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 silly but one of the introductory examples doesn't make sense to me.
(\d)\d\1
Sample text:
123-456-7890
(should capture the first number, 123
)
Can anyone explain what is going on in here?
As far as I can figure out, the first \d
captures the number 123. The \1
backreferences (marks) the group for later use. The parenthesis limit the scope of the group. But what does the second \d
does?
Simple explanation, like to a small child or a golden retriever are prefered.