Inserting variable text - partial filename

Our filenames are in this format:

12345_Something_Something.indd

InDesign can insert the entire filename as a text variable.

What we want is for it to only insert the first 5 numbers – everything before the underscore — as a text variable. It would need to change every time the template is saved with a new filename. I’m thinking it’s possible to write a Javascript extension to do this in InDesign. I just don’t know Javascript at all.

1) Is this even possible?

2) What resources are available to help me get this accomplished?

3) What is considered reasonable compensation for this project?

Thanks for your help!