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.regex.replace.aspx
 
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