FilteredTextBox is an extender which prevents a user from entering invalid characters into a text box. Note that since this effect can be avoided by deactivating JavaScript, you should use this extender as a convenience for your users, but you must never expect that the data being sent to the server consists of "valid" chars only.

The math-symbols example above is initialized with this code:

 <ajaxToolkit:FilteredTextBoxExtender ID="ftbe" runat="server"
        TargetControlID="TextBox3"         
        FilterType="Custom, Numbers"
        ValidChars="+-=/*()." />

• TargetControlID - The ID of the text box for this extender to operate on
• FilterType - A the type of filter to apply, as a comma-separated combination of Numbers, LowercaseLetters, UppercaseLetters, and Custom. If Custom is specified, the ValidChars field will be used in addition to other settings such as Numbers.
• ValidChars - A string consisting of all characters considered valid for the text field, if "Custom" is specified as the field type. Otherwise this parameter is ignored.