SOURCE

function checkRegisterForm() {
    var password_input = document.getElementById('password');
    var username = document.getElementById('username');
    var password_input2 = document.getElementById('password-2');

    // check username
    var re = /^\w{3,10}$/;
    if (!re.test(username.value)) {
        alert('用户名必须是3-10位英文字母或数字');
        return false;
    }

    // check password
    if ((password_input.value.length < 6) || (password_input.value.length>20)) {
        alert("口令必须是6-20位");
        return false;
    }

    if (password_input.value !== password_input2.value) {
        alert('两次输入的口令不一致');
        return false;
    }

    return true;
}

<form id="test-register" action="#" target="_blank" onsubmit="return checkRegisterForm()">
    <p id="test-error" style="color:red"></p>
    <p>
        用户名: <input type="text" id="username" name="username">
    </p>
    <p>
        口令: <input type="password" id="password" name="password">
    </p>
    <p>
        重复口令: <input type="password" id="password-2">
    </p>
    <p>
        <button type="submit">提交</button> <button type="reset">重置</button>
    </p>
</form>