- S-->AB
A-->aA|bA|e
B-->abB|e
this grammar is generating
(a+b)* {a^n b^n | n>=0}
S-->AaB
A-->aC|e
B-->aB|bB|e
C-->aCb|e
this grammar is generating
[a.{a^n b^n | n>=0}]*.a.(a+b)*
S-->AB
A-->aA|e
B-->aBb|e
this grammar is generating
a*. {a^n b^n | n>=0}
only 3rd is generating CFL not Regular.