Range checking using regular expressions?

How to perform range checking using regular expressions?

Take a 4-bit number (i.e. "dddd") as an example, how can I check whether it is within given range, say [1256-4350] or not?

To check whether the input is a 4 digit number use regex_match, and then convert the string to an integer using std::stoi to check the range.

std::regex expr(R"(\d{4})");

if(std::regex_match(input, expr)) {
    int num = std::stoi(input);

    if(num >= 1256 && num <= 4350) {
        // input is within range
    }
}   

As Jarod42 mentions in the comments, since you've already validated the input is a 4 digit number, it's not necessary to convert it to an integer. Assuming input is an std::string, this would work too

if(input >= "1256" && input <= "4350") {
    // input is within range
}