title: Replace all characters other than english letters and numbers to underscore
body: <p>I have a string, and I would like to replace all special characters with underscores.</p>
  
  <p>In other words, I just want 26 english letters (lower and upper cases) and 0-9 and the "_" character.</p>
  
  <p>Also note that there are the non-english characters and they need to be replaced with "_" as well.</p>
  
  <p>What is the most elegant way to do this in Ruby?</p>