Boost regex in Perl-compatible mode has \U escape sequence to convert characters to upper case. Example: doc abc, replace from ^. to \U$0 - result Abc.

However, this doesn't work for Cyrillic symbols (doc фыв = фыв). Boost regex is normally fully Unicode-aware, do you know if \U is hardcoded to only work on Latin symbols or there's some flag that enables it for all Unicode ranges? Notepad++ has the same issue so it seems to be the first case.