title: en_US or en-US, which one should you use?
body: <p>Assume you want to store the locale of user preference in database, which value you will use?</p>
  
  <p>en_US or en-US</p>
  
  <p>They are two standards, but which one you prefer to use as part of your own application?</p>
  
  <p><strong>Updated</strong>: Is seems many web sites use dash instead of underscore, e.g.</p>
  
  <p><a href="http://zh.wikipedia.org/zh-tw" rel="noreferrer">http://zh.wikipedia.org/zh-tw</a>
  <a href="http://www.google.com.hk/search?hl=zh-TW" rel="noreferrer">http://www.google.com.hk/search?hl=zh-TW</a></p>