Validate CF7 textfield with 11 check for Dutch BSN

I'm using CF7 on a Wordpress website and would like to validate a textfield if it contains 9 digits and with the so called 11-check. That's an algorithm to validate Dutch SSN (BSN). It works likes this: Let's say we perform the 11-check on BSN 123456782 1st digit = 1, 9 * 1 = 9 2nd digit = 2, 8 * 2 = 16 3rd digit = 3, 7 * 3 = 21 4th digit = 4, 6 * 4 = 24 5th digit = 5, 5 * 5 = 25 6th digit = 6, 4 * 6 = 24 7th digit = 7, 3 * 7 = 21 8th digit = 8, 2 * 8 = 16 9th digit = 2, -1 * 2 = -2 (last digit is not added but subtracted) Total is 154 Because 154 can be divided by 11 and it's a 9 digit number we can assume it is a valid BSN (154/11=14). It's important that there also is a validation of the length of 9 digits and that it's only possible to enter digits. I think this can be done with Javascript. The validation should work like any other validation in Contact Form 7. So that the form cannot be submitted when the input is invalid.