javascript - Some questions about regular expressions-PHP Chinese Network Q&A

    <!DOCTYPE html>
    <html lang="en">
    <head>
    <meta charset="utf-8" />
    <title>简易网页时钟</title>
    <style type="text/css">
    body,p {margin: 0;padding: 0;}
    body {color: #fff;font: 16px/1.5}
    #clock {width: 300px;text-align: center;background: #1a1a1a;margin: 10px auto;padding: 20px 0;}
    span {color: #000;width: 80px;line-height: 2;background: #fbfbfb;border: 2px solid #b4b4b4;margin: 0 10px;padding: 0 10px;}
    </style>
    <script type="text/javascript">
    window.onload = function() {
        var oClock = document.getElementById("clock");
        var aSpan = oClock.getElementsByTagName("span");
        setInterval(getTimes,1000);
        getTimes();
        function getTimes() {
            var oDate = new Date();
            var aDate = [oDate.getHours(),oDate.getMinutes(),oDate.getSeconds()];
            for(var i in aDate) aSpan[i].innerHTML = format(aDate[i])
        }
        function format(a) {
            return a.toString().replace(/^(\d)$/,"0")
        }
    }
    </script>
    
    
    <body>
    <p id="clock">
        <span></span>点<span></span>分<span></span>秒
    </p>
    </body>
    </html>

1.return a.toString().replace(/^(\d)$/,"0")这句表达的是什么意思呢?
2.setInterval(getTimes(),1000)这样写不行吗？为什么？

reply all(1)

世界只因有你2017-07-05 10:56:43 1 floor

1, /^(d)$/ are used to match a string with only one number, such as 2, 3, etc. The function of the brackets is to store the number as group 1,
replace( /^(d)$/,"0$1") is used to add 0 before the number, $1 represents the string stored in group 1,
such as 2 is replaced by 02, 3 Replace with 03,
You can enter '2'.replace(/^(d)$/,"0$1")in the console and run to see the result

2. No, the first parameter of setInterval must be a function, and getTimes() is the value after running the function. Only getTimes is a function

I answered two similar questions to the questioner in succession. I suggest that the questioner read the book carefully first and lay a solid foundation