Help! Where is the bracket error?

Question

I have the code:

#include <iostream>
#include <regex>
using namespace std;
int main()
{
string var; getline(cin,var);
const regex pattern ("^[a-z_A-Z]\\w*

Answer 1

Thank You everyone.
When the code failed on Sololearn's Code Playground, I ran it on CppDroid. But even there the code failed. Perhaps its because it uses gcc4.8. Ill run it on Code::Blocks...