我试图在由字符串组成的数据框中标识唯一的unicode值。我尝试使用grep函数，但是遇到以下错误
Error: '\U' used without hex digits in character string starting ""\U"
数据框示例
time sender message
1 2012-12-04 13:40:00 1 Hello handsome!
2 2012-12-04 13:40:08 1 \U0001f618
3 2012-12-04 14:39:24 1 \U0001f603
4 2012-12-04 16:04:25 2 <image omitted>
73 2012-12-05 06:02:17 1 Haha not white and blue... White with blue eyes \U0001f61c
40619 2015-05-08 10:00:58 1 \U0001f631\U0001f637
grep("\U", dat$messages)
数据
dat <-
structure(list(time = c("2012-12-04 13:40:00", "2012-12-04 13:40:08",
"2012-12-04 14:39:24", "2012-12-04 16:04:25", "2012-12-05 06:02:17",
"2015-05-08 10:00:58"), sender = c(1L, 1L, …