private static final Pattern PASSWORD_PATTERN =
            Pattern.compile("^" +
                    //"(?=.*[0-9])" +         //at least 1 digit
                    //"(?=.*[a-z])" +         //at least 1 lower case letter
                    //"(?=.*[A-Z])" +         //at least 1 upper case letter
                    "(?=.*[a-zA-Z])" +      //any letter
                    "(?=.*[@#$%^&+=])" +    //at least 1 special character
                    "(?=\\S+$)" +           //no white spaces
                    ".{4,}" +               //at least 4 characters
                    "$");