Have trouble understanding capturing groups and back references

Question

Wishing to put some order into my knowledge of regular expressions I decided to go through a book about them, Introducing Regular Expressions. And I know it\'s sill

Answer 1

\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...