Mastering std::regex in Modern C++ (C++11 – C++23)

1. Why Use std::regex?

Regular expressions allow you to search, match, and manipulate strings based on patterns, not fixed characters. They are powerful for:

• Validating email addresses or phone numbers

• Extracting words or numbers from a document

• Replacing patterns within text

• Parsing logs, configuration files, and even code

2. Required Headers and Setup

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

3. Main Types in <regex>

4. regex_search vs regex_match

Example: regex_search (Searches for pattern anywhere in the text)

std::string text = "Call me at 555-1234";
std::regex pattern(R"(\d{3}-\d{4})");

std::smatch match;
if (std::regex_search(text, match, pattern)) {
    std::cout << "Phone found: " << match[0] << "\n";
}

Example: regex_match (Full string must match)

std::string input = "abc123";
std::regex pattern(R"([a-z]+[0-9]+)");

if (std::regex_match(input, pattern)) {
    std::cout << "Valid format\n";
}

5. regex_replace Example

std::string html = "<b>Bold</b>";
std::regex tag(R"(<.*?>)");

std::string plain = std::regex_replace(html, tag, "");
std::cout << plain;  // Output: Bold

6. Regex Pattern Syntax (Cheat Sheet)

Character Classes

Note: Use raw strings (R"(pattern)") in C++ to avoid escaping backslashes.

Quantifiers

Anchors and Boundaries

Grouping and Alternation

7. Real-World Examples

Validate Email

std::regex email(R"((\w+)(\.\w+)*@(\w+)\.(\w+))");
std::string s = "user.name@example.com";
std::smatch m;
if (std::regex_match(s, m, email)) {
    std::cout << "Valid email!\n";
}

Extract All Words from a Sentence

std::string sentence = "C++ is powerful and fast";
std::regex word(R"(\w+)");
auto begin = std::sregex_iterator(sentence.begin(), sentence.end(), word);
auto end = std::sregex_iterator();

for (auto it = begin; it != end; ++it) {
    std::cout << "Word: " << it->str() << "\n";
}

Replace URLs with Placeholder

std::string input = "Visit http://example.com now!";
std::regex url(R"(http[s]?://\S+)");
std::string clean = std::regex_replace(input, url, "[link]");
std::cout << clean;  // Output: Visit [link] now!

8. Error Handling

try {
    std::regex broken("[A-Z");  // Missing closing ]
} catch (const std::regex_error& e) {
    std::cerr << "Regex error: " << e.what() << '\n';
}

9. Using Regex Flags

std::regex pattern("hello", std::regex_constants::icase);  // Case-insensitive

10. Practice Tasks for Beginners

1. Write a regex to validate phone numbers like +966-55-1234567

2. Extract all IP addresses from a log file line

3. Build a small command-line tool that replaces banned words in a sentence

4. Validate hexadecimal color values like #FF5733 or #abc