code: private boolean passesExpression(LoggingEvent event) {
      if (event != null) {
        if (expressionRule != null) {
          return (expressionRule.evaluate(event, null));
        }
      }
      return true;
    }
docs: Helper method that supports the evaluation of the expression
  
  @param event
  @return true if expression isn't set, or the result of the evaluation otherwise