Question

I have a Bug at my simple REGEX.
I have been trying to write some simple regular expressions at C++ using std::regex. Here is my code so far.

#include <iostream>
#include <regex>
#include <string>

int main(void)
{
    std::string str = "Hello world";
    std::regex rx("\w+\s\w+"), rx2("ello");
    std::cout << std::boolalpha << std::regex_match(str.begin(), str.end(), rx) << "\n";
    std::cout << std::boolalpha << std::regex_search(str.begin(), str.end(), rx2) << "\n";
    return 0;
}

This program should print (according to a tutorial)

true
true

but it prints

false
false

Where am I making a mistake? Thanks in advance.

Note: I’m using g++ -std=c++0x %file.cpp% -o %file% if it helps

Answer 1

As already stated g++ (GCC) does not have a proper implementation for regex (it is unimplemented but still compiles).

The Boost library has an implementation for regexes that is almost completely compatible with regexes in C++11. You could use that with minimal changes in your code (just using boost:: instead of std::).

Here is a code that compiles and works:

#include <iostream>
#include <boost/regex.hpp>
#include <string>

int main(void)
{
    std::string str = "Hello world";
    boost::regex rx("\\w+\\s\\w+"), rx2("ello");
    std::cout << std::boolalpha << boost::regex_match(str.begin(), str.end(), rx) << "\n";
    std::cout << std::boolalpha << boost::regex_search(str.begin(), str.end(), rx2) << "\n";
    return 0;
}

Note that I have also fixed missing escapes for the backslashes for rx because it does not work without it.

To compile this you have to install libboost-regex-dev package (or something similar if not using Ubuntu/Debian) and execute this:

g++ -std=c++0x main.cpp -lboost_regex -o test