Conversion from Char array to unsigned int array

I have an char array (fileNames_images)saving timestamps of sensor data..

fileNames_images (1,:) = '520944046594172889.png'
fileNames_images (2,:) = '520944046594172789.png'
...
..
..

I want to convert this character array into numeric array but matlab returns empty array.

camera_time = str2num( strcat('uint64(',fileNames_images(:,1:end-4),')') )

If I try to use loop;

for a = 1:1000
    camera_time(a,:) = str2num( strcat('uint64(',fileNames_images(a,1:end-4),')') );
end
It says, 
Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 0-by-0

Answer 1

[Walter Roberson]

use sscanf with a %lu format