How can I get list of domains registered in a particular geographical region? For instance, I want list of .ke domains in Eastern Africa
Access to that will be dependent on the policies of the TLD registrar; I would be very surprised if any of them permitted it (knowingly), although you might find one or two TLDs that permit AXFRs of the zone from one or more nameservers.
In your case, looking at the list of nameservers for
.KE reveals that
mzizi.kenic.or.ke permits zone transfers. So, running
dig @mzizi.kenic.or.ke. ke. axfr at the command line gives a useful list of .KE's top-level zones (co.ke, ne.ke, etc), and digging farther down with, for example,
dig @ns1.coza.net.za. co.ke. axfr turns up a complete list.
(Edit: I'm noticing that AXFRs from mzizi are failing now, but ns1.coza.net.za is still fine.)
Note that you won't be able to rely on this; it's very likely that this is a misconfiguration on their part.
This can not be accomplished without the aid/permission of the registrar that manages the ccTLD in question.
It is possible that some registrars make this information public, but I'm not aware of any.
I should note that I'm somewhat familiar with this topic as I run domain-wide web harvesting of the .is domain for the National Library of Iceland. Fortunately, in our case, the registrar has been most cooperative (as required by law). I know that some other countries (Australia and New Zealand amongst them IIRC) have been unable to procure such lists and have relied on information from the Internet Archive instead.
The closest you could come is to find out where the servers are located. If you retrieve the ip where the domain points to, you could run them through a list of ip ranges per county. This way you can find the location of the server.(a quick way to find the ip of one domain = cmd> ping www.example.com)
An example of a list like this can be found at: http://www.countryipblocks.net/
It's not a foolproof solution though. The owners of the site could be located in the netherlands, while the server is located in the UK, or even further: America.
Netcraft has a search by domain name feature that takes into account ~1,340,568 web sites visited by users of the Netcraft Toolbar. It also allows wild-card characters. Searching for "*.ke" returned 115 sites.