Understanding Form Validation

HTML forms are used to collect data from users by using several field types. Forms are used for data entry, as front-end search engines, for filling out orders, for signing guest books, for providing user names and passwords to secure applications, and much more. Although forms have become one of the most important features in HTML, these forms provide almost no data validation tools.

This becomes a real problem when developing Web-based applications. As a developer, you need to be able to control what data users can enter into what fields. Without that, your programs will constantly be breaking due to mismatched or unanticipated data. And thus far, you have used forms only as search front ends—when forms are used to insert or update database tables (as you’ll see in Chapter 14, “Using Forms to Add or Change Data”), this becomes even more critical.