title: How to use regular expressions do reverse search?
body: <p>For example:<br>
  My string is: <code>123456789  nn  nn  oo nn  nn mlm nn203</code>.<br>
  My target is: <code>nn</code>.</p>
  
  <p>Then, I match string from the end to the beginning and return the first match result and its postion.<br>
  In this examlpe, the result is <code>nn</code> start in [-5] end in [-3].<br>
  I wrote the simple funcitonto  do this process, but how to use regular expressions to do this job?</p>