/**
 * Returns the domain name of the given e-mail address.
 * 
 * @param email 
 *           the given e-mail address.
 * @return the domain name of the given e-mail address.
 * @throws IllegalArgumentException
 *           if the given e-mail address is not valid.
 */
public String getDomainName(String email) {
    /*
     * If we are using J2SE 1.4 (or later) the factory will create an
     * instance of JdkRegex, otherwise it will create an instance of Perl5Regex.
     */
    Regex regex = RegexFactory.createRegex("^(.+)@(.+)$");
    Match match = regex.match(email);
    
    if (!match.isSuccessful()) {
        throw new IllegalArgumentException("E-mail address is not valid");
    }

    String[] groups = match.getGroups();
    String domainName = groups[2];
    return domainName;
}