Visual Basic.NET How to Program (PDF)

Chapter 20 ASP .NET, Web Forms and Web Controls 969
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Fig. 20.20 AdvertisementFile used in AdRotator example (part 3 of 3).
20.5.3 Validation Controls
This section introduces a different type of Web control, called a validation control (or validator),
which detremines whether the data in another Web control are in the proper format.
For example, validators could determine whether a user has provided information in a required
field or whether a ZIP-code field contains exactly five digits. Validators provide a
mechanism for validating user input on the client. When the HTML for our page is created,
the validator is converted into ECMAScript 3 that performs the validation. ECMAScript is
a scripting language that enhances the functionality and appearance of Web pages. ECMA-
Script is typically executed on the client. However, if the client does not support scripting
or scripting is disabled, validation is performed on the server.
The example in this section prompts the user to input a phone number, in the form 555–
4567. After the user enters a number, validators ensure that the phone-number field is filled
and that the number is in the correct format before the program sends the number to the
Web server. Once the phone number is submitted, the Web Server responds by sending an
HTML page containing all possible letter combinations that represent the phone number.
The letters used for each digit are the letters found on a phone’s key pad. For instance, the
5-button displays the letters j, k and l. For the position in the phone number where there is
a 5, we can substitute one of these three letters. Businesses often use this technique to make
their phone numbers easy to remember. Figure 20.21 presents the ASPX file.
The HTML page sent to the client browser accepts a phone number in the form 555–
4567 and then lists all the possible words that can be generated from both the first three
digits and the last four digits. This example uses a RegularExpressionValidator
3. ECMAScript (commonly known as JavaScript) is a scripting standard created by the ECMA (European
Computer Manufacturer’s Association). Both Netscape’s JavaScript and Microsoft’s
JScript implement the ECMAScript standard, but each provides additional features beyond the
specification. For information on the current ECMAScript standard, visit www.ecma.ch/
stand/ecma-262.htm.