+TODO: Document this
+
+=item Character set
+
+The character set semantics are determined by an enum that is contained
+in this field. This is still experimental and subject to change, but
+the current interface returns the rules by use of the in-line function
+C<get_regex_charset(const U32 flags)>. The only currently documented
+value returned from it is REGEX_LOCALE_CHARSET, which is set if
+C<use locale> is in effect. If present in C<< rx->extflags >>,
+C<split> will use the locale dependent definition of whitespace
+when RXf_SKIPWHITE or RXf_WHITE is in effect. ASCII whitespace
+is defined as per L<isSPACE|perlapi/isSPACE>, and by the internal
+macros C<is_utf8_space> under UTF-8, and C<isSPACE_LC> under C<use
+locale>.
+