JavaScript正则表达式练习详解编程语言

var reg = /^/d{6}$/; 
        var str = "130400"; 
        var b = str.match(reg); 
        if (b === null) { 
            alert("格式错误"); 
        }else{ 
            document.write(b); 
        }

var reg = /^[a-zA-Z]{1}([a-zA-Z0-9]|[_.]){4,19}$/; 
        var str = "as_12asasa"; 
        document.write(reg.test(str));