equivalence class - set of characters in a codeset all having the same collation value.

Each character belongs to exactly one equivalence class.

base character - the member of an equivalence class having the lowest unique collation value.

Usually, all of the members of an equivalence class have the same appearance or are to be collated identically.

Equivalence classes find application in regular expressions. NCeqvmap() - AIX 3.1 macro, no longer supported, which determines if a given character is the base character of some equivalence class.

Applications wishing to use equivalence class features must use standard functions for manipulating regular expressions.

There are no standard functions for determining equivalence class membership.

Each platform has functions for determining equivalence class membership. These functions are non-standard and their interfaces are not published.