Form validation problem can only enter pure digital

For chestnut:
In order to verify the information submitted is pure digital input, I define a click event check information submitted (), as follows:

    <form action="InsertServlet?opr=insert" name="form"  method="post">
    	<p>只能输入数字:<input type="text" name="b" ></p>
    	<p><input type="button" value="保存" οnclick="check()">
    	</p>
    </form>

check () method is as follows, where / ^ \ d $ / 0-9 represents any one of a regular expression authentication information input in this way to meet the requirements:

function check(){
		var reg = /^\d$/;
		//获取到输入的值
        var b = document.getElementsByName('b')[0].value;
        //判断是否是正则表达式的格式
        var flag = reg.test(b);
        if(!flag){
			alert("必须是数字");
			
		}else {		
		
		window.location.href="javascript:document.form.submit()"; 
		 
		}