G'day bbs2web,

There may be more to your data than you show; however, from what's in your OP, you may be overthinking the solution.

A cut-down version of your data:

$ cat fred
id pool     image         snap device
0  rbd_hdd  vm-209-disk-1 -    /dev/rbd0
1  rbd_hdd  vm-107-disk-1 -    /dev/rbd1
10 rbd_hdd  vm-144-disk-1 -    /dev/rbd10
[download]

Removing ... except:

$ cat fred | perl -ne 'print unless /^[^\d]/'
0  rbd_hdd  vm-209-disk-1 -    /dev/rbd0
1  rbd_hdd  vm-107-disk-1 -    /dev/rbd1
10 rbd_hdd  vm-144-disk-1 -    /dev/rbd10
[download]

Which is basically a double-negative for:

$ cat fred | perl -ne 'print if /^\d/'
0  rbd_hdd  vm-209-disk-1 -    /dev/rbd0
1  rbd_hdd  vm-107-disk-1 -    /dev/rbd1
10 rbd_hdd  vm-144-disk-1 -    /dev/rbd10
[download]