What does \1 mean in a regular expression?

Question

Python

Hello!
What does \1 mean in a regular expression ^\W\d{2}\s?\.([abc\d])\1\1$?
What !12.awill match \1 in the string?

Answer 1

\1 means backreference, that is, a reference to a group construct. In this case, your group is ([abc\d]), so \1 will denote what fell under this group (letter a,b,c or a number).
In your line !12.a it will be 'a'