Return a regex pattern matching valid email addresses. Uses the same
        logic as the django validator, with the folowing exceptions:

        - Internationalized domain names not supported
        - IP addresses not supported
        - Strips lookbehinds (not supported in javascript regular expressions)

def get_email_regex(self):
        """
        Return a regex pattern matching valid email addresses. Uses the same
        logic as the django validator, with the folowing exceptions:

        - Internationalized domain names not supported
        - IP addresses not supported
        - Strips lookbehinds (not supported in javascript regular expressions)
        """
        validator = self.default_validators[0]
        user_regex = validator.user_regex.pattern.replace('\Z', '@')
        domain_patterns = ([re.escape(domain) + '$' for domain in
                            validator.domain_whitelist] +
                           [validator.domain_regex.pattern.replace('\Z', '$')])
        domain_regex = '({0})'.format('|'.join(domain_patterns))
        email_regex = user_regex + domain_regex
        return re.sub(r'\(\?\<[^()]*?\)', '', email_regex)