Let 's try
a = {'Scenario_1_mode_1', 'Scenario_2_mode_1', 'Scenario_3_mode_1', 'Scenario_4_mode_1'};
a = a';
for i=1:length(a)
b = regexp(a{i},'\d*','Match');
if ~isempty(b)
out(i)=str2double(strjoin(b, ''));
else
out(i)=NaN;
end
end