13.1.6 Ordering of Characters

The total ordering on characters is guaranteed to have the following properties:

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If two characters have the same implementation-defined attributes, then their ordering by char< is consistent with the numerical ordering by the predicate < on their code attributes.

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If two characters differ in any attribute, then they are not char=.

[Reviewer Note by Barmar: I wonder if we should say that the ordering may be dependent on the implementation-defined attributes.]

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The total ordering is not necessarily the same as the total ordering on the integers produced by applying char-int to the characters.

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While alphabetic_1 standard characters of a given case must obey a partial ordering, they need not be contiguous; it is permissible for uppercase and lowercase characters to be interleaved. Thus (char<= #\a x #\z) is not a valid way of determining whether or not x is a lowercase character.

Of the standard characters, those which are alphanumeric obey the following partial ordering:

 A<B<C<D<E<F<G<H<I<J<K<L<M<N<O<P<Q<R<S<T<U<V<W<X<Y<Z
 a<b<c<d<e<f<g<h<i<j<k<l<m<n<o<p<q<r<s<t<u<v<w<x<y<z
 0<1<2<3<4<5<6<7<8<9
 either 9<A or Z<0
 either 9<a or z<0                                                      

This implies that, for standard characters, alphabetic_1 ordering holds within each case (uppercase and lowercase), and that the numeric characters as a group are not interleaved with alphabetic characters. However, the ordering or possible interleaving of uppercase characters and lowercase characters is implementation-defined.