Question

I’m trying to use regular expressions in order to validate strings so before I go any further let me explain first how the strings looks like: optional number of digits followed by an ‘X’ and an optional (‘^’ followed by one or more digits).

Here are some exmaples: “2X”, “X”, “23X^6” fit the pattern while strings like “X^”, “4”, “foobar”, “4X^”, “4X44” don’t.

Now where was I: using ‘egrep’ and the “^[0-9]{0,}\X(\^[0-9]{1,})$” regex I can validate just fine those strings however when trying this in C++ using the C++11 regex library it fails.

Here’s the code I’m using to validate those strings:

#include <iostream>
#include <regex>
#include <string>
#include <vector>

int main()
{
    std::regex r("^[0-9]{0,}\\X(\\^[0-9]{1,})$",
                 std::regex_constants::egrep);
    std::vector<std::string> challanges_ok {"2X", "X", "23X^66", "23X^6",
                                            "3123X", "2313131X^213213123"};
    std::vector<std::string> challanges_bad {"X^", "4", "asdsad", " X",
                                             "4X44", "4X^"};
    std::cout << "challanges_ok: ";
    for (auto &str : challanges_ok) {
        std::cout << std::regex_match(str, r) << " ";
    }

    std::cout << "\nchallanges_bad: ";
    for (auto &str : challanges_bad) {
        std::cout << std::regex_match(str, r) << " ";
    }
    std::cout << "\n";

    return 0;
}

Am I doing something wrong or am I missing something? I’m compiling under GCC 4.7.

Answer 1

Your regex fails to make the '^' followed by one or more digits optional; change it to:
"^[0-9]*X(\\^[0-9]+)?$".

Also note that this page says that GCC’s support of <regex> is only partial, so std::regex may not work at all for you (‘partial’ in this context apparently means ‘broken’); have you tried Boost.Xpressive or Boost.Regex as a sanity check?