Answer by tom for masking of email address in java

This will try to mask even if the character count before the @ is less than or equal to 3.

    public static String maskEmail(String fieldValue) {        if (StringUtils.isBlank(fieldValue)) {            return "";        }        int pos = fieldValue.indexOf('@');        if (pos == -1) {            return "*".repeat(fieldValue.length());        }        int chars = Math.min(pos-1, 3);        String pattstr = "(^[^@]{%d}|(?!^)\\G)[^@]".formatted(chars);        return fieldValue.replaceAll(pattstr, "$1*");    }