Checkout Field Validation (different than others)

Hi,
I did a search on google and on the forums and couldn't find anything regarding this.

I have a site built on 1.5.6.4
Currently utilizing a one page checkout module. (MarketInSG)

When customers are checking out or creating an account, they have the option to select their customer group. If they select a customer group then a field pops up that was originally the "Company ID" field. I have renamed the field to Member #. I need a way of verifying the alphanumeric member# is valid. I have a list of 26,000 member numbers I need to validate it against. I need to validate the number exists in the list and that the member expiry date for that member number has not passed.

Any help is appreciated. Website is located at a temporary domain, "www.canadafishingtackle.com

Hi,
any help is appreciated.

I'd like to have the validation when they click the checkout button. I can add the list of member numbers to the opencart database via phpmyadmin, but don't know where to put them.

Hi,
Anyone? Is it possible to just have it verify the Company ID field is a certain length?