Thanks for the excellent library John and others. I could not easily find an option to specify a callback function that somehow returns a custom built replacement string for use with regex_replace. The .NET framework utilises the MatchEvaluator mechanism: http://msdn2.microsoft.com/en-us/library/system.text.regularexpressions.rege... While there is no one definition of a string to return in the case of boost regex's, perhaps a Range or a pair of iterators would be sufficient (although temporaries may cause trouble for the unwary). Anyway, I have had need of of this mechanism and couldn't use the excellent regex_replace as a consequence. Neil