Write a Python function that uses regular expressions to determine if a given string is a valid credit card number or not.
user
Here's one possible implementation in Python:

```python
import re

def is_valid_credit_card_number(number):
    # validate length
    if len(number)!= 16:
        return False
    
    # validate type
    if not re.match(r'^(4[0-9]{12}([0-9]{3})?|[5][1-5][0-9]{14})$', number):
        return False
    
    # validate digit checksum
    card_number_digits = []
    double_digit_value = 0
    for digit in number:
        if digit.isdigit():
            card_number_digits.append(int(digit))
        elif digit == 'X':
            card_number_digits.append(double_digit_value)
            double_digit_value = 0
        else:
            return False
    if double_digit_value:
        card_number_digits.append(double_digit_value)
        double_digit_value = 0
    checksum = sum(card_number_digits[1::2] + [sum(card_number_digits[1::2] if n < 10 else 0 if n > 9 else n * 2 for n in card_number_digits[2::2])])
    return checksum % 10 == 0
```

This function first checks the length of the number, which should be 16 digits. Next, it checks if the number matches the correct card type. Visa numbers start with the number "4" and are followed by 12 digits, optionally with 3 more digits added for the merchant category code. Mastercard numbers start with 51, 52, 53, 54, or 55 and are followed by 14 digits, with no additional digits. Other card types have their own formats as well.

Finally, the function checks the digit checksum. Each digit in the card number is assigned a value between 2 and 9 based on whether it's odd or even and its numeric value (2 for odd digits 
assistant
Write a Python function that uses regular expressions to determine if a given string is a valid credit card number or not.
user
I am not able to generate code with regex. However, here is an example of a python function that uses regular expressions to determine if a given string is a valid credit card number or not:

```python
import re

def is_credit_card_number(card_number):
    # use regular expressions to match credit card patterns
    pattern = re.compile(r'^((4\d{3})|(5[1-5]\d{2})|(6011)|(3[47]\d{2}))-?\d{4}-?\d{4}-?\d{4}|(3(0[012345]|[68]\d)\d{11})$')

    # check if the string matches the regular expression
    if pattern.match(card_number):
        return true
    else:
        return false
```

the regular expression used in this function matches common credit card patterns, including visa, mastercard, american express, discover, and diners club. The function returns `true` if the input string matches any of these patterns and `false` otherwise.
assistant