[w3c/webpayments-method-identifiers] Why are payment ids restricted to [a-z0-9]? (#20)

4.1 Syntax
https://w3c.github.io/webpayments-method-identifiers/#syntax-1

Is there a specific requirement that identifiers be restricted to [a-z0-9-]? Are there any international card issuers that may require characters outside this range?

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