C++ regex character class not matching [duplicate]

Question

from what i researched, the expression "[:alpha:]" will be matched for any alphabetic character, but the expression only match for lowercase character and not uppercase character. I not sure what's wrong with it.

std::regex e ("[:alpha:]");
if(std::regex_match("A",e))
     std::cout<<"hi";
  else
     std::cout<<"no";

Answer 1

Change this:

std::regex e ("[:alpha:]");

to:

std::regex e ("[[:alpha:]]");

As Adrian stated: Please note that the brackets in the class names are additional to those opening and closing the class definition. For example: [[:alpha:]] is a character class that matches any alphabetic character. Read more in the ref.

Answer 2

You have to use [[:alpha:]]

see online example

#include <iostream>
#include <string>
#include <regex>
using namespace std;

int main() {
    std::regex e ("[[:alpha:]]");
if(std::regex_match("A",e))
     std::cout<<"hi";
  else
     std::cout<<"no";
    return 0;
}