find_list$X[which.max(sapply(find_list$find_list, function(myX)
attr(gregexpr(myX, lookup_value)[[1]], "match.length")))]
#[1] "f"
DATA
find_list = structure(list(X = c("a", "b", "c", "d", "e", "f", "g"), find_list = c(1L,
12L, 123L, 124L, 125L, 1234L, 1235L)), .Names = c("X", "find_list"
), class = "data.frame", row.names = c(NA, -7L))
lookup_value = 12345678