我们可以使用 .input-group 类来向表单输入框中添加更多的样式，如图标、文本或者按钮。
使用 .input-group-prepend 类可以在输入框的的前面添加文本信息， .input-group-append 类添加在输入框的后面。
最后，我们还需要使用 .input-group-text 类来设置文本的样式。
Bootstrap4 实例
<form>
<div class="input-group mb-3">
<div class="input-group-prepend">
<span class="input-group-text">@</span>
</div>
<input type="text" class="form-control" placeholder="Username">
</div>
<div class="input-group mb-3">
<input type="text" class="form-control" placeholder="Your Email">
<div class="input-group-append">
<span class="input-group-text">@json.cn</span>
</div>
</div>
</form>
输入框大小
使用 .input-group-sm 类来设置小的输入框， .input-group-lg 类设置大的输入框：
Bootstrap4 实例
<form>
<div class="input-group mb-3 input-group-sm">
<div class="input-group-prepend">
<span class="input-group-text">Small</span>
</div>
<input type="text" class="form-control">
</div>
</form>
<form>
<div class="input-group mb-3">
<div class="input-group-prepend">
<span class="input-group-text">Default</span>
</div>
<input type="text" class="form-control">
</div>
</form>
<form>
<div class="input-group mb-3 input-group-lg">
<div class="input-group-prepend">
<span class="input-group-text">Large</span>
</div>
<input type="text" class="form-control">
</div>
</form>
多个输入框和文本
Bootstrap4 实例
<!-- 多个输入框 -->
<form>
<div class="input-group mb-3">
<div class="input-group-prepend">
<span class="input-group-text">Person</span>
</div>
<input type="text" class="form-control" placeholder="First Name">
<input type="text" class="form-control" placeholder="Last Name">
</div>
</form>
<!-- 多个文本信息 -->
<form>
<div class="input-group mb-3">
<div class="input-group-prepend">
<span class="input-group-text">One</span>
<span class="input-group-text">Two</span>
<span class="input-group-text">Three</span>
</div>
<input type="text" class="form-control">
</div>
</form>
复选框与单选框
文本信息可以使用复选框与单选框替代：
Bootstrap4 实例
<div class="input-group mb-3">
<div class="input-group-prepend">
<div class="input-group-text">
<input type="checkbox">
</div>
</div>
<input type="text" class="form-control" placeholder="JSON">
</div>
<div class="input-group mb-3">
<div class="input-group-prepend">
<div class="input-group-text">
<input type="radio">
</div>
</div>
<input type="text" class="form-control" placeholder="GOOGLE">
</div>
输入框添加按钮组
Bootstrap4 实例
<div class="input-group mb-3">
<div class="input-group-prepend">
<button class="btn btn-outline-secondary" type="button">Basic Button</button>
</div>
<input type="text" class="form-control" placeholder="Some text">
</div>
<div class="input-group mb-3">
<input type="text" class="form-control" placeholder="Search">
<div class="input-group-append">
<button class="btn btn-success" type="submit">Go</button>
</div>
</div>
<div class="input-group mb-3">
<input type="text" class="form-control" placeholder="Something clever..">
<div class="input-group-append">
<button class="btn btn-primary" type="button">OK</button>
<button class="btn btn-danger" type="button">Cancel</button>
</div>
</div>
设置下拉菜单
输入框中添加下拉菜单不需要使用 .dropdown 类。
Bootstrap4 实例
<div class="input-group mt-3 mb-3">
<div class="input-group-prepend">
<button type="button" class="btn btn-outline-secondary dropdown-toggle" data-toggle="dropdown">
选择网站
</button>
<div class="dropdown-menu">
<a class="dropdown-item" href="https://www.google.com">GOOGLE</a>
<a class="dropdown-item" href="">JSON</a>
<a class="dropdown-item" href="https://www.taobao.com">TAOBAO</a>
</div>
</div>
<input type="text" class="form-control" placeholder="网站地址">
</div>
输入框组标签
在输入框组通过在输入框组外围的 label 来设置标签，标签的 for 属性需要与输入框组的 id 对应，点击标签后可以聚焦输入框：
Bootstrap4 实例
<label for="demo">Write your email here:</label>
<div class="input-group mb-3">
<input type="text" class="form-control" placeholder="Email" id="demo" name="email">
<div class="input-group-append">
<span class="input-group-text">@json.cn</span>
</div>
</div>