Search for locations of subsequences?

Is there a function to efficiently search for a subsequence within a vector?

For example, with

x <- 1:100

I'd like to search for the sequence c(49,50,51), and be told that it 
occurs exactly once, starting at location 49.  (The items in the vectors 
might be numeric or character, and there might be repetitions within the 
search pattern or within the vector I'm searching.)

Duncan Murdoch