Regex Evaluate Part Of A String

I am currently working on creating a dialogue which will allow the user to input a range of worksheets to use for a pivot table.

I want the user to be able to input a variable into the dialogue. For example, I want the input "n-5 - n-1" to mean the 6th from last to the 2nd from last sheets. I am trying to find n - ([0-9]+)? and replace it with Worksheets.Count - $1. In other words, if there are 10 sheets, I want to transform "n-5 - n-1" into "5 - 9".

Here is the code I have so far:

[Code] ....

This has been working for strings like "n" or "n-4" but not "4 - n-3" or "n-5 - n-1".

Here is the code I use afterwards to interpret the string once the n's have been replaced:

[Code] ....

I'm new to Regex and VBA in general.