Instead of a regular expression, you could use isstrprop() to identify which characters in each element of the cell array are digits and extract those digits. This assumes that there are no other digits in the char arrays other than those you'd like to extract.
'C' is an example cell array of chars.
'digits' is a numeric vector where digits(n) is the number stored in C{n}.
C(1) = {'<string_attribute name="EEE_Level_LTT"><s>34</s></string_attribute>'};
C(2) = {'<string_attribute name="EEE_Level_LTT"><s>9</s></string_attribute>'};
digits = str2double(cellfun(@(x)x(isstrprop(x,'digit')), C, 'UniformOutput',false));
