This parameter will only be used if "Preserve Local Directory Structure" is chosen above. The field accepts a regular expression. If the regular expression evaluates to a prefix if the upload path, the prefix will be removed from the path.
To continue the example from above help, if this field contains:
"upload_stuff/" or ".*_stuff/"
Then the resulting file path will be:
https://mgr.cubit.example.com/pbl/project_name/priv/my/path/sub_folder/my_file.txt
If left blank, the path will not be altered.
For more information on regular expressions see this wikipedia article