I modified my question in "search and convert string function "

Hi ,all R-helper!
I modified my  question  in "search and convert string function".
I want to do this kind of operation In R enviroment :
For example :
R <- 4
testString <- "I love %R%"
then search this testString, when find beginning and end symbol"%",get a new 
substring "%R%".
then get rid of symbole"%",replace string"R" by R(numeric )
So the final string is "I love 4"

How can I implement? if there is not this kind of function ,how can I write 
function in C and implement in R.
Thanks advance

Michael