RFC 3966, "The tel URI for Telephone Numbers", December 2004

Errata ID: 4376
Status: Held for Document Update
Type: Editorial
Publication Format(s) : TEXT
Reported By: OKUMURA Shinji
Date Reported: 2015-05-26
Held for Document Update by: Ben Campbell
Date Held: 2015-06-09

Section 3 says:

phonedigit           = DIGIT / [ visual-separator ]
phonedigit-hex       = HEXDIG / "*" / "#" / [ visual-separator ]

It should say:

phonedigit           = DIGIT / visual-separator;
phonedigit-hex       = HEXDIG / "*" / "#" / visual-separator;

Notes:

An optional and alternative rule is typically meaningless.

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