伪PHP正则表达式

我正在尝试使用php找到正确的正则表达式来检查伪代码。它应该在4到64个字符之间，具有字母数字字符（区分大小写）和下划线。我尝试了这些：

preg_match ( "/^([[[:alnum:]]_]){4, 64}$/", $pseudo )
preg_match ( "/^([a-zA-Z0-9_]){4, 64}$/", $pseudo )
preg_match ( "/^[[[:alnum:]]_]{4, 64}$/", $pseudo )
preg_match ( "/^[a-zA-Z0-9_]{4, 64}$/", $pseudo )
preg_match ( "/^([[[:alnum:]]_]){4,64}$/", $pseudo )
preg_match ( "/^([a-zA-Z0-9_]){4,64}$/", $pseudo )
preg_match ( "/^[[[:alnum:]]_]{4,64}$/", $pseudo )
preg_match ( "/^[a-zA-Z0-9_]{4,64}$/", $pseudo )
preg_match ( "#^([[[:alnum:]]_]){4, 64}$#", $pseudo )
preg_match ( "#^([a-zA-Z0-9_]){4, 64}$#", $pseudo )
preg_match ( "#^[[[:alnum:]]_]{4, 64}$#", $pseudo )
preg_match ( "#^[a-zA-Z0-9_]{4, 64}$#", $pseudo )
preg_match ( "#^([[[:alnum:]]_]){4,64}$#", $pseudo )
preg_match ( "#^([a-zA-Z0-9_]){4,64}$#", $pseudo )
preg_match ( "#^[[[:alnum:]]_]{4,64}$#", $pseudo )
preg_match ( "#^[a-zA-Z0-9_]{4,64}$#", $pseudo )
...

还有更多，但没人能用，我不知道为什么。

编辑：我的功能是这样的：

function pseudoIsAllowed($pseudo) {
    $res = true;
    echo "$pseudo\n";
    if (! preg_match ( "/^[[[:alnum:]]_]{4,64}$/", $pseudo ))
        $res = false;
    if ($res)
        echo "$pseudo is allowed.\n";
    else
        echo "$pseudo is NOT allowed.\n";
    return $res;
}