permutation of two lists based on criteria

i hope someone can help me, how to achieve this. I have to files with list of data, i am trying to calculate possible permutations between the two lists and save them to a new file. i realized that my output file is very big ( more than 30 Gb ). i would like to know how to make permutation only between the data that meets specific criteria. F.eks if :

Data 1: VHxBxVVxPx255x98x Data 2: VHxBxVVxPx255x98x

only permutate if char(6 and 7) from data1 = char(6 and 7) from data2.

my code so far :

    fid = fopen( 'file1.txt' );
    cac = textscan( fid, '%20s' );
    fclose( fid );
    num = cac{1};
    fid = fopen( 'file2.txt' );
    cac = textscan( fid, '%20s' );
    fclose( fid );
    str = cac{1};
    fid = fopen( 'file3.txt', 'w' );
    for ii = 1 : length( num )
        for jj = 1 : length( str )
            fprintf( fid, '%1s - %1s\n', num{ii}, str{jj} );
        end
    end   
    fclose( fid );