text=c("hello, please keep THIS","THIS is important","all THIS should be done","not exactly This","not THHIS", "THAT is something I need, too")
res <- regmatches(text, gregexpr("\\b(THIS|THAT)\\b", text))
unlist(res)
dGV4dD1jKCJoZWxsbywgcGxlYXNlIGtlZXAgVEhJUyIsIlRISVMgaXMgaW1wb3J0YW50IiwiYWxsIFRISVMgc2hvdWxkIGJlIGRvbmUiLCJub3QgZXhhY3RseSBUaGlzIiwibm90IFRISElTIiwgIlRIQVQgaXMgc29tZXRoaW5nIEkgbmVlZCwgdG9vIikKcmVzIDwtIHJlZ21hdGNoZXModGV4dCwgZ3JlZ2V4cHIoIlxcYihUSElTfFRIQVQpXFxiIiwgdGV4dCkpCnVubGlzdChyZXMp