Conditionally replace regex matches in string

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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.

c++11 visual-studio-2010 visual-c++ c++ regex
2 Answers

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;
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