This possible by using fwrite or fprintf when the number of characters do not change. If it does or could change, there is no other way than reading the complete file, modifying the data and re-write it:
Str = fileread(FileName);
Str = strrep(Str, char([13, 10]), char(10)); % Care for linebreaks
CStr = regexp(Str, char(10), 'split');
CStr{23} = strrep(CStr{23}, 'short string', 'much longer string');
FID = fopen(FileName, 'w'); % or wt
if FID == -1, error('Cannot open file for writing: %s', FileName); end
fprintf(FID, '%s\n', CStr{:});
fclose(FID);
