Pattern matching doubt

[Sen George]

The following code

import java.util.regex.*;
import static java.lang.System.out;

public class test_match1 {
   public static void main(String... args) {

     String mSource  = "x12y4_z6";
     String position = "01234567";

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

     Pattern p = Pattern.compile("\\d*?");

     Matcher m = p.matcher(mSource);

     boolean b = false;

     out.println("\n\n      Matcher Program \n");
   //  out.println("\n" + "   Pattern string: " + args[0] + "\n"); 
     out.println("\n" + "   Source string: " + mSource); 
     out.println("\n" + "   Position:      " + position+ "\n\n"); 

     while (b = m.find()) {    
       out.println();
       out.println("  Match found at position: " + m.start() + "\n" + "  Data Group: " + m.group() + "\n" );
     }

  }
} 

compiles and runs fine.

Program produces an output of 'match' found in positions 0, 1, 2, 3, 4, 5, 6, 7, and 8. But no data groups. How is this explained-matches found but no data groups?

[Henry Wong]

Program produces an output of 'match' found in positions 0, 1, 2, 3, 4, 5, 6, 7, and 8. But no data groups. How is this explained-matches found but no data groups?

Well, look at the pattern....

Pattern p = Pattern.compile("\\d*?");   

This reluctantly matches zero or more digits. Since, zero digits is a valid match, an zero length string is a valid match -- which is what you are matching.

Henry