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.

itemprop="text">

class="normal">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"