Help with password validation regex

[darkart]

Hi!

I have a problem regarding the password validation. I want my users to input a password consisting of alphanumeric characters which is at least 6 characters and must consist of upper and lower chars. Here is my code

if (!eregi('^{(?=.\d)(?=.[a-z])(?=.*[A-Z]).{6,16}$',$reg_password))}
{
$error_msg[] = "<font color='red'>Error! </font>Invalid password format.";
}

It gives out an error message:

Warning: eregi() [function.eregi]: REG_BADRPT in ...

I hope someone will be kind enough to help me with my problem. Thanks

[Weedpacket]

eregi() doesn't recognise PCRE assertions. Also, because you're using eregi(), it can't tell the difference between upper case and lower case.
But I wouldn't bother getting so fancy unless I had to. I can't think of many reasons why the following wouldn't be a better approach:

if(strlen($reg_password)<6 || !preg_match('/[A-Z]/', $reg_password) || !preg_match('/[a-z]/', $reg_password))
{
    $error_msg[] = "<span class='error'>Error!</span> Invalid password format.";
}