reentrant_format.rb

class String
  old_format = instance_method(:%)

  define_method(:%) do |arg|
    return self unless self =~ /%/
    if arg.is_a?(Hash)
      (self.gsub(/%\{([^%]*?)\}/) { 
        a = $1
        if a =~ /([^\[]+)\[([^\]]*)\](.*)/
          h = arg[$1.to_sym][$2.to_sym]
          h_args = $3.gsub(/\[([^:\]][^\]]*)\]/, '[:\1]')
          h_args.nil? ? h : eval("h#{h_args}")
        else
          arg[a.to_sym]
        end
      }) % arg
    else
      old_format.bind(self).call(arg)
    end
  end
end

puts '%{a}, %{b[%{id}][:d]}' % { :a => 'hello', :b => { :c => { :d => 'world' } }, :id => 'c' } 

puts '%{a}, %{b[%{id}][d]}' % { :a => 'hello', :b => { :c => { :d => 'world' } }, :id => 'c' } 

puts '%{a}, %{b}' % { :a => 'hello', :b => 'world' } 

puts '%{a}, %{b[id]}' % { :a => 'hello', :b => { :id => 'world' } }