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# How to validate Visa Card number using Regular Expression?

Visa credit or debit cards are assigned a 16-digit unique identifier known as a Visa card number. The number—which is ordinarily stamped on the front of the card—is used to find the cardholder's account while making purchases or carrying out transactions. The first six digits of a Visa card number reflect the issuing bank in contrast to the remaining digits, which are unique to the account number and the cardholder. For the purpose of validating the number's accuracy and preventing fraud, Visa card numbers have an extra check digit.

## Methods

The methods are as follows for validating a Visa card number with regular expression −

Explain the regular expression

Determine the pattern

## Method 1: Explain the regular expression

To make a regular expression that complies with the Visa card number pattern, we can use character classes, quantifiers, and grouping.

### Syntax

The following syntax can be used in C++ to validate a Visa card number using regular expressions −

Use regular expressions by include the regex> header file.

Create a pattern for regular expression that resembles structure of Visa card number. A Visa card number always includes 16 digits and 4 as first digit. This is the way pattern can explain −

std::regex pattern("^4[0-9]{15}$");

Any string with a starting digit of 4 and a subsequent 15 digits will fit this pattern.

Using std::regex_match() function, find out that user-provided Visa card number matches regular expression pattern. The two inputs to std::regex_match() method are string to be tested and the regular expression pattern that will be used to test it. If string matches desired pattern, method returns true. In absence of that, it returns false.

std::string visa Number = "1234567890123456"; if (std::regex_match(visa Number, pattern)) { std::cout << "Valid Visa card number." << std::endl; } else { std::cout << "Invalid Visa card number." << std::endl; }

### Algorithm

A possible regular expressions algorithm to verify a Visa card number is as follows −

**Step 1** − To match Visa card numbers in a regular expression, first create a pattern. The pattern's first character should be "" to signify the beginning of the string and "4" to signify a Visa card. The final 15 characters all have the possibility of being digits. The string's end should be indicated by the symbol "$" at end of pattern. A Visa card number is represented by the regular expression pattern "4[0-9]15$".

**Step 2** − Take string representation of entered Visa card number.

**Step 3** − Make use of a regular expression engine to compare the Visa card number to the pattern in the regular expression.

**Step 4** − If the Visa card number matches regular expression pattern, then it is valid Visa card number. A non-matching Visa card number is considered invalid.

**Step 5** − The Visa card number can also be checked to see if it is still valid by calculating a cheque sum using the card's digits. Before being added to the other digits to create the checksum, each extra digit must first be multiplied by two. It's acceptable to use any card number as long as the total is a multiple of 10. The card number must be a multiple of 10, or it is ineligible.

**Step 6** − As the result of the validation, return "Valid" or "Invalid".

### Example 1

Here is an example in C++ for validating a Visa card number using Regular Expression −

The regular expression pattern 4[0-9]12(? :[0-9]3])?, which corresponds to the dollar amount on a Visa card, is first defined in this example. To match this pattern, the string must start with 4, be followed by 12 digits, and then either have 3 more digits or not. The std::regex_match() function that is used to find whether the pattern matches supplied Visa card number std::regex object has been constructed using same pattern. "Valid Visa card number" is outputted if the card number resembles the pattern, otherwise, "Invalid Visa card number."

#include <iostream> #include <regex> #include <string> using namespace std; int main() { std:: regex visa_regex("^4[0-9]{12}(?:[0-9]{3})?$"); std:: string visa_card_number = "4123456789012345"; if (std::regex_match(visa_card_number, visa_regex)) { cout << "Valid Visa card number" << endl; } else { cout << "Invalid Visa card number" << endl; } return 0; }

### Output

Valid Visa card number

## Method 2: Determine the pattern

Visa card numbers usually begin with a '4' and are 13 to 16 digits long. The first digit is constant or fixed, and the remaining digits can be different. On the basis of this information, one can make a regular expression pattern.

### Syntax

To validate a Visa card number using regular expressions, you can use the following syntax for the match method.

^4[0-9]{12}(?:[0-9]{3})?$

In this regular expression pattern −

^ represents the start of the input.

**4**indicates that the card number must start with the digit 4, as Visa card numbers always do.**[0-9]{12}**specifies that the next 12 characters must be digits ranging from 0 to 9.**(?:[0-9]{3})?**allows for an optional group of 3 digits at the end of the card number, represented by**(?: ... )?.**This group is enclosed in**(?: ... )**to make it non-capturing.**$**denotes the end of the input.

### Algorithm

To validate a Visa card number using a regular expression, you can follow the algorithm outlined below −

**Step 1** − Define the regular expression pattern for a Visa card number. Visa card numbers typically start with the digit 4 and have a total length of 16 digits.

**Step 2** − Create a function or method to implement the validation algorithm. Let's call it **validateVisaCardNumber.**

**Step 3** − Inside the **validateVisaCardNumber **function, take the card number as input.

**Step 4** − Utilise a regular expression matching function in your programming language to apply the regular expression pattern to the card number. If the card number matches the pattern, this function should return a boolean result reflecting that fact.

**Step 5** − Return true to show that the card number is legitimate for a Visa card if the regular expression pattern matches the card number.

**Step 6** − Return false to indicate that the card number is invalid for a Visa card if the regular expression pattern does not match the card number.

### Example 2

An example that shows how to validate a Visa card number by using a regular expression −

In this example, the isVisaCardValid function checks whether a card number is a valid Visa card number using a regular expression pattern (4[0-9]12(?:[0-9]3)?$). The primary function then validates the function using two sample card numbers before displaying the findings.

Please take note that only Visa card numbers can be used with this regular expression pattern. Different patterns should be same for other card kinds, also for the Mastercard and American Express.

#include <iostream> #include <regex> bool isVisaCardValid(const std::string& cardNumber) { // Define the regular expression pattern for Visa card numbers std::regex pattern("^4[0-9]{12}(?:[0-9]{3})?$"); // Match the card number against the pattern return std::regex_match(cardNumber, pattern); } int main() { // Test cases std::string cardNumber1 = "4532015112890367"; // Valid Visa card number std::string cardNumber2 = "378282246310005"; // Not a Visa card number // Validate the Visa card numbers bool isValid1 = isVisaCardValid(cardNumber1); bool isValid2 = isVisaCardValid(cardNumber2); // Display the results std::cout << "Card Number 1 is " << (isValid1 ? "valid" : "invalid") << std::endl; std::cout << "Card Number 2 is " << (isValid2 ? "valid" : "invalid") << std::endl; return 0; }

### Output

Card Number 1 is valid Card Number 2 is invalid

## Conclusion

In conclusion, developers and organizations can take advantage of handy technique of validating a Visa card number using regular expressions to make sure that the credit and debit card numbers submitted by customers are secure and legitimate. The regular expression for validating a Visa card number normally includes checking the length of the number, making sure it begins with the right digit, and making sure it passes the Luhn algorithm check. Businesses can lower the risk of fraudulent transactions and increase the security of their payment systems by using regular expression validation for Visa card numbers.