Credit Card Validation --- Luhn Check

/**
 * Checks if the value is a valid credit card number
 * @param value a string
 * @return true if the value is a valid credit card number
 */
public static boolean isValid(String value){
    return luhnCheck(value.replaceAll("\\D", "")); //remove non-digits
}
 
 /**
 * Checks if a cardNumber passes LUHN check
 * @param cardNumber A string contains only digits
 * @return true if the cardNumber passes LUHN check
 */
private static boolean luhnCheck(String cardNumber){
    int sum=0;
    for (int i=cardNumber.length()-1; i>=0; i-=2){
    sum+=Integer.parseInt(cardNumber.substring(i, i+1));
        if (i>0){
            int d=2*Integer.parseInt(cardNumber.substring(i-1, i));
            if (d>9) d-=9;
            sum+=d;
        }
    }
    return sum%10==0;
}