Executing a R-string

Hello...

In other languages (e.g. php, perl), you have the ability to create a  
valid string and execute the string to get the result. For example (in  
pseudo-R):

S<-"which(m==4)"
R<-exec(S)

I know this does not work, but was wondering if there was an  
equivalent mechanism that I cannot find in the docs anywhere to make  
it work.

Thanks,
Joe