First some examples: -- std::string input = "u"; std::string expression = "[^UuMmFf]*"; std::string replacement = "Unknown"; boost::regex regular_expression(expression); std::string output = boost::regex_replace(input, regular_expression, replacement, boost::match_default); Boost: output = "UnknownuUnknown" $ echo 'u' | sed 's/[^UuMmFf]*/Unknown/g' UnknownuUnknown Expected. -- std::string input = "x"; std::string expression = "[^UuMmFf]*"; std::string replacement = "Unknown"; boost::regex regular_expression(expression); std::string output = boost::regex_replace(input, regular_expression, replacement, boost::match_default); Boost 1.33.1: output = "UnknownUnknown" Prior: "Unknown" $ echo 'x' | sed 's/[^UuMmFf]*/Unknown/g' Unknown The result generated by Boost 1.33.1 is not expected! -- std::string input = " "; std::string expression = "[^UuMmFf]*"; std::string replacement = "Unknown"; boost::regex regular_expression(expression); std::string output = boost::regex_replace(input, regular_expression, replacement, boost::match_default); Boost 1.33.1: output = "UnknownUnknown" Prior: "Unknown" $ echo ' ' | sed 's/[^UuMmFf]*/Unknown/g' Unknown The result generated by Boost 1.33.1 is not expected! -- The expression "[^UuMmFf]*" will greedily match the input string. In the case of "x" or " " (space) the expression will match the whole string! There should not be a match on an empty string! Considering these examples I would mark this as a bug! Thanks, P. Sliepenbeek __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com