I just found a code to build a contact form:

This part works fine for me, but now I did add an RequiredFieldValidator.#
Here's the code e.g. txtEmailAddress:

RequiredFieldValidator FieldEmailValidieren = new RequiredFieldValidator();
FieldEmailValidieren.ID = "_FieldEmailValidieren";
FieldEmailValidieren.ControlToValidate = txtEmailAddress.ID;
FieldEmailValidieren.Text = "<br>The E-Mail you have entered is not valid!";
FieldEmailValidieren.ErrorMessage = "Field 'Email' is empty";
FieldEmailValidieren.Display = ValidatorDisplay.Static;

This works fine for me, too, but the problem is that I need an RegularExpressionValidator.

Can somebody help me herewith?