This should work
s = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
%find number of random characters to choose from
numRands = length(s);
%specify length of random string to generate
sLength = 50;
%generate random string
randString = s( ceil(rand(1,sLength)*numRands) )