regex - gsub() everything except specified characters?

How do I gsub() everything except a specified character in R?

In my problem I have the following string...

"the quick brown fox jumps over a lazy dog"

I have to generate a new string with by removing all characters except 'r' and 'o' and get the following output...

"roooro"

Assuming all characters are lower case how do I go about. I tried...

gsub(!"r","",gsub(!"o","",str1))

But the '!' doesn't work.

Answer

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"

---

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"