regex - gsub() everything except specified characters?

We need to use ^ inside the [ to match all characters except the ro. Here, the [^ro]+ implies matching one or more characters that are not a 'r' or 'o' and replace it with blank ("").

gsub("[^ro]+", "", str1)
#[1] "roooro"

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If we have a vector of values, we can create the pattern with paste

v1 <- c("r", "o")
gsub(paste0("[^", paste(v1, collapse=""), "]+"), "", str1)
#[1] "roooro"