[YouTube-API] Entirely numerical video id?

I'm looking into finding an easy way to differentiate between YouTube and Vimeo videoIDs. 

It seems that all Vimeo IDs are entirely numerical and YouTube's are a mixture of numbers, letters, and punctuation.

I can't seem to find any documentation saying whether this is the case or if I have just not seen an entirely numerical YouTube ID.

Does anyone know whether this is true or not?

Thank you

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