Prompt characters in a MaskedTextBox

I have the need to create a customized text entry box that can accept
various types of geographic coordinates. The problem=20

The MaskedTextBox only allows a single prompt character to be displayed
upon user input. I need the ability to display different prompt
characters for certain character positions. How can I do this? Also, due
to the nature of the text being inputted, I suspect that I will need to
use a regex to validate this properly. So I'm assuming because of these
two limitations, that the MaskedTextBox isn't the way to go.

Does anyone have any suggestions on how I can go about this? Use a
regular text box and validate against a reg expression? But how do I
handle displaying the custom prompt characters?

The information contained in this e-mail message is PRIVATE. It may =
contain confidential information and may be legally privileged. It is =
intended for the exclusive use of the addressee(s). If you are not the =
intended recipient, you are hereby notified that any dissemination, =
distribution or reproduction of this communication is strictly =
prohibited. If the intended recipient(s) cannot be reached or if a =
transmission problem has occurred, please notify the sender immediately =
by return e-mail and destroy all copies of this message.=20
Thank you.=20