x <- "123456789
kfasdf 3456789asdf
a 1
12345 789
1 9
a 678a"
regmatches(x, gregexpr("(?<![\\d\\h])(?>\\d|(?<!\\d)\\h){8}\\d", x, perl=TRUE))
eCA8LSAiMTIzNDU2Nzg5CmtmYXNkZiAgMzQ1Njc4OWFzZGYKYSAgICAgICAgMQoKMTIzNDUgNzg5CjEgICAgICAgOQphICAgICA2NzhhIgogICAgcmVnbWF0Y2hlcyh4LCBncmVnZXhwcigiKD88IVtcXGRcXGhdKSg/PlxcZHwoPzwhXFxkKVxcaCl7OH1cXGQiLCB4LCBwZXJsPVRSVUUpKQ==