I want to generate numbers for each placeholders in range, like:

12XXXXXXXX generates:

1200000000
1200000001
1200000002
...
1299999998
1299999999

or 12X0X generates:

12000
12001
12002
...
12009
12100
12101
12102
...
12109
12200
12201
12202
...
...
...
12909

How can I do that easily?