<html>
<head>
<style>
.box{
padding: 50px;
}
.left ,.top{
float: left;
}
.left{
margin-right: 10px;
}
.tip{
font-size: 12px;
color: red;
display: none;
}
</style>
</head>
<body>
<div class="box">
<div class="left">
<input type="text" id="phone" placeholder="输入手机号">
</div>
<div class="tip" id="tip">
请输入正确的手机号
</div>
</div>
</body>
<script>
window.onload = function(){ //窗口加载事件
var phone = document.getElementById("phone");
var tip = document.getElementById("tip");
phone.onfocus = function(){ //当输入框开始获得焦点，显示后面的提示
tip.style.display = "block";
}
phone.onblur = function(){ //当鼠标移开输入框，说明输入完成，开始校验格式是否正确
var relPhone = this.value;
const reg = /^1\d{10}$/; //手机号正确表达式
if (reg.test(relPhone)){
tip.innerHTML = "正确";
}else {
tip.innerHTML = "手机号格式有误";
}
}
}
</script>
</html>
