Diffing "defgh" and "defghij"

[m n]=size(a);
b=0;
[m n b]=size(a);
for i=1:m
  for j=1:m-i
    for k=1:n-j
      c = a(i:j+i,k:k+j);
      if c==c(1)
	b=max(b,sum(c(:)));
      end
    end
  end
end