modify wordpress optin plugin

looking to modify wordpress plugin for the number entry people now enter all sort of number in the telephone entry line to make the optin complete which is not good. What i would like is to add a function in the plugin where only 4 area code can be enter (xxx or yyy or ddd or ggg) plus 7 digits (xyc-fghj)max repeated numer time 3 time for each digits. if the form does not contain the good area code plus 7 digits max digit repeated time is 3 for the 7 numbers. the form cannot be submitted.