Welcome to the Make community!
To do this, you can use the built-in function replace — “Replaces the search string with the new string.”
{{ replace(text; search string; replacement string) }}
e.g.: (paste this into the field)
{{replace(filename; "/^[\w\W]+\//"; emptystring)}}
For more information, the function’s documentation can be found in the Help Centre. You should also complete the tutorials in the Make Academy.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Investing some effort into the Make Academy will save you lots of time and frustration using Make.