- 1 while s/B<((?:[^<>]|I<[^<>]*>)*)R<([^>]*)>/B<$1>${2}B</g;
- 1 while (s/B<([^<>]*)I<([^>]+)>/B<$1>I<$2>B</g);
- 1 while (s/I<([^<>]*)B<([^>]+)>/I<$1>B<$2>I</g);
+
+ # Now un-nest all B<>, I<>, R<>. Theoretically we could have
+ # indefinitely deep nesting; in practice, one level suffices.
+ 1 while s/([BIR])<([^<>]*)([BIR])<([^<>]*)>/$1<$2>$3<$4>$1</g;
+
+ # Replace R<...> with bare ...; eliminate empty markup, B<>;
+ # shift white space at the ends of [BI]<...> expressions outside
+ # the expression.
+ s/R<([^<>]*)>/$1/g;