Question marks in regular expressions

Question

I\'m reading the regular expressions reference and I\'m thinking about ? and ?? characters. Could you explain me with some examples their usefulness? I don\'t understand the

Answer 1

Running the test harness from Oracle documentation with the reluctant quantifier of the "once or not at all" match X?? shows that it works as a guaranteed always-empty match.

$ java RegexTestHarness

Enter your regex: x?
Enter input string to search: xx
I found the text "x" starting at index 0 and ending at index 1.
I found the text "x" starting at index 1 and ending at index 2.
I found the text "" starting at index 2 and ending at index 2.

Enter your regex: x??
Enter input string to search: xx
I found the text "" starting at index 0 and ending at index 0.
I found the text "" starting at index 1 and ending at index 1.
I found the text "" starting at index 2 and ending at index 2.

https://docs.oracle.com/javase/tutorial/essential/regex/quant.html

It seems identical to the empty matcher.

Enter your regex:     
Enter input string to search: xx
I found the text "" starting at index 0 and ending at index 0.
I found the text "" starting at index 1 and ending at index 1.
I found the text "" starting at index 2 and ending at index 2.

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

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