regex - Have trouble understanding capturing groups and back references -

To know about any order regular expression, I decided to go through a book about them, Regular introduction . And I know that it is silly but I do not understand one of the introductory examples.

  (\ d) \ d \ 1  

sample text:

  123-456-7890  

(should be occupied by the first number, 123 )

Can anyone say what is happening here?

As far as I can understand, the first \ d occupies the number 123. For later use, the group backfruit (points) limits the brackets group's scope. But what else does \ d do?

Simple explanation, like a small child or a golden dog.

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matches string "123-456-7890" but this is "323 " (which can be part of a larger string, for example " 323- first digit ("3") \ d: second digit ("2") \ 1: The first group (which was "3")

Now, if your book shows that (\ d) \ d \ 1 \ / code> is passed to "123" should be Captured "123-456-7890" , then there may be an error ...