A problem of Redcarpet

test.rb

require 'redcarpet'
require 'redcarpet/render/review'

render = Redcarpet::Render::ReVIEW.new()
mk = Redcarpet::Markdown.new(render)

str1 = "$t_1$"
str2 = "$t_1$ to $t_2$"

puts "#{str1} is converted to "
puts mk.render(str1)
puts "#{str2} is converted to "
puts mk.render(str2)

$ ruby test.rb
$t_1$ is converted to 


$t_1$

$t_1$ to $t_2$ is converted to 


$t@<b>{1$ to $t}2$