Using Backreferences in Regex

Backreferences in regex allow you to refer to previously captured groups within the same pattern. This is useful for matching repeated substrings or enforcing certain constraints.

Here's a simple example where we use a backreference to match a word that repeats:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "hello hello world";
6  std::regex pattern(R"((\w+)\s+\1)");
7  std::smatch match;
8
9  if (std::regex_search(input, match, pattern)) {
10    std::cout << "Match: " << match.str() << "\n";
11    std::cout << "Repeated word: "
12      << match[1].str();
13  }
14}

1Match: hello hello
2Repeated word: hello

In this example, (\w+) captures a word, and \1 refers to the same word captured by the first group. The pattern matches repeated words.

You can use multiple backreferences to enforce patterns with more complexity. For example, matching repeated sequences:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "abc abc def def";
6  std::regex pattern(R"((\w+)\s+\1\s+(\w+)\s+\2)");
7  std::smatch match;
8
9  if (std::regex_search(input, match, pattern)) {
10    std::cout << "Match: " << match.str() << "\n";
11    std::cout << "First repeated word: "
12      << match[1].str() << "\n";
13    std::cout << "Second repeated word: "
14      << match[2].str() << "\n";
15  }
16}

1Match: abc abc def def
2First repeated word: abc
3Second repeated word: def

Here, (\w+) captures a word and \1 refers to this word. The second capture group (\w+) captures another word, and \2 refers to this second word.

Backreferences in C++ regex are powerful for matching patterns where certain parts need to be repeated or constrained. By using \1, \2, etc., you can refer to previously captured groups, allowing for complex pattern matching and validation.