Namespaced IDs' paths can have ASCII lowercase alphabets, ASCII digits, dots, underscores, slashes, and hyphens. However, the path segment of a directory-based resource pack only allows ASCII lowercase alphabets, ASCII digits, dots, or underscores, but not hyphens.

This can potentially interfere with overriding assets/data using extracted resource pack/data pack if the path contains a hyphen. However, I am not aware of a vanilla stuff with a hyphen in the path. This mostly affects mods and potentially custom data packs.

(Slashes are directory separators and thus does not cause issues.)

See the following code:

public class x {

private static final Pattern d = Pattern.compile("[._a-z0-9]+"); // notice there is no hyphen