Regex.Replace from char position to another char position

Regex.Replace from char position to another char position

Good afternoon. I am a VB.Net programmer that has made every attempt to
implement regular expressions into my applications as much as possible. I
choose regex over Net methods because practice makes perfect. This is only
for my regex knowledge benefit.
Take a string like this for example ignoring the quotes.
":1Af404080A83hfndsgt4u47", the part of the string i am looking at is
these 8 values. "04080A83" These can change. The values are not important
but the position. Starting from 0 the first char position will be 5 to 12.
I know we can match any char until {5} but it possible to replace a range.
Example {5,12}. Final output would be "":1Af4Hello123hfndsgt4u47""
Hello123
Thank you for your time. It might not be possible like i said for my own
Benita.