如何使用gsub在R中的向量中僅保留前幾個字符/數字？

Question

基本上，我只想在向量中保留第一個字符，我知道這可以在substr（）中輕松完成，但是我想知道如何在gsub（）中做到。

例如，

codes <- c("02Q","4E (1)","4S (1)","A0","A2","A4")

我想要一個像這樣的結果向量

c("0","4","4","A","A","A")

謝謝

Answer 1

你可以做

sub("^(\\w).*$", "\\1", codes)
#[1] "0" "4" "4" "A" "A" "A"

說明：

• ^：表示字符串的開頭
• \\ w：表示一個字母數字項目，用括號將其捕獲，然后通過將“ \\\\ 1”作為替換參數進行檢索
• 。*：表示0次或多次
• $：表示字符串的結尾

Answer 2

似乎您只想保留第一個字符。

gsub("(?<!^).", "", codes, perl=TRUE)
# [1] "0" "4" "4" "A" "A" "A"

(?<!^)否定性向后看，它斷言匹配將以行邊界的任何開頭（但不是開頭）開始。

要么

codes <- c("02Q","4E (1)","4S (1)","A0","A2","A4")
sub("(?<!^).*", "", codes, perl=T)
[1] "0" "4" "4" "A" "A" "A"

再多一點..

> sub("(?!^.).*", "", codes, perl=T)
[1] "0" "4" "4" "A" "A" "A"
> sub("\\B.*", "", codes, perl=T)
[1] "0" "4" "4" "A" "A" "A"