Is there an equivalent of "&" in R's regular expressions for backreference to entire match?

In base R sub/gsub functions: The answer is NO, see this reference:

There is no replacement text token for the overall match. Place the entire regex in a capturing group and then use \1 to insert the whole regex match.

In stringr package: YES you can use \0:

> library(stringr)
> str_replace_all("123 456", "\\d+", "START-\\0-END")
[1] "START-123-END START-456-END"