{Chunky <%= food %>!}
food = "bacon"
template.result(binding)

food = "peanut butter"
template.result(binding)

puts template.result

Chunky peanut butter!

Error! (Exception?) Here's stdout:
NameError: undefined local variable or method `food' for main:Object
	from (erb):1

template = %q{
<% if problems.empty? %>
  Looks like your code is clean!
<% else %>
  I found the following possible problems with your code:
  <% problems.each do |problem, line| %>
    * <%= problem %> on line <%= line %>
  <% end %>
<% end %>}.gsub(/^\s+/, '')
template = ERB.new(template, nil, '<>')
problems = [["Use of is_a? instead of duck typing", 23],
	    ["eval() is usually dangerous", 44]]
template.run(binding)

I found the following possible problems with your code:
* Use of is_a? instead of duck typing on line 23
* eval() is usually dangerous on line 44

I found the following possible problems with your code:
* Use of is_a? instead of duck typing on line 23
* eval() is usually dangerous on line 44

problems = []
template.run(binding)

Looks like your code is clean!

Looks like your code is clean!

class String
  def substitute(binding=TOPLEVEL_BINDING)
    eval(%{"#{self}"}, binding)
  end
end
template = %q{Chunky #{food}!}

food = 'bacon'
template.substitute(binding)

food = 'peanut butter'
template.substitute(binding)

food = '#{system("dir")}'
puts template.substitute(binding)

Chunky #{system("dir")}!

Chunky #{system("dir")}!