List of Valid TLDs: http://newgtlds.icann.org/en/program-status/delegated-string...
Some tlds can have single dot; like .com & some might have multiple; like firm.ind.in or co.uk or ac.in or such.
Check more discussion here at SO question: https://stackoverflow.com/questions/10306690/what-is-a-regul...
Thanks for looking at my photo website :)