Email Address Validation: Explained in Detail, Code for Production Quality WPF TextBox

Validating email addresses is difficult, because the specification what constitutes a valid email address allows for an immense variety. Trying to translate these specifications into proper Regex leads to huge and complicated expressions which still might not cover all cases. This article describes how email address validation can be reasonably done and provides a production quality code WPF TextBox, including letting the user enter only valid keystrokes, controlling if an email address must be entered (Required field) and alerting the user if he tries to close the window without saving the changes made.