Basic IBAN check in JAVA following the rules as laid out on (http://en.wikipedia.org/wiki/International_Bank_Account_Number#Validating_the_IBAN)

IbanTest.java

import java.math.BigInteger;

public final class IbanTest {

    public static final int IBANNUMBER_MIN_SIZE = 15;
    public static final int IBANNUMBER_MAX_SIZE = 34;
    public static final BigInteger IBANNUMBER_MAGIC_NUMBER = new BigInteger("97");

    public static boolean ibanTest(String accountNumber) {
        String newAccountNumber = accountNumber.trim();

        // Check that the total IBAN length is correct as per the country. If not, the IBAN is invalid. We could also check
        // for specific length according to country, but for now we won't
        if (newAccountNumber.length() < IBANNUMBER_MIN_SIZE || newAccountNumber.length() > IBANNUMBER_MAX_SIZE) {
            return false;
        }

        // Move the four initial characters to the end of the string.
        newAccountNumber = newAccountNumber.substring(4) + newAccountNumber.substring(0, 4);

        // Replace each letter in the string with two digits, thereby expanding the string, where A = 10, B = 11, ..., Z = 35.
        StringBuilder numericAccountNumber = new StringBuilder();
        for (int i = 0;i < newAccountNumber.length();i++) {
            numericAccountNumber.append(Character.getNumericValue(newAccountNumber.charAt(i)));
        }

        // Interpret the string as a decimal integer and compute the remainder of that number on division by 97.
        BigInteger ibanNumber = new BigInteger(numericAccountNumber.toString());
        return ibanNumber.mod(IBANNUMBER_MAGIC_NUMBER).intValue() == 1;

    }
}

Hi @dandydev

From the documentation Character.getNumericValue():

If the character does not have a numeric value, then -1 is returned. If the character has a numeric value that cannot be represented as a nonnegative integer (for example, a fractional value), then -2 is returned.

In case of a negative value returned by this method the BigInteger initialisation will crash. Here is my quick fix:

        // Replace each letter in the string with two digits, thereby expanding the string, where A = 10, B = 11, ..., Z = 35.
        StringBuilder numericAccountNumber = new StringBuilder();
        int numericValue;
        for (int i = 0;i < newAccountNumber.length();i++) {
            numericValue = Character.getNumericValue(newAccountNumber.charAt(i));
            if(-1 >= numericValue) {
                return false;
            } else {
                numericAccountNumber.append(numericValue);
            }
        }

Using regex can be sensible faster.
Moreover using a bit of binary math can avoid using BigInteger.

        // Replace each letter in the string with two digits, thereby expanding the string, where A=10, B=11, ..., Z=35.
        string modifiedIban = iban.Substring(4) + iban.Substring(0, 4);
        modifiedIban = Regex.Replace(modifiedIban, @"\D", m => ((int)m.Value[0] - 55).ToString());
        int remainer = 0;
        while (modifiedIban.Length >= 7) {
            remainer = int.Parse(remainer + modifiedIban.Substring(0, 7)) % 97;
            modifiedIban = modifiedIban.Substring(7);
        }
        remainer = int.Parse(remainer + modifiedIban) % 97;
        return (remainer == 1);

I hope it helps