删除包含和在字符串后面的行

say we have:

说我们有：

dat <-  data.frame( x1 = c(rep(c("A","B"), each = 4),"B"),
                x2 = rep(c("a","a","b","b", "a","a","a","a","a")),
                x3 = c("cow","pig","cow","cow",
                       "cow","pig","pig","pig",
                       "cow"
                       )
                  ) 
dat <-  data.frame( x1 =