PARSER_BEGIN(parens2)

public class parens2 {

    public static void main(String args[]) throws ParseException {
       parens2 parser = new parens2(System.in);
       int pairs_of_parens = parser.parentheses();
       System.out.println("Pairs of parentheses = " + pairs_of_parens);
    }

}
PARSER_END(parens2)

SKIP : 
{
        " "
|       "\n"
}

TOKEN : 
{
        < LPAREN: "("   >
|       < RPAREN: ")"   >
}

int parentheses():
{int nestedParens;}
{
   <LPAREN> nestedParens = parentheses() <RPAREN> { return nestedParens + 1; }
|                                                 { return 0;      }
}