Visual Basic.NET How to Program (PDF)

Chapter 20 ASP .NET, Web Forms and Web Controls 971
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Fig. 20.21 Validators used in a Web Form that generates possible letter combinations
from a phone number (part 2 of 2).
to match another Web control’s content against a regular expression. (The use of regular
expressions is introduced in Chapter 15, Strings, Characters and Regular Expressions.)
Lines 35–41 create a RegularExpressionValidator named phoneNumber-
Validator. Property ErrorMessage’s text (lines 37–38) is displayed on the Web Form
if the validation fails. The regular expression that validates the input is assigned to property
ValidationExpression in line 38. The input is valid if it matches the regular expression
^\d{3}-\d{4}$ (i.e., if exactly 3 digits are followed by a hyphen and exactly 4
digits, where the 3 digits are at the beginning of the string and the 4 digits are at the end of
the string).
The clicking of property ValidationExpression in the Properties window
displays a dialog that contains a list of common regular expressions for phone numbers, ZIP
codes and other formatted information. However, we write our own regular expression in
this example, because the phone number input should not contain an area code. Line 39
associates phoneTextBox with phoneNumberValidator by setting property
ControlToValidate to phoneTextBox. This indicates that phoneNumber-
Validator verifies the phoneTextBox’s contents. If the user inputs text that does not
have the correct format and attempts to submit the form, the ErrorMessage text is displayed
in red.
This example also uses a RequiredFieldValidator to ensure that the text box
is not empty when the HTML form is submitted. Lines 43–49 define RequiredField-
Validator phoneInputValidator, which confirms that phoneTextBox’s content
is not empty. If the user does not input any data in phoneTextBox and attempts to
submit the form, validation fails, and the ErrorMessage for this validator is displayed
in red. If the validator is successful, a multiline TextBox named outputTextBox (lines
29–33) displays the words generated from the phone number. Notice that the Visible
property initially is set to False when the server returns its HTML response.
Figure 20.22 depicts the code-behind file for the ASPX file in Fig. 20.21. Notice that
this code-behind file does not contain any implementation related to the validators. We say
more about this soon.