x <- c("[ghjg6] [fdg5] [113gi4lki] great work", "[xzswedc: acf] [xzt8] [111eerrh5]", "[asd2] [1] [113vu17hg 115er5lgr 112cgnmbh ] get out", "Some text with no brackets")
df <- data.frame(x)
df$x = sub(".*\\[([^][]+)].*", "\\1", df$x)
df