Help in validating an IP address

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C#

public class IPAddressTextBox : RegExTextBox
        {
            public IPAddressTextBox()
            {
                InitializeComponent();
                this.ValidationExpression = @"^((25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]|[0-9])\.){3}(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]|[0-9])$";
                this.ErrorMessage = "The IP address must be in the form of 111.111.111.111";
            }
        }

I have used this code for validating an IPaddress in my project and i have got an error as

"The type or namespace name "RegExTextBox" could not be found( are you using directive or an assembly reference)

i am not aware of this C# how to use..

[edit]Unchecked "Ignore HTML in text" option to let the "Code Block" work - in future try using the preview to check the formatting![/edit]