Write a Java program (using JFIex/Cup) to interpret WAE expressions. The CFG for

Question

Write a Java program (using JFIex/Cup) to interpret WAE expressions. The CFG for WAE expressions is given below: SEMI ::= I {+ ) I (- } I {with { ) > I You will write a main program, called WAE.java, that prompts the user with wae>. The user can enter a WAE expression at the prompt or type exit to exit the WAE interpreter. If the user enters a WAE expression, your program should verify that it is a valid expression. If the expression is valid, it should be evaluated and the value of the expression should be printed; Otherwise and error message should be reported. $ java WAE WAE> (with {x 3} {with {y 4} {with (Z 5} {+ x (+ y z)}}}} The value is 12 WAE> (with (x 3) (with {y 4} {+ y z}} Syntax Error WAE> (with (x 3) (with (y 4) {+ y 2}}} Semantic Error

Explanation / Answer

import java.util.regex.*;

class RegexDemo

{

   public static void main (String [] args)

   {

      if (args.length != 2)

      {

          System.err.println ("java RegexDemo regex text");

          return;

      }

      Pattern p;

      try

      {

         p = Pattern.compile (args [0]);

      }

      catch (PatternSyntaxException e)

      {

         System.err.println ("Regex syntax error: " + e.getMessage ());

         System.err.println ("Error description: " + e.getDescription ());

         System.err.println ("Error index: " + e.getIndex ());

         System.err.println ("Erroneous pattern: " + e.getPattern ());

         return;

      }

      String s = cvtLineTerminators (args [1]);

      Matcher m = p.matcher (s);

      System.out.println ("Regex = " + args [0]);

      System.out.println ("Text = " + s);

      System.out.println ();

      while (m.find ())

      {

         System.out.println ("Found " + m.group ());

         System.out.println (" starting at index " + m.start () +

                             " and ending at index " + m.end ());

         System.out.println ();

      }

   }

   static String cvtLineTerminators (String s)

   {

      StringBuffer sb = new StringBuffer (80);

      int oldindex = 0, newindex;

      while ((newindex = s.indexOf (" ", oldindex)) != -1)

      {

         sb.append (s.substring (oldindex, newindex));

         oldindex = newindex + 2;

         sb.append (' ');

      }

      sb.append (s.substring (oldindex));

      s = sb.toString ();

      sb = new StringBuffer (80);

      oldindex = 0;

      while ((newindex = s.indexOf (" ", oldindex)) != -1)

      {

         sb.append (s.substring (oldindex, newindex));

         oldindex = newindex + 2;

         sb.append (' ');

      }

      sb.append (s.substring (oldindex));

      return sb.toString ();

   }

}

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