title: Can I build a serializer that validates username and email existence on is_valid call?
body: <p>guys!
  I have this serializer class for the user model</p>
  <pre><code>class UserSerializer(serializers.ModelSerializer):
      username = serializers.RegexField(&quot;^(?=.{6,20}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?&lt;![_.])$&quot;)
      full_name = serializers.RegexField(&quot;^([a-zA-Z]{2,}\s[a-zA-Z]{1,}'?-?[a-zA-Z]{2,}\s?([a-zA-Z]{1,})?)&quot;)
      email = serializers.EmailField()
      phone = serializers.RegexField(&quot;^(\+\d{1,2}\s?)?1?\-?\.?\s?\(?\d{3}\)?[\s.-]?\d{3}[\s.-]?\d{4}$&quot;)
  
      class Meta:
          model = User
          fields = ['username', 'full_name', 'email', 'password', 'phone']
  </code></pre>
  <p>This is my serializer class for the moment. I want to validate that the username and email does not exist before saving an user. Can I do it without manually writing the validations? And I also need to validate the regex for the username and the email field for the email</p>
  <p>Thank you!</p>