Задачка Python3

In [1]: import re

In [2]: re.compile('\d{4}').search('payment_0113_10032015_msk.txt').group()
Out[2]: '0113'

In [3]: re.compile('\d{4}').search('order-spb-canceled-2210.rtf').group()
Out[3]: '2210'

In [4]: re.compile('\d{4}').search('заявка берлин сентябрь №9043.txt').group()
Out[4]: '9043'

re.compile('(?<=[\W_])\d{4}(?=[\W_])').search('01_2014-02-15_113_2451.csv').group()

strings = ['payment_0113_10032015_msk.txt',
                'order-spb-canceled-2210.rtf',
                'заявка берлин сентябрь №9043.txt',
                '01_2014-02-15_113_2451.csv', 
                '45f2m56Qfd_1122_000000001.html']
import re
r = re.compile(r'\D(\d{4})\D(?!.*\D\d{4}\D)')
[r.search(s).groups()[0] for s in strings]

import os, re

files = ['payment_0113_10032015_msk.txt',
'order-spb-canceled-2210.rtf',
'заявка берлин сентябрь №9043.txt',
'01_2014-02-15_113_2451.csv',
'45f2m56Qfd_1122_000000001.html']





def extract_and_print(s):
    size = 4;
    name, ext = os.path.splitext(s)
    nums = re.split('[^\d]+', name)
    nums_with_length = filter(lambda s: len(s) == size, nums)
    
    if len(nums_with_length) == 0:
        print('***** Problems with line ' + s);
    else:
        num = nums_with_length[-1] # the last of us
        print("{0} {1}".format(num, s)) 


for s in files: extract_and_print(s)

0113 payment_0113_10032015_msk.txt
2210 order-spb-canceled-2210.rtf
9043 заявка берлин сентябрь №9043.txt
2451 01_2014-02-15_113_2451.csv
1122 45f2m56Qfd_1122_000000001.html

# -*- coding: utf-8 -*-
files = ['payment_0113_10032015_msk.txt',
'order-spb-canceled-2210.rtf',
'заявка берлин сентябрь №9043.txt',
'01_2014-02-15_113_2451.csv',
'45f2m56Qfd_1122_000000001.html']


def digits(s):
    return ''.join([a for a in s if  '0'<=a<='9'])

[digits(a)[:4] for a in files]

xxxx-xx-xx => {date}