Insert Mode with MaskedEditExtender

Question

I am using asp.net with ajaxcontroltoolkit. I use the MaskedEditExtender frequently. In one case, I have a field that must be exactly 17 characters. If the user is typing in the string and they miss a character is the middle, they would like to go into insert mode to add a character. But insert mode does not appear to work. Is there some way to get insert mode to work with MaskedEditExtender? If not can you suggest an alternative?

Thanks in advance.

Bob Avallone