title: username regex in rails
body: <p>I am trying to find a regex to limit what a person can use for a username on my site.  I don't need to have it check to see how many characters there are in it, as another validation does this.  Basically all I need to make it do is make sure that it allows: letters (capital and lowercase) numbers, dashes and underscores.</p>
  
  <p>I came across this: /^[-a-z]+$/i </p>
  
  <p>But it doesn't seem to allow numbers.</p>
  
  <p>What am I missing?</p>