Your best option for pattern matching is usually regular expressions (regexp).
In this particular situation where the pattern is simple, you can go for a simpler solutions though, as follows:
>> buffer = fileread('A.txt') ;
>> n = numel(strfind(buffer, 'AAA')) ;
Let me know if the pattern is indeed more complicated than a simple string, or if you must differentiate 'AAA' from 'AAAA' for example or avoid matching 'BAAABBB'.
