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 "\u005C".

(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?

score 11 · Accepted Answer · edited Jun 13 '14 at 22:22

11

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 :(

edited Jun 13 '14 at 22:22

Daniel Frey

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answered Jun 13 '14 at 22:11

Jon Skeet

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Doesn't that just refer to the grammar of writing other grammars? I mean it allows `HEXDIG` and `heXdIG` to be used interchangeably, but the surely can't refer to all characters in the grammar like `"A"`, otherwise you could not define **any** case-sensitive language!? – Daniel Frey Jun 13 '14 at 22:14
@DanielFrey: I don't think so - that's a separate part of the RFC: "Rule names are case-insensitive". I agree that it's far from clear though. – Jon Skeet Jun 13 '14 at 22:16
+1 I wrote the first comment before your edit. With the edit it makes sense now and yes, I agree it's not the clearest spec :) – Daniel Frey Jun 13 '14 at 22:19
And all the case-sensitive keywords in the JSON defining RFC 4627 are provided using individual characters (with ASCII literal in a comment after it), like `false = %x66.61.6c.73.65 ; false`. – silmeth Oct 14 '17 at 09:37

JSON Unicode escape sequence - lowercase or not?

1 Answers1