Replacing tokens that match pieces of a regex - c++

Question

I would like to use a regex both as a pattern to search and a template to construct a string. (I'm using boost::regex because I'm on gcc 4.8.4 where apparently regex is not fully supported (until 4.9)):
That is, I want to construct a regex, pass it to a function, use the regex to match some files, then construct an output file name following the same pattern. For example:
Regex: "file_.*\.txt"
to match things like "file_1.txt", "file_2.txt", etc.
and then would like to construct from it
Output: "file_all.txt"
That is, I want to match files starting with "file_" and ending with ".txt", then I want to fill in "all" between the "file_" and the ".txt", all from a single regex object.
We'll skip the matching to the regex as that is straightforward, but rather focus on the replacement:
#include <iostream>
#include <iterator>
#include <string>
#include <boost/regex.hpp>
std::string constructOutput(const boost::regex& myRegex)
{
// How to replace the match to the center of the filenames here?
// return boost::regex_replace(?, myRegex, "all");
}
int main()
{
// We can do something like this, but it requires us to manually separate the "center" of the regex from the string, as well as keep around a string object and a regex object:
// std::string myText = "File_.*.txt";
// boost::regex myRegex("_.*\\.");
// std::cout << '\n' << boost::regex_replace(myText, myRegex, "_all.") << '\n';
// Want to do this:
boost::regex myRegex("File_.*\\.txt");
std::string outputString = constructOutput(myRegex);
std::cout << outputString << std::endl;
}
Is something like this possible?