strings <- c("d1 x2 d2x4 5 specimens collected the smallest is 4 x 1 x 3 mm",
" stomach x3 d2x4 8 pieces with the largest measuring 4 x 1 x 3 mm")
strings <- gsub("\\b(\\d+)(\\s+(?:specimens|pieces))", ".\\1\\2", strings, perl = TRUE)
strings
CnN0cmluZ3MgPC0gYygiZDEgeDIgZDJ4NCA1IHNwZWNpbWVucyBjb2xsZWN0ZWQgdGhlIHNtYWxsZXN0IGlzIDQgeCAxIHggMyBtbSIsCgkJCSIgc3RvbWFjaCB4MyBkMng0IDggcGllY2VzIHdpdGggdGhlIGxhcmdlc3QgbWVhc3VyaW5nICA0IHggMSB4IDMgbW0iKQoJCQkKc3RyaW5ncyA8LSBnc3ViKCJcXGIoXFxkKykoXFxzKyg/OnNwZWNpbWVuc3xwaWVjZXMpKSIsICIuXFwxXFwyIiwgc3RyaW5ncywgcGVybCA9IFRSVUUpCnN0cmluZ3M=