C++11 regex doesn't match the string [duplicate]

Question

I want to parse a token that looks like this:

1111111111111111:1384537090:Gl21j08WWBDUCmzq9JZoOXDzzP8=

I use a regular expression ([0-9]{16}):([0-9]{5,20}):([a-zA-Z0-9\\+/=]{28}), and it does the job when I try it.

Then I try it with C++:

std::regex regexp(R"(([0-9]{16}):([0-9]{5,20}):([a-zA-Z0-9\\+/=]{28}))", 
     std::regex_constants::basic);
std::smatch match;

if (std::regex_search(stringified, match, regexp)) {
    cout << match[0] << ',' << match[1] << ',' << match[2] << endl;
} else {
    cout << "No matches found" << endl;
}

I compile it on Ubuntu 13.10 x64 using GCC 4.8.1 with -std=c++11 flag. But I always get No matches found. What am I doing wrong?

Answer 1

You were specifying POSIX basic regex, in that format you must escape () and {}

I was able to get get matches with a few changes:

 int main(int argc, const char * argv[]){
    using std::cout;
    using std::endl;
    std::regex regexp(R"(\([0-9]\{16\}\):\([0-9]\{5,20\}\):\([a-zA-Z0-9\\+/=]\{28\}\))",std::regex_constants::basic);
    std::smatch match;
    std::string stringified = "1111111111111111:1384537090:Gl21j08WWBDUCmzq9JZoOXDzzP8=";
    if (std::regex_search(stringified, match, regexp)) {
        cout << match[1] << "," << match[2] << "," << match[3]<< endl;
    } else {
        cout << "No matches found" << endl;
    }
    return 0;
}

Or you could use:

std::regex_constants::extended

If you use std::regex_constants::extended you should not escape () and {}

If you don't want to use a raw string, you can do that as well:

std::regex regexp("([0-9]{16}):([0-9]{5,20}):([a-zA-Z0-9\\\\+/=]{28})",std::regex_constants::extended);

You'll just have to double up on the \\ to properly escape them. The above regex also works with the default regex grammar std::regex_constants::ECMAScript

std::regex regexp("([0-9]{16}):([0-9]{5,20}):([a-zA-Z0-9\\\\+/=]{28})");

It looks like GCC just added regex supported in their development branch of GCC 4.9.

Answer 2

It appears that you need to use 'extended' syntax. Change regex_constants::basic to regex_constants::extended and it will match.

You need extended syntax in order to perform capturing.

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04