val.py

class _BaseDomainNameValidator(BaseValidator):
    def __call__(self, form, field):
        labels = get_labels(value)
        if not labels:
            return False
        is_valid = True
        for index, label in enumerate(labels):
            if index == 0 and label == '*':
                continue
            if not label and index + 1 != len(labels):
                is_valid = False
            label_str = get_label_str(label)
            if re.search(self.regex, test_str):
                field.errors.append('Must be a valid domain name.')
                is_valid = False
        return is_valid

    def get_labels(self, value):
        if not value or value in ['@', '*']:
            return []
        return value.split('.')

    def get_label_str(label):
        """Can be overridden"""
        return label


class DomainNameValidator(BaseValidator):
    regex = '[^a-zA-Z0-9-_]'


class TxtRecordDomainNameValidator(BaseValidator):
    regex = '[^a-zA-Z0-9-]'

    def get_label_str(self, label):
        return label[1:] if label.startswith('_') else label