try {
if (expression == null || expression.length() == 0) {
// always return true for null or empty expressions
_evaluator = Literal.booleanLiteral(true);
} else {
SelectorLexer lexer = new SelectorLexer(
new StringReader(expression));
lexer.initialise();
SelectorParser parser = new SelectorParser(lexer);
parser.initialise();
parser.selector(); // start parsing at the selector rule