I'm just learning about regular expressions and I need to read in a text file and find every instance of a number and find the sum of all the numbers. 

```
import re

sum = 0
list_of_numbers = list()
working_file = open("sample.txt", 'r')
for line in working_file:
    line = line.rstrip()
    working_list = re.findall('[0-9]+', line)
    if len(working_list) != 1:
        continue
    print(working_list)
    for number in working_list:
        num = int(number)
        list_of_numbers.append(num)
for number in list_of_numbers:
    sum += number
print(sum)

```

I put the `print(working_list)` in order to try and debug it and see if all the numbers are getting found correctly and I've seen, by manually scanning the text file, that some numbers are being skipped while others are not. I'm confused as to why as I thought my regular expression guaranteed that any string with any amount of digits will be added to the list.

[Here is the file](http://py4e-data.dr-chuck.net/regex_sum_42.txt).