Well the "\d" in your expression means a digit so when you say s/(\d{6})(\b{1}) you are saying find 6 digits and assign them to $1 then find 1 digit and assign it to $2. Your string contains 6 digit followed by an "_" so you expression will fail. if you wish to go this way you would want s/^(\d{6})_(\d{2}):(\d{2})/$1$2$3.monday/ This should give you what you want. You'll notice that I added the "^" to the beginning of the expression to anchor my search to the leftmost position.