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```def cc_type(cc_number):
"""
Function determines type of CC by the given number.

WARNING:
Creditcard numbers used in tests are NOT valid credit card numbers.
You can't buy anything with these. They are random numbers that happen to
conform to the MOD 10 algorithm!

>>> # Unable to determine CC type
>>> print cc_type(1234567812345670)
None

>>> # Test 16-Digit Visa
>>> print cc_type(4716182333661786), cc_type(4916979026116921), cc_type(4532673384076298)
Visa Visa Visa

>>> # Test 13-Digit Visa
>>> print cc_type(4024007141696), cc_type(4539490414748), cc_type(4024007163179)
Visa Visa Visa

>>> # Test Mastercard
>>> print cc_type(5570735810881011), cc_type(5354591576660665), cc_type(5263178835431086)
Mastercard Mastercard Mastercard

>>> # Test American Express
>>> print cc_type(371576372960229), cc_type(344986134771067), cc_type(379061348437448)
American Express American Express American Express

>>> # Test Discover
>>> print cc_type(6011350169121566), cc_type(6011006449605014), cc_type(6011388903339458)
Discover Discover Discover
"""
AMEX_CC_RE = re.compile(r"^3[47][0-9]{13}\$")
VISA_CC_RE = re.compile(r"^4[0-9]{12}(?:[0-9]{3})?\$")
MASTERCARD_CC_RE = re.compile(r"^5[1-5][0-9]{14}\$")
DISCOVER_CC_RE = re.compile(r"^6(?:011|5[0-9]{2})[0-9]{12}\$")

CC_MAP = {"American Express": AMEX_CC_RE, "Visa": VISA_CC_RE,
"Mastercard": MASTERCARD_CC_RE, "Discover": DISCOVER_CC_RE}

for type, regexp in CC_MAP.items():
if regexp.match(str(cc_number)):
return type
return None

if __name__ == "__main__":
import doctest
doctest.testmod(optionflags=doctest.ELLIPSIS | doctest.NORMALIZE_WHITESPACE)
```