Hybrid validation support

[DXist]

Validation of email address is not possible using lxml. I think it may be a good idea to enforce lxml validation with some soft validators.

We may need extra method in validation API that is called in both validation strategies.

[plq]

You can validate email addresses using regular expressions up to a point. That doesn't cover your use case?

[DXist]

In my case I want to use Django email validator that uses extra checks after regex validation to support RFC 3490. I can't just copy and paste email_re from Django code because XmlSchema regex language is limited.