rm(list=ls())
A_1 <- letters[1]
A_2 <- letters[1]
A_3 <- letters[1]
B_1 <- letters[1]
B_2 <- letters[1]
B_3 <- letters[1]
ls()
# [1] "A_1" "A_2" "A_3" "B_1" "B_2" "B_3"

N <- c(1, 2)

ls(pattern=paste0("A_", 1))
# [1] "A_1"

sapply(N, function(i) ls(pattern=paste0("A_", i)))
# [[1]]
# character(0)

# [[2]]
# character(0)