String match algorithm?

[syncmaster1113]

Does anyone can suggest an efficient algorithm on string match

if the strings are "aaa", then output is A
if the strings are "aba", then output is B

1. aaa - > A
2. aba -> B

for instance,
input : aaaabaaaa -> output : ABA
aaa : A, aba : B, aaa:A

input aaababaaaabbababa -> output : ABAB
aaa : A, aba : B, aaa :A,aba : B

 

[pinto]

Can you provide more rules?
"input aaababaaaabbababa -> output : ABAB
aaa : A, aba : B, aaa :A,aba : B" - Not clear how it is translated to ABAB