RFC 2047, "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", November 1996

Note: This RFC has been updated by RFC 2184, RFC 2231

Errata ID: 5469
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Peter Occil
Date Reported: 2018-08-17
Held for Document Update by: Barry Leiba
Date Held: 2019-04-30

Section 5 says:

    In this case the set of characters that may be used in a "Q"-encoded
    'encoded-word' is restricted to: <upper and lower case ASCII
    letters, decimal digits, "!", "*", "+", "-", "/", "=", and "_"
    (underscore, ASCII 95.)>

It should say:

In this case the set of characters that may be used in a "Q"-encoded
'encoded-text' is restricted to: <upper and lower case ASCII
letters, decimal digits, "!", "*", "+", "-", "/", "=" (only if
followed by two hexadecimal digits), and "_" (underscore, ASCII 95.)>

Notes:

This sentence discusses the use of encoded words within a phrase. However, the original text is wrong in at least two ways:

1. The production "encoded-word" also allows "?" to appear, as well as characters allowed in MIME "token"s.
2. Even in the "encoded-text" portion the "=" sign can't appear freely in Q-encoding, but must be followed by two hexadecimal characters (as stated later in section 5).

The correction given here changes "encoded-word" to "encoded-text" and adds text restricting where "=" can appear. Another plausible correction would be to add "?" and the other characters allowed in "tokens" (and leave the text "'q'-encoded 'encoded-word'" alone), but that would interfere with later RFCs, notably RFC 2231, that restrict the syntax of the charset component of encoded-words.

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