<div class="main">
     <input id="userName" type="text" placeholder="请输入用户名">
     <input id="rule" type="button" value="过滤">
   </div>

html,body,div,input{margin:0;padding:0;}
    .main{width:400px;height:auto;padding:0 15px;text-align:center;}
    .main input{width:100%;height:35px;border:none;margin-top:20px;border-radius:5px;}
    input[type="text"]{text-align:left;padding-left:15px;box-sizing:border-box;border:1px solid blue;}
    input[type="button"]{width:50%;background:blue;}
    @media only screen and (max-width: 415px) {
      .main{width:100%;box-sizing:border-box;}
    }

var userName = document.getElementById('userName'),
        rule   = document.getElementById('rule'),
        regexEmpty = /^(\s|\u00A0)+|(\s|\u00A0)+$/g;

    rule.onclick = function (){
      userName.value = userName.value.replace(regexEmpty,''); //正则替换
      console.log(userName.value);
    }