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'))
bGlicmFyeShzdHJpbmdyKQogCnggPC0gYygnMTIzNDU2Nzg5JywgJ2tmYXNkZiAgMzQ1Njc4OWFzZGYnLCAnYSAgICAgICAgMScpCnkgPC0gYygnMTIzNDUgNzg5JywgJzEgICAgICAgOScsICdhICAgICA2NzhhJykKIApzdHJfZXh0cmFjdCh4LCByZWdleCgnWyBcXGRdKD86KD88PVsgXSlbIF18XFxkKXs3fVxcZCcpKQpzdHJfZXh0cmFjdCh5LCByZWdleCgnWyBcXGRdKD86KD88PVsgXSlbIF18XFxkKXs3fVxcZCcpKQ==