Adding custom bbcode with no-value parameter

[Aradmey]

I added a custom bbcode to subs.php lately. Now, I want to add a parameter which will have no value, something like this:
If this is the regular use:

Code Select Expand

[mem][/mem]

I want this to work too:

Code Select Expand

[mem all][/mem]

How do I do it? I don't want the parameter to have any value whatsoever.
Thank you.

I currently do it like this without parameter:

Code Select Expand

array(
    'tag' => 'members',
    'type' => 'unparsed_content',
    'content' => '$1',
    'validate' => my function,
    'disabled_content' => '$1',
);