Hi, I would like to change a regular expression in a compatible way. Here is the use case. Initially, in the configuration I defined a reg-exp for validation of data before persisting it. Some data was created and persisted, later the reg-exp required a change, however, the change must be done is such a way that existing data should not become invalid. Example: Initial validation says "no ; (semi-colon) and : (colon) in the text". However, later we decide to relax the constraint and want to allow : (colon). By allowing this we are not invalidating any existing data and hence this change should be allowed. However, we should not be changing the configuration and add additional constraint, like now I don't want to allow _ (underscore) since there will be existing data that has _ (underscore). I am using: boost_1_46_1 Pseudo-code of APIs I am using: #include <boost/xpressive/xpressive.hpp> string regexPattern("^[^\\/?%*:|\"<>\.]+$"); boost::xpressive::sregex tmpRegex = boost::xpressive::sregex::compile(regexPattern); bool match = boost::xpressive::regex_match("value to validate", tmpRegex); Now I decide that I would start allowing . (dot) from now on. Is there a way that will allow me to validate that whatever successfully matches the old expression will also match the new expression Regards, Prakash Bande Director - Hyperworks Enterprise Software Altair Eng. Inc. Troy MI Ph: 248-614-2400 ext 489 Cell: 248-404-0292 My Secure Drop Box: https://ftam1.altair.com/dropbox/~Ursfnk