As Daniel says, there must be more going on. Can you copy the code directly? strfind returns the indices; if there are no matches, it returns []. Hence, if line was 'root/folder/file', strfind(line,'/') returns [5,12].
So in this case you'd get an indexing (out of bounds) error on the next line. If there was three slashes in the string, you'd get a vector like [13, 14, 15, 16].
Are you perhaps trying to extract a portion of the string and str2num it? In which case, you want
caseNo = str2double(line(no_slash(3)+1 : no_slash(3)+4))
(Note: if you leave out the line in the above command, you get NaN. Or if line isn't a string.)
