Regex help

[Joshua Ebarvia]

Please explain the output of this. I'm having hard time on regex

 import java.util.regex.*;
public class Test{
        public static void main(String[] args){
                Pattern p = Pattern.compile("a?a");
                Matcher m = p.matcher("abaaba");

                System.out.println("Pattern: " + p.pattern());
                System.out.println("Source: abaaba");
                while(m.find()){
                        System.out.println("start: " + m.start() + " group:" + $
                }
        }
}
   

Output

  Pattern: a?a
Source: abaaba
start: 0 group:a
start: 2 group:aa
start: 5 group:a
  

And if I change the pattern to only a?
the output is

   start: 0 group:a
start: 1 group:
start: 2 group:a
start: 3 group:a
start: 4 group:
start: 5 group:a
start: 6 group: 

Please explain both,

Regards,

[Henry Wong]

The regex "a?a" means to match either zero or one character "a", followed by another character "a". This means that it can match either "a" or "aa".

The regex "a?" means to match either zero or one character "a". This means that it can match either "" or "a". Or a zero-length string, or a single character of "a".

If you now go back to the application, you should see how the regex is found using your source string.

Henry

[Joshua Ebarvia]

Thanks