f1 = fopen('test2.txt'); % your data in file test2.txt
c = textscan(f1,'%s','Delimiter','\n');
fclose(f1);
out = str2double(c{1}(find(strcmp(c{1},'BEGIN DATA'))+1:end));
out = out(~isnan(out));
or
out = regexp(c{1},'^\d*$','match');
out = str2double(cat(1,out{~cellfun(@isempty,out)}));
or
out = str2double(c{1}(cellfun(@(y)all(~isletter(y))&~isempty(y),c{1})));
