R: $0 regex operator in stringr-CodePudding

I want to use the $0 regex operator via the str_replace() function (stringr package).

For instance, I have a vector:

test <- c("1111", "2222", "3333")
test
[1] "1111" "2222" "3333"

and I want to get the next vector: "111_1" "222_2" "333_3"

When I try to do it via str_replace, I get the following:

str_replace(test, "^\\d{3}", "\\$0_")
[1] "\\111_1" "\\222_2" "\\333_3"

How does $0 work in stringr?

P.S.: I don't want to use (?<=) and (?=) bulky operators.

CodePudding user response：

It seems that \0 with no $ refers to the match so:

str_replace(test, "^\\d{3}", "\\0_")
## [1] "111_1" "222_2" "333_3"

In base R it could be done like this where \1 refers to the first (and here only) capture group:

sub("^(\\d{3})", "\\1_", test)
## [1] "111_1" "222_2" "333_3"