captcha验证中的问题

<?php
session_start();
$_SESSION['n1'] = rand(1,20);
$_SESSION['n2'] = rand(1,20);
$_SESSION['expect'] = $_SESSION['n1']+$_SESSION['n2'];

$str='';
if($_SESSION['errStr'])
{
    $str='<div class="error">'.$_SESSION['errStr'].'</div>';
    unset($_SESSION['errStr']);
}
$success='';
if($_SESSION['sent'])
{
    $success='<h1>Thank you!</h1>';
    $css='<style type="text/css">#contact-form{display:none;}</style>';
    unset($_SESSION['sent']);
}
?>
<script>
function formValidate()
{
var captcha=document.form.captcha.value;
var num=/^[0-9]+$/;
if(captcha=="")
{
document.getElementById("errCaptcha").innerHTML="Enter Captcha";
}
else if(!captcha.match(num))
{
document.getElementById("errCaptcha").innerHTML="Enter Valid Captcha";
}
else
{
document.getElementById("errCaptcha").innerHTML="";
var ez=1;
}
if(ez==1)
{
return true;
}
else
{
return false;
}
}
</script>
<form id="form3" name="form" action="" onsubmit='return formValidate()' method="post">  
<fieldset class="last">                                     
                <p>
                    <label for="captcha"><?php echo $_SESSION['n1']; ?> + <?php echo $_SESSION['n2'];?> =<span class="style1">*</span><span style="color:red" id="errCaptcha"></span></label></td>
                    <input type="text" name="captcha" id="captcha" />               
            </fieldset>                 
            <p class="submit"><button type="submit">Send</button></p>           
        </form> 

这就是我正在做的，但无法验证captcha例如：2+2=4但是如果我写2+2=10它不会验证