Assign parts of FileName to xmp:UserComment

[User_]

My files are named like

"2019_10_28_112702_IMG_2898.mp4"

. How can assign

"IMG_2898"

to xmp:UserComment?

Tried variations of

"-XMP-exif:UserComment<${FileName;s/\d{17}_(d{8})/$1/}"

but did not succeed :-)

Thank you for your help!

[StarGeek]

Will all the files have "IMG_(number)" as the data you're trying to pull or might there be something else.

Assuming the former, you could use
exiftool "-XMP:UserComment<${Filename;m/(img_\d+)/i;$_=$1}" /path/to/files/

This attempts to match and capture "img(underscore)(1 or more numbers)", case insensitively.

To break down what your original attempt actually did, the \d tried to match numbers only and the {17} required 17 of them.  The underscores blocked that match.  Assuming the second part was supposed to be \d{8}, that would have been the same except it was looking for 8 numbers.

[User_]

All files ended with "IMG_(number)" and your proposed command worked good!

Thank you very much!