Input Mask help request

[jgraves]

I have a custom field in my user profile fields. It is a required field for new users that has a string in a format like this:

X-XX-XXX-XX-XX-XXXXX

I only need the first 3 and last 5 characters. The rest are unneeded but a part of the key they were supplied.

How would I do an input mask which required them to enter the first 3 and last five, using a placeholder (like a '#') for the not-required parts? The input would look like this:

1-23-###-##-##-45678

Can anyone assist?