How to check date format by reg in c++?

Question

I want to check date format in a string, so I use std::regex in c++;

I goggled some demo, but they all failed to me, includes:

#include <iostream>                                                                                                                                                                         
#include <regex>

using namespace std;

int main() {
    string text = "future2018-7-12";
    // regex  pattern("[0-9]{4}-[0-9]{1,2}-[0-9]{1,2}");  // these three all failed
    // regex  pattern("[0-9]{4}-[0-9]{1,2}-[0-9]{1,2}");
    std::regex pattern("\\b\\d{4}[-]\\d{2}[-]\\d{2}\\b");

    smatch results;
    if ( regex_match(text, results, pattern) ) { 
        smatch::iterator it = results.begin();
        int i = 0;
        for(; it != results.end(); ++it, ++i)
            cout<<i<<": "<<*it<<endl;
    } else {
        cout << "match failed: " <<text<< endl;
    }   
    return 0;
}

my compile command is:

g++ -std=c++11 reg.cpp -o reg

execute command is:

./reg

my machine is:

centos 7 in vmware

they all crashed with the error:

terminate called after throwing an instance of 'std::regex_error'
  what():  regex_error

that confuses me, because i can only find these demo about regex.

So, what wrong with my code?