Preg_match confusion

Question

While developing a login section , i got an error .

The result always shows up the false statement.what's wrong with this code

Could anyone guide me through "preg_match",Wolud like to know preg_match in detail.

The code is :

if (isset($_POST['submit'])) 
 {

if (preg_match('%^[A-Za-z0-9]\S{8,20}$%', stripslashes(trim($_POST['userid'])))) 
{
    $u = $_POST['userid'];
    echo $u.'<br />';
} 
else 
{
    $u = FALSE;
    echo '<p><font color="red" size="+1">Please enter a valid User ID!</font></p>';
}

    if (preg_match ('%^[A-Za-z0-9]\S{8,20}$%', stripslashes(trim($_POST['password'])))) 
{
    $p =$_POST['password'];
    $p=sha1($p);
    echo $p;
} 
else 
{
    $p = FALSE;
    echo '<p><font color="red" size="+1">Please enter a valid Password!</font></p>';
}

}

Answer 1

%^[A-Za-z0-9]\S{8,20}$%

Means:

1 of A-Z or a-z or 0-9 8 through 20 of \S

I doubt that's what you meant?

Perhaps try the \S in the []