How to replace all occurrences of a certain character, but not if the character is the first in string?

Question

I have the following vector

x = c("AXX", "XAX", "XXA")

I would like to replace all the "A" to "B" in x, but not if "A" is at the beginning of the string. Desired:

c("AXX", "XBX", "XXB")

Answer 1

Use a negated start anchor in a regular expression:

x <- c("AXX", "XAX", "XXA")

Base R

gsub("(?!^)A", "B", x, perl=TRUE)
##[1] "AXX" "XBX" "XXB"

stringr

library(stringr)
str_replace_all(x, "(?!^)A", "B")
##[1] "AXX" "XBX" "XXB"