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, ...
