JSON Unicode escape sequence - lowercase or not?

Question

I was reading RFC 4627 and I can't figure out if the following is valid JSON or not. Consider this minimalistic JSON text:

["\u005c"]

The problem is the lowercase c .

According to the text of the RFC it is allowed:

Any character may be escaped. If the character is in the Basic Multilingual Plane (U+0000 through U+FFFF), then it may be represented as a six-character sequence: a reverse solidus, followed by the lowercase letter u, followed by four hexadecimal digits that encode the character's code point. The hexadecimal letters A though F can be upper or lowercase. So, for example, a string containing only a single reverse solidus character may be represented as "\\".

(Emphasis mine)

The problem is that the RFC also contains the grammar for this:

char = unescaped /
       escape (
           %x22 /          ; "    quotation mark  U+0022
           %x5C /          ; \    reverse solidus U+005C
           %x2F /          ; /    solidus         U+002F
           %x62 /          ; b    backspace       U+0008
           %x66 /          ; f    form feed       U+000C
           %x6E /          ; n    line feed       U+000A
           %x72 /          ; r    carriage return U+000D
           %x74 /          ; t    tab             U+0009
           %x75 4HEXDIG )  ; uXXXX                U+XXXX

where HEXDIG is defined in referenced RFC 4234 as

HEXDIG         =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

which includes only uppercase letters.

FWIW, from what I researched most JSON parsers accept both upper and lowercase letters.

Question(s) : What is actually correct? Is there a contradiction and the grammar in the RFC should be fixed?

Answer 1

I think it's explained by this part of RFC 4234:

ABNF strings are case-insensitive and the character set for these strings is us-ascii.

Hence:

  rulename = "abc" 

and:

  rulename = "aBc" 

will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC", and "ABC".

On the other hand, the follow-on part is not terribly clear:

To specify a rule that IS case SENSITIVE, specify the characters individually.

For example:

  rulename = %d97 %d98 %d99 

or

  rulename = %d97.98.99 

In the case of the HEXDIG rule, they're individual characters to start with - but they're specified literally as "A" etc rather than %d41 , so I suspect that means they're case-insensitive. It's not the clearest spec I've read :(