How to get text between two indexes in C++

A lot of my CS homework involves getting what lies between two characters, sometimes between two words.

For example, I needed to get the number in between the round brackets and all the text in between the square brackets of this string to do operations on them.

Add (0) [id:7583;username:eevee;score:92;grade:A]

The common “obvious” solution to this would be to get the indices of the characters that have the text you want in between them, and use substring(charA, charB).

The issue with that approach is that C++ doesn’t use the fun, simple substring like JavaScript where you can use two indices as arguments. C++’s substring takes the starting indice and then the number of characters you want after that starting indice.


The number of characters we want is just the distance between the two indices. I wrote two functions to calculate said distance and return the desired substring:

#include <iostream>

std::size_t length_between_two(std::size_t a, std::size_t b)
    if (b < a) return 0;

    return b - a;

std::string text_between_two(std::string s, std::string a, std::string b)
    std::size_t aPos = s.find(a);
    std::size_t bPos = s.find(b);

    // The + a.length() is necessary here so we don't get the text that makes up a
    std::size_t length = length_between_two(aPos + a.length(), bPos);

    // See previous comment.
    std::string sub = s.substr(aPos + a.length(), length);
    return sub;

int main(int argc, char *argv[])
    std::string inputString = "Add (0) [id:7583;username:eevee;score:92;grade:A]";

    std::string betweenCurly = text_between_two(inputString, "(", ")");
    std::string betweenSquare = text_between_two(inputString, "[", "]");

    std::cout << "curly: " << betweenCurly << std::endl;
    // 0

    std::cout << "square: " << betweenSquare << std::endl;
    // id:7583;username:eevee;score:92;grade:A