Can you populate a $variable$ within a string?

Question

I have what I'll call a template I pull out of some XML that shows the format the remote files are stored.  So for example:

 

https://<url>/whatever/file$number$.mp4

 

So I have this in a string.  I also know that the $number$ starts with 1 and ends with 300.  So if I do a loop to retrieve all the files do I need to strip it down to say 

 

https://<url>/whatever/file

 

Then as part of the loop apend the number and the .mp4?  Or can you automatically replace $number$ within the existing string with each number as you loop thru?

 

Thanks.

 

JR