Documentation missing on custom fields

-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.5.1
-- PHP version -- : 5.6.17
-- Browser(s) name and version -- : Firefox 47.0

I am trying to create a custom field that will be validated against a range of numbers.
I would assume that's the intent of the "Regular Expression Check".
I've gone through the documentation and every forum posting trying to figure out how to setup this type of validation and found nothing that documents the intended use. Nor could I find an example on the demo site to try to figure out how it should be setup.

I am familiar with the creation and testing of regular expressions, but it's the expected setup in Hikashop I don't understand, now what is supposed to happen on the front-end in validation.

I created a custom field of type "Text" on the "item" table and entered a variety of valid regular expression for testing, including virtually every regex I found in this forum referencing expressions in other inquiries. No matter what I tried, the newly added custom field on the front-end lets me enter anything. It's as if the check isn't being fired.

Is there documentation on how to setup an expression-validated field in Hikashop?
(I don't need documentation on regular expressions themselves)

Once setup, when is the validation performed - on field exit, when the user tries to add to cart or some other event?
If the value entered fails the check, is it the "Custom Error Message" that is displayed?
Do I need to do anything else to get the expression check to be run to validate the input?