Have trouble understanding capturing groups and back references

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\d is just one digit.

This regular expression doesn’t match the "123-456-7890" string but it would match "323" (which could be part of a greater string, for example "323-456-7890") : 

 (\d) : first digit ("3")
 \d   : another digit ("2")
 \1   : first group (which was "3")

Now, if your book pretends that (\d)\d\1 should capture "123" in "123-456-7890", then it might contain an error…

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