title: What characters in this string throw the error invalid character constant?
body: <p>I'm following an example online, and in one of the lines of code it has this string: </p>
  
  <pre><code>Pattern screenNamePattern = Pattern.compile('(@[a-zA-Z0-9_]+)');
  </code></pre>
  
  <p>When I put it into Eclipse, it throws the error "invalid character constant." What strings in here throw that error? I'm confused...</p>