Try
x = {'overcomplete';'phellandrene';'phenanthrene';'pneumatocele';'pneumonocele'
'preintercede';'preinterfere';'pseudocumene';'pseudosphere';'spermatocele' };
y = 'aer';
xpr = regexprep( y, '(\w{1})(?=\w)', '$1\|' ); % remove the letters of y
cac = regexprep( x, xpr, '' );
str = cat( 2, cac{:} ); % concatenate all words
asc = double( str ); % convert to ascii-numbers
edges = [min(asc):1:max(asc)];
[ n, bin ] = histc( asc, edges );
ixm = find( n == max(n) );
fprintf(['highest occurring letters in x, which doesn''t appear in y, are ' ...
, '"%s" with %i occurrences\n'], char(edges(ixm)), n(ixm(1)) )
outputs
highest occurring letters in x, which doesn't
appear in y, are "np" with 11 occurrences
