According to support: The Variables list does not support Regex. Those variables should be literally added to the list.
This seems like a major shortcoming as I would like to add things like {\d} instead of individual values {0}, {1}, {2}... et cetera.
In another case, I have shortcode pointing to a disclaimer like [d:7441]. This one is not as easy as tying out all of the possible values but could easily be handled by: \[d\:\d+?\]
I cannot understand why regex is not valid here.