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boost::regex re("x", boost::regex_constants::icase); That should be: boost::regex re("x", boost::regex::perl|boost::regex_constants::icase);

Or boost::regex re("x", boost::regex::normal | boost::regex::icase) Ok, I have overlooked this statement in the documentation: "A valid value of type syntax_option_type will always have exactly one of the elements normal, basic, extended, awk, grep, egrep, sed or perl set". My fault - Thank you! Jan