php表单,带有javascript验证

i'm working on a registration page. im stuck in this javascript form validation. what i want to do is first validate the entered data then pass the value to action page. but the validation never works. i tried removing action and also removing (form) in function and tried passing form but none works.

here is my code

<html>

<head>
    <title>Registeration</title>
    <script language="javascript">
    function regval(form) {
        var a = document.forms["regform"]["username"].value;
        var b = document.forms["regform"]["password"].value;

        if (a == null || a == "") {
            alert("Enter username");
            return false;
        }
        if (b == null || b == "") {
            alert("enter password");
            return false;
        }

    }
    </script>
</head>

<body>
    <form name="regform" id="regform" action="registration.php" method="post" onSubmit="return regval();">
        <table width="274" border="0" align="center" cellpadding="2" cellspacing="0">
            <tr>
                <td width="95">
                    <div align="right">Username:</div>
                </td>
                <td width="171">
                    <input type="text" name="username" />
                </td>
            </tr>
            <tr>
                <td>
                    <div align="right">Password:</div>
                </td>
                <td>
                    <input type="password" name="password" />
                </td>
            </tr>
            <td>
                <div align="right"></div>
            </td>
            <td>
                <input name="submit" type="submit" value="register" onClick="validateForm()" />
            </td>
            </tr>
        </table>
    </form>
</body>

</html>
if(b==nul||b="")

Should be:

if(b==null||b=="")

Or shorter:

if(!a) { alert("Enter username"); return false; }
if(!b) { alert("Enter password"); return false; }
<html>
<head>
<title>Registeration</title>
<script language="javascript">
function validateForm(form)
{
var a=document.forms["regform"]["username"].value;
var b=document.forms["regform"]["password"].value;
alert(a);

if(a==null || a=="")
{
alert("Enter username");
return false;
}
if(b==nul||b=="")
{
alert("enter password");
return false;
}

}
</script>
</head>
<body>
<form name="regform" id="regform" action="#"  method="post" onSubmit="return regval();">
<table width="274" border="0" align="center" cellpadding="2" cellspacing="0">
<tr>
<td width="95"><div align="right">Username:</div></td>
<td width="171"><input type="text" name="username" /></td>
</tr>
<tr>
<td><div align="right">Password:</div></td>
<td><input type="password" name="password" /></td>
</tr>
<td><div align="right"></div></td>
<td><input name="submit" type="submit" value="register" onClick="validateForm()"/></td>
</tr>
</table>
</form>
</body>
</html>

try this code

function regval(form)
    {
        var a=document.forms["regform"]["username"].value;
        var b=document.forms["regform"]["password"].value;

        if(a=="")
        {
            alert("Enter username");
            return false;
        }
        if(b == "")
        {
            alert("enter password");
            return false;
        }

    }