Doubt about Regex and Quantifiers

[Nya Iwa]

Hi!
I need some help!
(code below from Sun´s tutorial)

 
import java.io.Console;
import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class RegexTestHarness {

    public static void main(String[] args){
        Console console = System.console();
        if (console == null) {
            System.err.println("No console.");
            System.exit(1);
        }
        while (true) {

            Pattern pattern = 
            Pattern.compile(console.readLine("%nEnter your regex: "));

            Matcher matcher = 
            pattern.matcher(console.readLine("Enter input string to search: "));

            boolean found = false;
            while (matcher.find()) {
                console.format("I found the text \"%s\" starting at " +
                   "index %d and ending at index %d.%n",
                    matcher.group(), matcher.start(), matcher.end());
                found = true;
            }
            if(!found){
                console.format("No match found.%n");
            }
        }
    }
}

So I run this code.

  
C:\>java RegexTestHarness

Enter your regex: aa?
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.

Enter your regex: aa?
Enter input string to search: ab
I found the text "a" starting at index 0 and ending at index 1.

Enter your regex: aa?
Enter input string to search: aaba
I found the text "aa" starting at index 0 and ending at index 2.
I found the text "a" starting at index 3 and ending at index 4.

Enter your regex: a?
Enter input string to search: aabbaba
I found the text "a" starting at index 0 and ending at index 1.
I found the text "a" starting at index 1 and ending at index 2.
I found the text "" starting at index 2 and ending at index 2.
I found the text "" starting at index 3 and ending at index 3.
I found the text "a" starting at index 4 and ending at index 5.
I found the text "" starting at index 5 and ending at index 5.
I found the text "a" starting at index 6 and ending at index 7.
I found the text "" starting at index 7 and ending at index 7.  

My question is: why only in the last string (aabbaba) there´s a match of zero length and others (a, ab, aaba) don´t have??

[Henry Wong]

My question is: why only in the last string (aabbaba) there´s a match of zero length and others (a, ab, aaba) don´t have??

Take a look at your regex. The last case has a different regex -- that can match a zero length string.

Henry

[Nya Iwa]

Hi Henry!
Could you explain to me why the last case "can match a zero length string" and the the others can't? I know the regex is different, but why when I try a? I can match a zero length string and when I try aa? I can't match it?

I'm so confused! Why this output?
When I try by myself (on paper) I don't get this output!
The relationship between quantifiers, zero-length string and matcher.find() isn't very clear to me..
I don´t know exactly where I'm doing wrong..

Could you please explain to me step by step following the source code how the third case works?
It would be of a great help!!

[Henry Wong]

Could you explain to me why the last case "can match a zero length string" and the the others can't? I know the regex is different, but why when I try a? I can match a zero length string and when I try aa? I can't match it?

The "?" means zero or one of the previous character. So....

aa? ... matches a letter "a", followed by zero or one "a"s.... basically, it can match "a" or "aa".

a? ... matches zero or one "a"s.... basically, it can match nothing or "a".

Henry

[Henry Wong]

When I try by myself (on paper) I don't get this output!
The relationship between quantifiers, zero-length string and matcher.find() isn't very clear to me..
I don´t know exactly where I'm doing wrong..

Well, in order to tell you what you are doing wrong, we need to know what you are doing. Please explain to us what you put on paper...

Henry