Rapid serial visual presentation paradigm

I am trying to run rapid serial visual presentation (RSVP) paradigm experiment for attentional blink phenomenon. for the experiment,

1) 13 to 21 letters is randomly drawn (without replacement) from the alphabet (I, O, Q, and S are left out because they resemble digits).

2) 2 of the letters were replaced with digits, randomly drawn (without replacement) from the digits 2 through 9.

3) the second digit (T2) was presented 3 to 6 temporal positions from the end of the stream. and the temporal distance between the first digit(T1) and the second digit(T2) is systematically varied from 1 to 5 positions.

numletters = randi([13, 21]);

s = ['ABCDEFGHJKLMNPRTUVWXYZ'];

rsvp = s(randperm(numel(s),1));

t2place = s(end-(3:6));

digits = randi([2, 9]);

t2 = t2place(randperm(numel(t2place),1));

for index = 1:numletters

rsvp = s(randperm(numel(s),1))

if length(rsvp) = t2place

rsvp = digits

end

I am new to the matlab program, and don't now how to place T1 and T2 in a loop meeting the conditions provided above.

thansk again for your help.