A language is reg if you can draw to fa for that language. since
(a + b)* is a language containing all possible string over ∑=(a,b) and it is regular as we can build a finite automata,
Now coming to you query regular language is the super-set of all languages ?
For proving that we have to prove regular language should be able to do some thing which other language (CFL,REC etc) can,t do.
If you want your assumption is valid so it should be universally valid.
But simply speaking this is not true.Whatever regular language do CFL can also do or even CSL can do.
However CFL can do more which regular language can,t be able to do .String comparision where memory is unlimited.
Even there are higher class of language rather than reg language for that refer Chomsky hierarchy.