The code contains a function that gets called to check or validate if a creditcard is either a mastercard,visa,american express or Discover card.
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Visa Card: The pattern starts with a "^" to signify the start of the string, followed by the digit "4", which is unique to Visa cards. It then expects exactly 12 digits (0-9) after the starting digit, with an optional group of three more digits at the end. This pattern matches typical Visa card numbers.
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Mastercard: Similar to Visa, the pattern starts with a "^" followed by the digit "5", which is characteristic of Mastercard. The second digit must be in the range 1-5, followed by exactly 14 digits (0-9). This pattern matches typical Mastercard numbers.
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American Express: The pattern begins with a "^" followed by the digit "3", specific to American Express cards. The second digit must be either 4 or 7, followed by exactly 13 digits (0-9). This pattern matches typical American Express card numbers.
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Discover Card: The pattern starts with a "^" followed by the digit "6", which is indicative of Discover cards. It then expects either "011" or "5" followed by any two digits (0-9), followed by exactly 12 digits (0-9). This pattern matches typical Discover card numbers.
Finally it returns "Unknown" if its invalid.