Email checker

Here's a simple email checker in C++ that checks if a given email is valid or not. This program uses a regular expression to validate the email.

```cpp
#include <iostream>
#include <regex>
#include <string>

// Function to check if an email is valid
bool isValidEmail(const std::string& email) {
    // Regular expression for a valid email
    std::regex pattern("^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$");
    return std::regex_match(email, pattern);
}

int main() {
    // Test emails
    std::string emails[] = {"test@example.com", "invalid_email", "test@example.co.uk", "test@example.com.au"};

    // Check each email
    for (const auto& email : emails) {
        if (isValidEmail(email)) {
            std::cout << email << " is a valid email." << std::endl;
        } else {
            std::cout << email << " is not a valid email." << std::endl;
        }
    }

    return 0;
}
```

This program uses the `std::regex` class to define a regular expression that matches a valid email. The `std::regex_match` function is then used to check if the given email matches this pattern.

Here's a breakdown of the regular expression used:

- `^` matches the start of the string.
- `[a-zA-Z0-9._%+-]+` matches one or more alphanumeric characters, dots, underscores, percent signs, plus signs, or hyphens.
- `@` matches the at symbol.
- `[a-zA-Z0-9.-]+` matches one or more alphanumeric characters, dots, or hyphens.
- `\\.` matches a period (escaped with a backslash because `.` has a special meaning in regular expressions).
- `[a-zA-Z]{2,}` matches the domain extension (it must be at least 2 characters long).
- `$` matches the end of the string.

This regular expression is not foolproof and may not match all valid email addresses, but it should cover most common cases.