library(stringr)
my_string <- "\r\nContent: base64\r\n\r\nDBhHB\r\nDGlV\r\nPAAHJ\r\nAwQU\r\n"
pat <- "(?m)^\\S+(?:\\R\\S+)*$"
unlist(str_extract_all(my_string, pat))
## => [1] "DBhHB\r\nDGlV\r\nPAAHJ\r\nAwQU"
my_string <- "\r\nNot This\r\n\r\nKeepThis\r\nKeepThis\r\nNot This\r\nKeepThis\r\n"
unlist(str_extract_all(my_string, pat))
## => [1] "KeepThis\r\nKeepThis" "KeepThis"
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