Custom contact forms has a built validation for fixed emails that is validated by default if the email string doesn't contain '@' or if they field is empty. Custom contact forms plugin also checks to see if the other fields are empty or not, which I think is awesome.
However, there isn't a built in validation feature for other important fields like phone numbers.
I would like to get in the code and make manual changes myself in validating phone numbers using regular expressions and php but can't find the correct file.
If there is an easier way to add validation, any suggestions appreciated.