C++ Extract number from the middle of a string

Question

I have a vector containing strings that follow the format of text_number-number

Eg: Example_45-3

Answer 1

updated for C++11

(important note for compiler regex support: for gcc. you need version 4.9 or later. i tested this on g++ version 4.9[1], and 9.2. cppreference.com has in browser compiler that i used.)

Thanks to user @2b-t who found a bug in the c++11 code!

Here is the C++11 code:

#include 
#include 
#include 

using std::cout;
using std::endl;

int main() {
    std::string input = "Example_45-3";
    std::string output = std::regex_replace(
        input,
        std::regex("[^0-9]*([0-9]+).*"),
        std::string("$1")
        );
    cout << input << endl;
    cout << output << endl;
}

---

boost solution that only requires C++98

Minimal implementation example that works on many strings (not just strings of the form "text_45-text":

#include 
#include 
using namespace std;
#include 

int main() {
    string input = "Example_45-3";
    string output = boost::regex_replace(
        input,
        boost::regex("[^0-9]*([0-9]+).*"),
        string("\\1")
        );
    cout << input << endl;
    cout << output << endl;
}

console output:

Example_45-3
45

Other example strings that this would work on:

• "asdfasdf 45 sdfsdf"
• "X = 45, sdfsdf"

For this example I used g++ on Linux with #include and -lboost_regex. You could also use C++11x regex.

Feel free to edit my solution if you have a better regex.

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Commentary:

If there aren't performance constraints, using Regex is ideal for this sort of thing because you aren't reinventing the wheel (by writing a bunch of string parsing code which takes time to write/test-fully).

Additionally if/when your strings become more complex or have more varied patterns regex easily accommodates the complexity. (The question's example pattern is easy enough. But often times a more complex pattern would take 10-100+ lines of code when a one line regex would do the same.)

---

[1]

[1] Apparently full support for C++11 was implemented and released for g++ version 4.9.x and on Jun 26, 2015. Hat tip to SO questions #1 and #2 for figuring out the compiler version needing to be 4.9.x.