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 !
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.
