Data Validation for Custom Fields?

I know that this isn't possible atm. But i need this for an custom field How can i add this? Anyone?

Example:
Customfield
Field Type = Textbox
7 digits ( from 1,000,000 to 9,999,999)
allowed sign: 0 - 9

How can i check the input? So that the customer can only put the right data to this custom field.

That's not possible in the current version. Working on a custom field libraries which support custom validation like that at the moment. Implementing for Membership Pro first, then if it all goes well, we will use it in Events Booking as well.

Features are coming, but need time to get it implemented into the core download package,

Regards,

Tuan