Additionally:
Although you can use \(digit) in the right side of a s/// operation, you shouldn't. This was included (i would guess) just to appease the sed junkies out there ;)

Generally the only time \(digit) should be used is on the LHS of a s///, or within a m//.
Example:

/(foo)\1/;
# is effectively the same as
/(foo){2}/;
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