我想在一些字符串后得到值，演示如下

dt <- data.table(col.1 = c("a1, b2, c3, d4"))
x <- c("a", "b", "c")
dt[, (x) := lapply(FUN = str_match(string = .SD, 
                                   pattern = paste0("(?<=", x, ")([\\d])"))[, 2], 
                   X = x),
   .SDcols = "col.1"]

理想的结果如下所示

desirable <- data.table(col.1 = c("a1, b2, c3, d4"),
                        a = c("1"),
                        b = c("2"),
                        c = c("3"))

我收到如下错误消息：

• www.example.com中的错误match.fun（有趣）：

c("'str_match(string = .SD, pattern = paste0(\"(?<=\", x, \")([\\\\d])\"))[, ' is not a function, character or symbol", "'    2]' is not a function, character or symbol")*

但是我想不出怎么解决这个问题。谁能给予我点建议？