a = "#'YOUR_ANSWER
As_samp2 = 36
As_samp3 = 38
#'END_ANSWER"
b <-gsub(pattern = "^(#'YOUR_ANSWER.*)(?:\\R(?!\\h*#'END_ANSWER$).*)*(\\R\\h*#'END_ANSWER)$", a, replace="\\1\nspace for answer\\2", per=T)
print(b)
YSA9ICIjJ1lPVVJfQU5TV0VSCiAgICAgICBBc19zYW1wMiA9IDM2CiAgICAgICBBc19zYW1wMyA9IDM4CgogICAgICAgIydFTkRfQU5TV0VSIgoKCmIgPC1nc3ViKHBhdHRlcm4gPSAiXigjJ1lPVVJfQU5TV0VSLiopKD86XFxSKD8hXFxoKiMnRU5EX0FOU1dFUiQpLiopKihcXFJcXGgqIydFTkRfQU5TV0VSKSQiLCBhLCByZXBsYWNlPSJcXDFcbnNwYWNlIGZvciBhbnN3ZXJcXDIiLCBwZXI9VCkKcHJpbnQoYik=