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

Regular Expressions in C++ with Examples: In Practice

Regular expressions (regex) are a powerful tool for text processing, allowing you to search, replace, and validate strings using complex patterns. In C++, support for regular expressions was introduced in the C++11 standard through the <regex> library. In this article, we will break down how to use std::regex in practice: from simple validations to data extraction and text replacement.

This material will be useful both for beginners just getting acquainted with C++ and for experienced developers looking to refresh their knowledge. We will cover the key functions: std::regex_match, std::regex_search, and std::regex_replace, as well as working with capture groups.

Core Components of the <regex> Library

Before moving on to examples, let's understand the main classes and functions:

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• std::regex — a class representing the regular expression itself (the pattern). It is created from a pattern string and an optional grammar flag (ECMAScript by default).
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• std::smatch — a container for storing search results (for strings of type std::string). An analog is std::cmatch for C-strings.
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• std::regex_match — checks if the entire string matches the pattern completely.
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• std::regex_search — searches for a substring matching the pattern anywhere in the string.
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• std::regex_replace — replaces all matches of the pattern with a specified string (with support for backreferences).
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All functions accept flags to control behavior (e.g., std::regex_constants::icase for case-insensitive search).

Example 1: Validating a String Against a Pattern (std::regex_match)

The simplest task is to check if a string matches a given format. For example, let's check if a string is a valid email address (simplified version):

#include <iostream>\n#include <regex>\n#include <string>\n\nint main() {\n    std::string email = "user@example.com";\n    // Pattern for email validation (simplified)\n    std::regex pattern(R"([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,})");\n\n    if (std::regex_match(email, pattern)) {\n        std::cout << "Email is valid." << std::endl;\n    } else {\n        std::cout << "Email is invalid." << std::endl;\n    }\n\n    // Check an invalid email\n    std::string bad_email = "user@.com";\n    if (std::regex_match(bad_email, pattern)) {\n        std::cout << "Email is valid." << std::endl;\n    } else {\n        std::cout << "Email is invalid." << std::endl;\n    }\n\n    return 0;\n}

Output:

Email is valid.\nEmail is invalid.

Explanation:

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• We use a raw string literal R"(...)" to avoid escaping backslashes.
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• The std::regex_match function returns true only if the entire string completely matches the pattern.
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• The pattern [a-zA-Z0-9._%+-]+ means "one or more characters from the specified set".
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Example 2: Searching for Substrings and Extracting Data (std::regex_search)

Often you need not just to validate, but to find all occurrences of a pattern and extract parts of the string. For this, std::regex_search is used in a loop. In the following example, we will find all phone numbers in the format +7-XXX-XXX-XX-XX in the text and extract the operator code:

#include <iostream>\n#include <regex>\n#include <string>\n\nint main() {\n    std::string text = "Contacts: +7-495-123-45-67 and +7-916-987-65-43. Additional number: +7-495-111-22-33.";\n    // Pattern: +7-code(3 digits)-... (capture groups for code and the rest)\n    std::rege