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std::sub_match<BidirIt>:: length

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std::sub_match
difference_type length ( ) const ;

Devuelve el número de caracteres en la coincidencia.

Valor de retorno

std:: distance ( first, second ) si la coincidencia es válida, ​ 0 ​ en caso contrario.

Complejidad

Constante.

Ejemplo

Ejecutar este código 
#include <iostream>
#include <regex>
#include <string>
int main()
{
    std::string sentence{"Quick red fox jumped over a lazy cow."};
    const std::regex re{"([A-z]+) ([a-z]+) ([a-z]+) ([a-z]+)"};
    std::smatch words;
    std::regex_search(sentence, words, re);
    for (const auto& m : words)
        std::cout << '[' << m << "], length = " << m.length() << '\n';
}

Salida: 

[Quick red fox jumped], length = 20
[Quick], length = 5
[red], length = 3
[fox], length = 3
[jumped], length = 6