Spigot permission node for specific functions?

If I want to give a certain group of people permission to run functions in a specific directory, for example:

world/data/functions/custom/admin <-- functions for admins world/data/functions/custom/moderator <-- functions for mods 

what is the permission node to assign to a group? I assumed it would be:

function.custom:moderator 

but that doesn't work. This permissions plugin is Permissions EX (PEX) which follows the standard syntax of any permissions plugin such as Essentials for Bukkit.