密码验证正则表达式

public final static String REG_PASSWORD_EXP="^[0-9a-zA-Z_]{6,8}$";
    regExp = ConstantUtil.REG_PASSWORD_EXP;
    p = Pattern.compile(regExp);
    m = p.matcher(cus.getPassword().trim());
    if (!m.find())
    {
        message.setCode(ConstantUtil.M_EXCEPTION);
        message.setMessage("请输入正确的密码");
        this.out(response, message);
        return;
    }