Non-Capture Groups

Non-capture groups in regex allow you to group parts of your pattern without saving the matched content. This can be useful for applying quantifiers or logical operations to parts of your regex without creating additional groups in the result.

Syntax

Non-capture groups are defined using (?: ...) instead of the regular capture group parentheses ( ... ). For example:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "apple orange banana";
6  std::regex pattern("(?:apple|orange) banana");
7  std::smatch match;
8  std::regex_search(input, match, pattern);
9  std::cout << match.str();
10}

1orange banana

Here, (?:apple|orange) banana matches either "apple banana" or "orange banana", but does not create a separate capture group for "apple" or "orange".

Use Case 1: To Group Without Capturing

Use non-capture groups when you need to group parts of your regex pattern without the overhead of capturing. This is useful for cleaner and more efficient regex patterns.

Use Case 2: To Apply Quantifiers

Non-capture groups are helpful when you want to apply quantifiers to a group of expressions without capturing the group:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "The quick brown fox";
6  std::regex pattern("(?:quick|slow) brown");
7  std::smatch match;
8  std::regex_search(input, match, pattern);
9  std::cout << match.str();
10}

1quick brown

Use Case 3: To Control Alternation

They are also used to control alternation (|) without capturing the alternatives:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "red fox";
6  std::regex pattern("(?:red|blue) fox");
7  std::smatch match;
8  std::regex_search(input, match, pattern);
9  std::cout << match.str();
10}

1red fox

Non-capture groups provide flexibility in crafting regex patterns that are both efficient and easy to read.