The format of credit card numbers mainly follows specific digital rules and algorithms. Here are some common credit card number formats and generation methods: ### Common credit card number formats 1. **Visa**: Starts with 4, a total of 16 digits. 2. **MasterCard**: Starts with a number between 51 and 55, a total of 16 digits. 3. **American Express (AMEX)**: Starts with 34 or 37, a total of 15 digits. 4. **Discover**: Starts with 6011, 644 or 65, a total of 16 digits. 5. **Diners Club**: Starts with a number between 300 and 305, or starts with 36 or 38, a total of 14 digits. ### Generation method When generating valid credit card numbers, you can use the "Luhn algorithm", which is an algorithm used to verify whether a credit card number is valid. Here are the basic steps: 1. **Generate a preliminary digital sequence**: Depending on the type of credit card required, randomly generate the first few digits (set the leading digits), and then fill in the remaining digits to form a 16-digit or 15-digit digital string. 2. **Calculate the check digit**: Use the Luhn algorithm to calculate the last digit (check digit) to ensure that the generated number is valid. 3. **Luhn algorithm steps**: - Process each digit from right to left. - If it is an odd digit, keep the original digit. If it is an even digit, multiply the number by 2. If the result is greater than 9, subtract 9. - Add all the processed digits. - If the sum is a multiple of 10, it means that the generated credit card number is valid. ### Example Suppose you want to generate a Visa card number: 1. First generate a 15-digit number starting with 4, for example: 4234 5678 9012 345. 2. Use the Luhn algorithm to calculate the last digit. Please note that generating credit card numbers is only for learning purposes, such as simulation, testing, etc., and is strictly prohibited for fraud or other illegal activities.