# Limiting Allowable Values

**RadTextBox** can be used with **RegularExpressionValidator** to validate the input for a specific format.

The following example shows how to use this approach to use **RadTextBox** for entering Email addresses:

```ASPNET
<telerik:RadTextBox RenderMode="Lightweight" ID="RadTextBox1" runat="server">
	</telerik:RadTextBox>
	<asp:RegularExpressionValidator
	id="emailValidator"
	runat="server"
	Display="Dynamic"
	ErrorMessage="Please, enter valid e-mail address."
	ValidationExpression="^[\w\.\-]+@[a-zA-Z0-9\-]+(\.[a-zA-Z0-9\-]{1,})*(\.[a-zA-Z]{2,3}){1,2}$"
	ControlToValidate="RadTextBox1">
</asp:RegularExpressionValidator>
<br/>
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Submit" />
```

When the user clicks the Submit button, the text box is validated and the error message is displayed if the entered text does not match the email format.

## See Also

* [Validation](https://www.telerik.com/products/aspnet-ajax/documentation/controls/textbox/features/validation)