Custom function ID naming requirements are unclear

[parched]

This page says

use characters A through Z, numbers zero through nine, underscores, and periods.

On https://learn.microsoft.com/en-us/office/dev/add-ins/excel/custom-functions-json it says

contains only alphanumeric characters and periods
... unique within the scope of the file. That is, no two function objects in the metadata file should have the same id value

• Are lowercase letters allowed?
• If so, are these distinct? "MYFUNC" vs "myfunc"
• I noticed in the sheetx.xml the periods are converted to underscores. Does that mean "MY.FUNC" is not distinct from "MY_FUNC"?

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• ID: 714da1e2-6053-0328-b31e-0bf514ecc774
• Version Independent ID: be1e524c-1198-c288-f71e-f9d7f84abeb7
• Content: Naming guidelines for custom functions in Excel - Office Add-ins
• Content Source: docs/excel/custom-functions-naming.md
• Service: excel
• Sub-service: add-ins
• GitHub Login: @o365devx
• Microsoft Alias: o365devx

[Rick-Kirkham]

@alison-mk Please take a look and see if the instructions need clarification.

[parched]

I've raised a script lab issue too OfficeDev/script-lab#1009.

I would prefer it if this were actually considered an Excel bug and fixed. I'm wanting to use period as a namespace separator in my custom function and have been assuming that IDs can't conflict with different ones in the parent namespace.

[parched]

Note, using underscore and period works fine in a LAMBDA function.

[alison-mk]

Hi @parched, thanks for submitting this question.

I've tested the ID naming requirements and my results are as follows.

Question: Are lowercase letters allowed as custom function ID values?
Answer: Yes, but they will be automatically converted to uppercase letters in the JSON output.

Question: Are lowercase letters distinct from uppercase letters as custom function ID values?
Answer: No. Custom function ID values are not case-sensitive. For example, “A” is the same as “a”.

Question: Are periods (.) distinct from underscores (_) in custom function ID values?
Answer: Yes.

I'm confirming these answers internally. I'll report back when I've updated the documentation, or if I have additional information.

Cheers,
Alison