Consider the Java program below, which is a recursive descent parser:

class Parens {
   GetChar getChar = new GetChar();
   char next;

   void P() {
      scan();
      Q();
      if (next == '#') System.out.println("Success!");
      else error(1);
   }

   void Q() {
      if (next == '(') {
         scan();
      }
      else error(2);
      R();
   }

   void R() {
      if (next == ')') {
         scan();
      }
      else {
         Q();
         R();
      }
   }

   void scan() {
      while (Character.isWhitespace(next = getChar.getNextChar()))
         ;
   }

   void error(int n) {
      System.out.println("*** ERROR: " + n);
      System.exit(1);
   }

   public static void main(String[] args) {
      Parens parens = new Parens();
      parens.P();
   }
}

Here are three separate runs (boldface = input):

(()())#
Success!
( () (( )) (() () ) () ) #
Success!
( () (( ))#
*** ERROR: 2