Why not to do something like this?
Q1 <- c(NA, NA, "A", NA, NA)
Q1[!is.na(Q1)]
#> [1] "A"
It would be easier to help you if you could provide a minimal REPRoducible EXample (reprex).
EDIT: Just in case you still want to follow the regex path
library(stringr)
str_extract(paste("A",NA,NA,NA,NA), "(?<!N\\s?)[A-E]")
#> [1] "A"