PHP Password validations using regexp

Passwords are very important security matters. How can the web developer be sure, that some security guidelines were implemented when passwords are set? Here is a password validation function, that might be useful:

public function isValidPasswordMsg($pwd)
{
    $error = "";
    if( strlen($pwd) < 6 ) {
        $error .= "Password too short! <br />";
    }

    if( !preg_match("#[0-9]+#", $pwd) ) {
        $error .= "Password must include at least one number! <br />";
    }

    if( !preg_match("#[a-z]+#", $pwd) ) {
        $error .= "Password must include at least one letter! <br />";
    }

    if( !preg_match("#[A-Z]+#", $pwd) ) {
        $error .= "Password must include at least one CAPS! <br />";
    }

    if( !preg_match("#\W+#", $pwd) ) {
        $error .= "Password must include at least one symbol! <br />";
    }

    if(empty($error))
        return TRUE;
    return $error;
}

The above function uses the php regular expressions function which is very useful in this situations.