How to find index of the first left appearance of character that belongs to regex?

How to find index of the first left appearance of character that belongs
to regex?

How to find index of the first left appearance of character that belongs
to regex ? I have arbitrary string and regex like invalid_tail =
re.compile(r'[^a-zA-Z0-9+/=\n\r].*$') and I need to find index of
character that is in invallid_tail reg.