I appreciate you adding in this capability with 2.5.0, however, I cannot get it to work. I changed my custom field from Text to Custom Text, put in the regex and saved. Now there is no textbox for the customer to enter a value into and the product cannot be added to the shopping cart. Only the field label and the asterisk is shown. I cannot even revert the field back to Text, as that dropdown is not shown anymore for that field.
EDIT: Actually the product was added to the cart, but there was not the usual add to cart notice. There was also no warning about an invalid entry. Since there is no textbox, and thus no value entered, I would have expected some validation error being shown, but there wasn't.