# The Luhn Algorithm to check a bank card's check sum.
# Plus a regular expression to narrow the options.

import re

def add_digits(string):
    '''Converts the chars of string into ints and adds them.
    
    string must only consist of digits
    '''
    return sum([int(c) for c in string])

def luhn(string):
    '''Print a result determined by the string and Luhn algorithm.
    
    'possible' if string is ok
    'invalid' if string is not.
    '''
    total = 0
    odd = False
    for c in reversed(string):
        if odd:
            n = int(c) * 2
            total += add_digits(str(n))
        else:
            total += int(c)
        odd = not odd
    print('possible' if total % 10 == 0 else 'invalid')    
    
# The regex will accept any 16 digit number which has 4 groups of 4 digits.
# Each of the groups is separated by a space or a dash '-'.
# There can be text before or after the card number.

card = 'my card number is 1234 5678 9012 3452 don\'t tell anyone.'

pattern = r'\b(\d{4}[ -]){3}\d{4}\b'
match = re.search(pattern, card)

if match:
    print(match.group(), '- ', end='')
    number = match.group().replace(' ','')
    luhn(number)