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

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