# example data
test_sentences <- c("The quick brown fox quickly", "jumps over the lazy dog")
Let's make the brown fox red:
sub("brown","red", test_sentences)
#[1] "The quick red fox quickly" "jumps over the lazy dog"
Now, let's make the "fast" fox act "fastly". This won't do it:
sub("quick", "fast", test_sentences)
#[1] "The fast red fox quickly" "jumps over the lazy dog"
sub only makes the first available replacement, we need gsub for global replacement:
gsub("quick", "fast", test_sentences)
#[1] "The fast red fox fastly" "jumps over the lazy dog"
See Modifying strings by substitution for more examples.