有两种方法可以实现，R 截取开头或结尾的子字符串。分别是：
- base R, substr
- stringr, str_sub, 支持负号索引，来截取字符结尾的字符串
substr
x <- "this is an example" # Create example character string
substr(x, 1, 3) # 截取前3个字符
# "thi"
n_last <- 3 # Specify number of characters to extract
substr(x, nchar(x) - n_last + 1, nchar(x)) # 截取后3个字符
# "ple"
stringr::str_sub
stringr::str_sub(x, - 3, - 1) # Extract last characters with str_sub
# "ple"
Reference
| [Extract First or Last n Characters from String in R (3 Examples) | Get Leading & Trailing Chars](https://statisticsglobe.com/r-extract-first-or-last-n-characters-from-string) |