Regular Expressions in $ 1, $ 2

$ 1, $ 2 is the expression of the content inside the parentheses

$ 1 is the contents of the first small brackets, $ 2 is a small parenthesis inside the contents of the second, and so on

 

For example (\\ d {4}) (\\ d {2}) (\\ d {2}) matches "20190919"

$ 1 is the first match of 2019 in brackets

$ 2 is the second matching brackets 09

$ 3 is the third of 19 matching brackets