The Java regular expression engine handles back references to non-existing groups in the same manner as back references to existing groups. Invalid or non-existing back references do not participate in the match. This means that a regular expression with an invalid back reference always fails, although it does not cause any exception.

For example, consider the following regex example:

    (apple|orange|banana)2 

The 2 back reference will be invalid because there is only one capturing group in this regular expression. Hence, the back reference of 2 always makes it a failed match against any input.

The same happens even when we have a regex pattern as follows:

    1 

Due to a similar rule, it will fail to match an empty string as well.