x <- "50A ABC DE 51A FG 52A HI 53A"
rx <- "\\G(?:(?! X ).)*?\\K\\b\\d+A\\b"
regmatches(x, gregexpr(rx, x, perl=TRUE))
x <- "50A ABC DE 51A FG X 52A HI 53A"
regmatches(x, gregexpr(rx, x, perl=TRUE))