Are the character digits ['0'..'9'] required to have contiguous numeric values?

Question

Must a C++ implementation set the chars '0'-'9' to have contiguous numeric values, i.e. so that:

'0' -> 0+n
'1' -> 1+n
 m  -> m+n
'9' -> 9+n

I cannot find it mentioned in the documentation of isdigit ([classification] (22.3.3.1 Character classification)) ^*, nor can I find it in the locale documentation (but maybe I did not look hard enough).

In 2.3 Character sets, we find that

The basic source character set consists of 96 characters: the space character, the control characters representing horizontal tab, vertical tab, form feed, and new-line, plus the following 91 graphical characters

But it doesn't mention any ordering (but maybe I did not look hard enough).

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^*: Interesting footnote there:

When used in a loop, it is faster to cache the ctype<> facet and use it directly [instead of isdigit() et al, end comment], or use the vector form of ctype<>::is.