df <- structure(list(t1 = c("Book (Pg 1)", "(Website) Online)", "Journal: 2018)", "Book1 (pg 2) book 3 (pg4)  something)")), class = "data.frame", row.names = c(NA, -4L))
gsub("\\([^()]*\\)(*SKIP)(*F)|[()]", "", df$t1, perl=TRUE)
# => [1] "Book (Pg 1)"                          "(Website) Online"                    
# =>  [3] "Journal: 2018"                        "Book1 (pg 2) book 3 (pg4)  something"