Try:

x <- 'ABCD'
gsub('\\B', ',', x, perl = T)

Prints:

[1] "A,B,C,D"

Might have misread the query; OP is looking to add comma's between letters only. Therefor try:

gsub('(\\p{L})(?=\\p{L})', '\\1,', x, perl = T)

• (\p{L}) - Match any kind of letter from any language in a 1st group;
• (?=\p{L}) - Positive lookahead to match as per above.

We can use the backreference to this capture group in the replacement.

您可以使用

> gsub("(.)(?=.)", "\\1,", x, perl=TRUE)
[1] "A,B,C,D"

(.)(?=.)正则表达式匹配将其捕获到组1中的任何字符（(.)），该组1后面必须跟有任何单个字符（(?=.)）是一个正前瞻，它需要紧接在当前位置右侧的字符）。
溶液的变化：

> gsub("(.)(?!$)", "\\1,", x, perl=TRUE)
## Or with stringr:
## stringr::str_replace_all(x, "(.)(?!$)", "\\1,")
[1] "A,B,C,D"

这里，如果存在字符串位置的结尾，则(?!$)匹配失败。
请参阅R demo online：

x <- "ABCD"
gsub("(.)(?=.)", "\\1,", x, perl=TRUE)
# => [1] "A,B,C,D"
gsub("(.)(?!$)", "\\1,", x, perl=TRUE)
# => [1] "A,B,C,D"
stringr::str_replace_all(x, "(.)(?!$)", "\\1,")
# => [1] "A,B,C,D"

非正则表达式友好的回答：

paste(strsplit(x, "")[[1]], collapse = ",")
#[1] "A,B,C,D"

另一种选择是使用肯定的look behind和look ahead来Assert存在前导字符和后续字符：

library(stringr)
str_replace_all(x, "(?<=.)(?=.)", ",")
[1] "A,B,C,D"