For various reasons I've needed to validate some credit card numbers in Python. For future reference, here's what I've come up with:

import re

def validate_cc(s):
    """
    Returns True if the credit card number ``s`` is valid,
    False otherwise.

    Returning True doesn't imply that a card with this number has ever been,
    or ever will be issued.

    Currently supports Visa, Mastercard, American Express, Discovery
    and Diners Cards.  

    >>> validate_cc("4111-1111-1111-1111")
    True
    >>> validate_cc("4111 1111 1111 1112")
    False
    >>> validate_cc("5105105105105100")
    True
    >>> validate_cc(5105105105105100)
    True
    """
    # Strip out any non-digits
    # Jeff Lait for Prime Minister!
    s = re.sub("[^0-9]", "", str(s))
    regexps = [
            "^4\d{15}$",
            "^5[1-5]\d{14}$",
            "^3[4,7]\d{13}$",
            "^3[0,6,8]\d{12}$",
            "^6011\d{12}$",
            ]

    if not any(re.match(r, s) for r in regexps):
        return False

    chksum = 0
    x = len(s) % 2

    for i, c in enumerate(s):
        j = int(c)
        if i % 2 == x:
            k = j*2
            if k >= 10:
                k -= 9
            chksum += k
        else:
            chksum += j

    return chksum % 10 == 0