Counting Regex Matches

Counting the number of regex matches in a string can be accomplished using std::sregex_iterator. This iterator allows you to iterate through all matches of a regex pattern in a string.

Using std::sregex_iterator

Here’s a simple example of how to count matches:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "one two three four five";
6  std::regex pattern("\\b\\w+\\b");
7
8  auto words_begin = std::sregex_iterator(
9    input.begin(), input.end(), pattern);
10  auto words_end = std::sregex_iterator();
11
12  int count = std::distance(words_begin, words_end);
13  std::cout << "Number of words: " << count;
14}

1Number of words: 5

In this example, the regex pattern "\b\w+\b" matches individual words in the string. We use std::sregex_iterator to iterate through all matches and std::distance to count them.

Detailed Example

Here’s another example that counts specific patterns, such as digits:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input =
6    "There are 3 cats, 4 dogs, and 5 birds.";
7  std::regex pattern("\\d");
8
9  auto numbers_begin = std::sregex_iterator(
10    input.begin(), input.end(), pattern);
11  auto numbers_end = std::sregex_iterator();
12
13  int count = std::distance(
14    numbers_begin, numbers_end);
15  std::cout << "Number of digits: " << count;
16}

1Number of digits: 3

In this case, the regex pattern "\d" matches individual digits. The process is the same: create an iterator for the matches and use std::distance to count them.

Summary

Counting regex matches in a string is straightforward with std::sregex_iterator. This iterator provides a way to traverse all matches of a pattern, and std::distance helps count them. This method is efficient and works well for both simple and complex patterns.