void regex_test()
{
cout << "Executing " << __func__ << endl;
std::string s ("This machine has c:\\ ,D:\\, E:\\ and z:\\ drives");
std::smatch m;
/**
* We have to use \\\\ so that we get \\ which means an escaped backslash.
*
* It's because there are two representations. In the string representation
* of the regex, we have "\\\\", Which is what gets sent to the parser.
* The parser will see \\ which it interprets as a valid escaped-backslash
* (which matches a single backslash).
*/
std::regex e ("[a-zA-Z]:\\\\"); // matches drive path
std::cout << "Target sequence: " << s << std::endl;
std::cout << "Regular expression: /[a-zA-Z]:\\\\\\\\/" << std::endl;
std::cout << "The following matches and submatches were found:" << std::endl;
while (std::regex_search (s,m,e)) {
for (auto x:m) std::cout << x << " ";
std::cout << std::endl;
s = m.suffix().str();
}
}
The code above shows that you need four backslashes to feed one literal backslash into the regex engine. Why is that? because you need four backslashes to produce two escaped backslashes in the regex string. The other two backslashes act as escape characters in the C/C++ compiler that you use.
Therefore, the "produced" two backslashes then act as a single escaped backslash for the regex engine which parses the input string.
Anyway, to give you an idea, this is the output of the function above in Linux:
Executing regex_test Target sequence: This machine has c:\ ,D:\, E:\ and z:\ drives Regular expression: /[a-zA-Z]:\\\\/ The following matches and submatches were found: c:\ D:\ E:\ z:\I hope this helps poor souls out there working with regex in C/C++.
