C++ regex Visual Studio Community 2015 <regex> gives unexpected results

Question

Using Visual Studio Community 2015 C++ Unexpected results using

Source code:

#include <regex>
int main()
{
    std::regex re("^(.)=(\\d{1,2})/(\\d{1,2})*$");
    std::cmatch cm;
    std::regex_match("f=12/64", cm, re);
    for (unsigned idxMatch = 0; idxMatch < cm.size(); idxMatch++)
    {
        printf("Found Match %d '%s'\n", idxMatch, cm[idxMatch]);
    }
    return 0;
}

Results:

Found Match 0 'f=12/64' Found Match 1 'f=12/64' Found Match 2 '12/64' Found Match 3 '4'

Expected results:

Found Match 0 'f=12/64' Found Match 1 'f' Found Match 2 '12' Found Match 3 '64'

Commentary:

The regex works correctly on multiple other regex systems including C, Perl, Java, and Javascript.

The regex works correctly on every multiple online tester that I attempted.

I have tried escaping the "/" with unexpectedly identical results.

I found no clues to possible incorrect regex at the Microsoft website.

Answer 1

The issue is that you are using printf with wrong format specifiers. The cm[idxMatch] is not a null-terminated string, thus the %s specifier will not work. The behavior is undefined when you provide the wrong data type to match the output specifier.

The easiest solution is to use std::cout , and typesafe output streams in general.

Live Example Using Visual Studio 2015 and std::cout

To round things out, here is your example using printf . Note the weirdness that is outputted:

Here is printf weirdness and Visual Studio 2015

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Edit: For g++, there is a runtime error when using printf .

g++ printf runtime error

while using std::cout , we get the desired output:

g++ using std::cout