title: Java regex error - Look-behind group does not have an obvious maximum length
body: <p>I get this error:</p>
  
  <pre><code>java.util.regex.PatternSyntaxException: Look-behind group does not have an
      obvious maximum length near index 22
  ([a-z])(?!.*\1)(?&lt;!\1.+)([a-z])(?!.*\2)(?&lt;!\2.+)(.)(\3)(.)(\5)
                        ^
  </code></pre>
  
  <p>I'm trying to match <code>COFFEE</code>, but not <code>BOBBEE</code>.</p>
  
  <p>I'm using java 1.6.</p>