Variables can be used in regular expression when doing variables extraction (advanced mode must be enabled for that).
However, for content validations, it is not possible to use a variable if the validation is done using a regexp.
This feature should really be added to the upcoming versions of NeoLoad !
This query has been sent to the NeoLoad product manager. But next time you should post in the idea section instead of answers section.
If the question is still open. You can use the JSONPath Function at the Validation window - that's quiet helpful! I searched for that function too and found that cool possability.