Credit card issuer checking

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public class Kata {
  //enum cardType = {"Unknown", "AMEX", "Discover", "Mastercard", "VISA"};
  public static String getIssuer(String cardNumber) {
    // AMEX 3400~3499
    // AMEX 3700~3799
    
    // Discover 6011
    
    // Mastercard 5100~5599
    // VISA 4000~4999
   
    int n = cardNumber.length();
    if(n < 4) return "Unknown";
    
    if(cardNumber.startsWith("34") || cardNumber.startsWith("37")) {
      if(n == 15) return "AMEX";      
    } else if(cardNumber.startsWith("6011")) {
      if(n == 16) return "Discover";      
    } else if(cardNumber.startsWith("51") 
             || cardNumber.startsWith("52")
             || cardNumber.startsWith("53")
             || cardNumber.startsWith("54")
             || cardNumber.startsWith("55")) {
      if(n == 16) return "Mastercard";      
    } else if(cardNumber.startsWith("4")) {
      if(n == 13 || n == 16) return "VISA";
    }
    
    return "Unknown";
    
  }
}

import static java.util.regex.Pattern.matches;

public class Kata { 

  public static String getIssuer(String cn) {
    if (matches("^3[4|7].{13}$", cn)) return "AMEX";
    if (matches("^6011.{12}$", cn)) return "Discover";
    if (matches("^5[1-5].{14}$", cn)) return "Mastercard";
    if (matches("^4.{12}(...)?$", cn)) return "VISA";
    return "Unknown";
  }
  
}