library(stringr)
 
x <- c('123456789', 'kfasdf  3456789asdf', 'a        1')
y <- c('12345 789', '1       9', 'a     678a')
 
str_extract(x, regex('[ \\d](?:(?<=[ ])[ ]|\\d){7}\\d'))
str_extract(y, regex('[ \\d](?:(?<=[ ])[ ]|\\d){7}\\d'))