Regular expression weird result [duplicate]

Code

gsub('101', '111', '110101101')
#[1] "111101111"

Would anyone know why the second 0 in the input isn't being substituted into a 1 in the output? I'm looking for the pattern 101 in string and replace it with string 111. Later on I wish to turn longer sub-sequences into sequences of 1's, such as 10001 to 11111.