How to apply Client-side validation by calling a server-side function in ASP.NET

You can use ReqularExpressionValidator control.

Here a regex

^(\{){0,1}[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}(\}){0,1}$

You can apply as many Validators as you need to the same control, in this case your TextBox.

In this scenario, a Custom Validator is the way to go because it enables you to do the validation with any function you develop to cover your needs. Please, have a look at this simple tutorial.

UPDATE 1: Server Side Validation

This is how it calls the server side function in the declaration of the CustomValidator:

<asp:CustomValidator runat="server" id="custPrimeCheck"
    ControlToValidate="txtPrimeNumber"
    OnServerValidate="PrimeNumberCheck"
    ErrorMessage="Invalid Prime Number" />

Example of the "PrimeNumberCheck" VB function:

Sub PrimeNumberCheck(sender as Object, args as ServerValidateEventArgs)
Dim iPrime as Integer = Cint(args.Value), iLoop as Integer, _
    iSqrt as Integer = CInt(Math.Sqrt(iPrime))

For iLoop = 2 to iSqrt
  If iPrime mod iLoop = 0 then
    args.IsValid = False
    Exit Sub
  End If
Next

args.IsValid = True
End Sub

UPDATE 2: Client Side Validation

This is how it calls the server side function in the declaration of the CustomValidator:

<asp:CustomValidator runat="server" id="custPrimeCheck"
    ControlToValidate="txtPrimeNumber"
    ClientValidationFunction="CheckPrime"
    ErrorMessage="Invalid Prime Number" />

Example of the "CheckPrime" JavaScript function:

function CheckPrime(sender, args)
{
    var iPrime = parseInt(args.Value);
    var iSqrt = parseInt(Math.sqrt(iPrime));

    for (var iLoop=2; iLoop<=iSqrt; iLoop++)
      if (iPrime % iLoop == 0) 
      {
          args.IsValid = false;
         return;
      }

    args.IsValid = true;
}

Thanks to @AdrianIftode for making me aware of this.