function SLyz(){
if(document.commentform.comname.value.length == 0){alert("请输入昵称");document.commentform.comname.focus();return false;}
var reg = /^([a-zA-Z0-9]+[_|\_|\.]?)*[a-zA-Z0-9]+@([a-zA-Z0-9]+[_|\_|\.]?)*[a-zA-Z0-9]+\.[a-zA-Z]{2,3}$/;if(document.commentform.commail.value.length == 0 && !reg.test(document.commentform.commail.value)){alert("1、邮箱地址是否填写 \r\n2 、邮箱地址是否正确");document.commentform.commail.focus();return false;}
var reg = /^(http:\/\/|https:\/\/)([a-zA-Z0-9]([a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{2,6}$/;if(!reg.test(document.commentform.comurl.value) && (document.commentform.comurl.value)){alert("网址格式必须加http://或https://且网址结尾不能有“/”;次选为选填项，可不填写");document.commentform.comurl.focus();return false;}
if(document.commentform.comment.value.length == 0){alert("请填写评论内容且评论内容需包含中文");document.xj.comment.focus();return false;}
}

<form method="post" name="commentform" action="<?php echo BLOG_URL; ?>index.php?action=addcom" id="commentform">

<form method="post" name="commentform" action="<?php echo BLOG_URL; ?>index.php?action=addcom" id="commentform" onSubmit="return SLyz();">