You can use 'strfind', which returns an empty array when the substring is not found. For example, the following code returns a value of TRUE when smallSubstring is not found within largeString, false when found. (So negate this expression if you want to know if it is in there or not).
>> smallSubstring = '0.0100';
>> largeString1 = 'Item0.0100';
>> largeString2 = 'Item0.0101';
>> isempty(strfind(largeString1, smallSubstring))
>> isempty(strfind(largeString2, smallSubstring))
You may find more information on 'strfind' in the documentation at the following link: