Renaming files keeping first 5 characters

Hi all

Just trying to use the string substitution option in the Batch Rename tool, but not having much luck. I'm just trying to rename all my files keeping only the first 5 characters of the filenames, so for example

From
17640 - Test item 1.png
18650 - Test item 2 with added extras.png

To

17640.png
18650.png

Can anyone point me in the right direction. Many thanks