a <- c("{.package:base73","for.package:base476",":.package:base9","print.package:graphics834")
unlist(regmatches(a, regexec("package:\\K\\D+", a, perl=TRUE)))
YSA8LSBjKCJ7LnBhY2thZ2U6YmFzZTczIiwiZm9yLnBhY2thZ2U6YmFzZTQ3NiIsIjoucGFja2FnZTpiYXNlOSIsInByaW50LnBhY2thZ2U6Z3JhcGhpY3M4MzQiKQp1bmxpc3QocmVnbWF0Y2hlcyhhLCByZWdleGVjKCJwYWNrYWdlOlxcS1xcRCsiLCBhLCBwZXJsPVRSVUUpKSk=