Conditionally replace regex matches in string - c++

Question

I am trying to replace certain patterns in a string with different replacement patters.
Example:
string test = "test replacing \"these characters\"";
What I want to do is replace all ' ' with '_' and all other non letter or number characters with an empty string. I have the following regex created and it seems to tokenize correctly, but I am not sure how to (if possible) perform a conditional replace using regex_replace.
string test = "test replacing \"these characters\"";
regex reg("(\\s+)|(\\W+)");
expected result after replace would be:
string result = "test_replacing_these_characters";
EDIT:
I cannot use boost, which is why I left it out of the tags. So please no answer that includes boost. I have to do this with the standard library. It may be that a different regex would accomplish the goal or that I am just stuck doing two passes.
EDIT2:
I did not remember what characters were included in \w at the time of my original regex, after looking it up I have further simplified the expression. Again the goal is anything matching \s+ should be replaced with '_' and anything matching \W+ should be replaced with empty string.

Answer 1

The c++ (0x, 11, tr1) regular expressions do not really work (stackoverflow) in every case (look up the phrase regex on this page for gcc), so it is better to use boost for a while.
You may try if your compiler supports the regular expressions needed:
#include <string>
#include <iostream>
#include <regex>
using namespace std;
int main(int argc, char * argv[]) {
string test = "test replacing \"these characters\"";
regex reg("[^\\w]+");
test = regex_replace(test, reg, "_");
cout << test << endl;
}
The above works in Visual Studio 2012Rc.
Edit 1: To replace by two different strings in one pass (depending on the match), I'd think this won't work here. In Perl, this could easily be done within evaluated replacement expressions (/e switch).
Therefore, you'll need two passes, as you already suspected:
...
string test = "test replacing \"these characters\"";
test = regex_replace(test, regex("\\s+"), "_");
test = regex_replace(test, regex("\\W+"), "");
...
Edit 2:
If it would be possible to use a callback function tr() in regex_replace, then you could modify the substitution there, like:
string output = regex_replace(test, regex("\\s+|\\W+"), tr);
with tr() doing the replacement work:
string tr(const smatch &m) { return m[0].str()[0] == ' ' ? "_" : ""; }
the problem would have been solved. Unfortunately, there's no such overload in some C++11 regex implementations, but Boost has one. The following would work with boost and use one pass:
...
#include <boost/regex.hpp>
using namespace boost;
...
string tr(const smatch &m) { return m[0].str()[0] == ' ' ? "_" : ""; }
...
string test = "test replacing \"these characters\"";
test = regex_replace(test, regex("\\s+|\\W+"), tr); // <= works in Boost
...
Maybe some day this will work with C++11 or whatever number comes next.
Regards
rbo

Answer 2

The way to do this has commonly been accomplished by using four backslashes to remove the backlash effecting the actual C code. Then you will need to make a second pass for the parentheses and escape them in your regex then and only then.
string tet = "test replacing \"these characters\"";
//regex reg("[^\\w]+");
regex reg("\\\\"); //--AS COMMONLY TAUGHT AND EXPLAINED
tet = regex_replace(tet, reg, " ");
cout << tet << endl;
regex reg2("\""); //--AS SHOWN
tet = regex_replace(tet, reg2, " ");
cout << tet << endl;
And in a single pass use;
string tet = "test replacing \"these characters\"";
//regex reg("[^\\w]+");
regex reg3("\\\""); //--AS EXPLAINED
tet = regex_replace(tet, reg3, "");
cout << tet << endl;