如下的代码为什么不能检查正确的邮件地址。

[caobin518]

要求：

当用户在useremail文本框中输完邮件地址后就检查是否合法。不合法就弹出：“错误的邮件格式”
<script language="javascript">
function isEmail (theStr) {
var atIndex = theStr.indexOf('@');
var dotIndex = theStr.indexOf('.', atIndex);
var flag = true;
theSub = theStr.substring(0, dotIndex+1)

if ((atIndex < 1)||(atIndex != theStr.lastIndexOf('@'))||(dotIndex < atIndex + 2)||(theStr.length <= theSub.length))
{ return(false); }
else { return(true); }
}
</script>
<form name="myform" method="post" action="">
email
<input name="useremail" type="text" id="useremail" onClick="isEmail(document.myform.useremail.value);">
<input type="submit" name="Submit" value="Submit">
</form>

[mysqlaping]

是正确的!!
<script language="javascript">
function isEmail (theStr) {
var atIndex = theStr.indexOf('@');
var dotIndex = theStr.indexOf('.', atIndex);
var flag = true;
theSub = theStr.substring(0, dotIndex+1)

if ((atIndex < 1)||(atIndex != theStr.lastIndexOf('@'))||(dotIndex < atIndex + 2)||(theStr.length <= theSub.length)) {
return false;
}
else {
return true;
}
}
</script>
<form name="myform" method="post" action="email.asp" onSubmit="return isEmail(document.myform.useremail.value)">
email
<input name="useremail" type="text" id="useremail" >
<input type="submit" name="Submit" value="Submit">
</form>

[mysqlaping]

<script language="javascript">
function isEmail (theStr) {
var atIndex = theStr.indexOf('@');
var dotIndex = theStr.indexOf('.', atIndex);
var flag = true;
theSub = theStr.substring(0, dotIndex+1)

if ((atIndex < 1)||(atIndex != theStr.lastIndexOf('@'))||(dotIndex < atIndex + 2)||(theStr.length <= theSub.length)) {
alert('不合法');return false;
}
else {
alert('合法');return true;
}
}
</script>
<form name="myform" method="post" action="email.asp" onSubmit="return isEmail(document.myform.useremail.value)">
email
<input name="useremail" type="text" id="useremail" >
<input type="submit" name="Submit" value="Submit">
</form>