im just gonna post some articles and stuff showing what nations are in South East Asia and what nations are in the Sub Indian Continent, or as some sources call it, South Asia.
but, before we go into nations in each geographical region in Asia, this is currently what the United Nations regards as regions.

blue = north asia
red = south asia
green = west asia
purple = central asia
yellow = east asia
orange = southeast asia
here are sources and articles stating what region Sri Lanka is in (notice how they don't say its in southeast asia)
http://en.wikipedia.org/wiki/Geography_of_Sri_Lanka
http://en.wikipedia.org/wiki/South_Asia
https://www.cia.gov/library/publication ... n_sas.html
http://www.mapsofworld.com/asia/regions ... a-map.html
here are articles stating what nations are in south east asia
http://en.wikipedia.org/wiki/Southeast_Asia
http://asiasociety.org/introduction-southeast-asia
http://www.seasite.niu.edu/crossroads/r ... eageog.htm
http://www.slideshare.net/cjcalvert/sou ... -geography (a cool powerpoint)
thank u and have a good night