Regular Expressions in C++ with Examples: A Practical Guide

Regular Expressions in C++ with Examples: From Basics to Practice

Regular expressions (regex) are a powerful tool for searching, extracting, and replacing text fragments by pattern. In C++, support for regular expressions appeared in the C++11 standard and is implemented in the <regex> library. In this article, we will analyze practical examples of using std::regex that will help you efficiently process strings in your projects.

1. Basics of std::regex: Creation and Simple Search

Three main classes are used to work with regular expressions in C++: std::regex (pattern), std::smatch (result for a string), and std::sregex_iterator (iterator for multiple matches).

Example 1: Checking if a string matches a pattern

#include <iostream>#include <regex>#include <string>

int main() {    std::string email = "user@example.com";    // Pattern for checking email (simplified)    std::regex pattern(R"([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,})");        if (std::regex_match(email, pattern)) {        std::cout << "Email is valid!" << std::endl;    } else {        std::cout << "Email does not match the format." << std::endl;    }    return 0;}

The std::regex_match function checks whether the entire string completely matches the pattern. Note the use of R"(...)" — this is a raw string literal, which allows you to avoid escaping backslashes.

Example 2: Searching for a substring

#include <iostream>#include <regex>#include <string>

int main() {    std::string text = "My number: +7 (123) 456-78-90. Call me!";    std::regex phone_pattern(R"(\\+7\\s?\\(\\d{3}\\)\\s?\\d{3}-\\d{2}-\\d{2})");    std::smatch match;        if (std::regex_search(text, match, phone_pattern)) {        std::cout << "Found number: " << match.str() << std::endl;        std::cout << "Position: " << match.position() << std::endl;    } else {        std::cout << "Number not found." << std::endl;    }    return 0;}

std::regex_search searches for the first match of the pattern in the string. The result is stored in a std::smatch object, from which you can extract the found substring and its position.

2. Extracting Data Using Capturing Groups

Groups allow you to extract individual parts of a match. They are denoted by parentheses ().

Example 3: Parsing a date

#include <iostream>#include <regex>#include <string>

int main() {    std::string date = "Today is 15.04.2025";    std::regex date_pattern(R"((\\d{2})\\.(\\d{2})\\.(\\d{4}))");    std::smatch match;        if (std::regex_search(date, match, date_pattern)) {        std::cout << "Full date: " << match[0] << std::endl;        std::cout << "Day: " << match[1] << std::endl;        std::cout << "Month: " << match[2] << std::endl;        std::cout << "Year: " << match[3] << std::endl;    }    return 0;}

Example 4: Extracting all matches using an iterator

#include <iostream>#include <regex>#include <string>

int main() {    std::string log = "ERROR: file not foundWARNING: low batteryINFO: operation completedERROR: connection timeout";    std::regex error_pattern(R"(^ERROR:\\s*(.+)$)");        // Use std::sregex_iterator to iterate over all lines    auto words_begin = std::sregex_iterator(log.begin(), log.end(), error_pattern);    auto words_end = std::sregex_iterator();        std::cout << "Found