Regexp matching fails with invalid special open parenthesis - c++

Question

I am trying to use regexps in c++11, but my code always throws an std::regex_error of Invalid special open parenthesis.. A minimal example code which tries to find the first duplicate character in a string:
std::string regexp_string("(?P<a>[a-z])(?P=a)"); // Nothing to be escaped here, right?
std::regex regexp_to_match(regexp_string);
std::string target("abbab");
std::smatch matched_regexp;
std::regex_match(target, matched_regexp, regexp_to_match);
for(const auto& m: matched_regexp)
{
std::cout << m << std::endl;
}
Why do I get an error and how do I fix this example?

Answer 1

There are 2 issues here:
std::regex flavors do not support named capturing groups / backreferences, you need to use numbered capturing groups / backreferences
You should use regex_search rather than regex_match that requires a full string match.
Use
std::string regexp_string(R"(([a-z])\1)");
std::regex regexp_to_match(regexp_string);
std::string target("abbab");
std::smatch matched_regexp;
if (std::regex_search(target, matched_regexp, regexp_to_match)) {
std::cout << matched_regexp.str() << std::endl;
}
// => bb
See the C++ demo
The R"(([a-z])\1)" raw string literal defines the ([a-z])\1 regex that matches any lowercase ASCII letter and then matches the same letter again.

Answer 2

http://en.cppreference.com/w/cpp/regex/ecmascript says that ECMAScript (the default type for std::regex) requires (?= for positive lookahead.

Answer 3

The reason your regex crashes for you is because named groups not supported by std::regex. However you can still use what is available to find the first duplicate char in string:
#include <iostream>
#include <regex>
int main()
{
std::string s = "abc def cde";
std::smatch m;
std::regex r("(\\w).*?(?=\\1)");
if (std::regex_search(s, m, r))
std::cout << m[1] << std::endl;
return 0;
}
Prints
c