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代码编辑器：

<!DOCTYPE htm>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet">
<script src="https://cdn.bootcss.com/jquery/1.12.4/jquery.js"></script>
<script src="https://cdn.bootcss.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<style>body {margin:20px}</style>
<form class="bs-example bs-example-form" role="form">
    <div class="input-group input-group-lg">
        <span class="input-group-addon">@</span>
        <input type="text" class="form-control" placeholder="Twitterhandle">
    </div>
    <br>
    <div class="input-group">
        <span class="input-group-addon">@</span>
        <input type="text" class="form-control" placeholder="Twitterhandle">
    </div>
    <br>
    <div class="input-group input-group-sm">
        <span class="input-group-addon">@</span>
        <input type="text" class="form-control" placeholder="Twitterhandle">
    </div>
</form>

代码编辑器：

运行结果：