std::regex_iterator

std::regex_iterator

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Prerequisites

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1. What is an std::regex_iterator

std::regex_iterator is an iterator used to iterate over all regex matches in a string.

Instead of finding just one match, it allows you to loop through every match

string + regex → multiple matches → iterate one by one

#include <regex>

std::regex pattern("...");
std::sregex_iterator it(begin, end, pattern);
std::sregex_iterator end;

Example

#include <iostream>
#include <regex>

int main()
{
    std::string text = "abc 123 def 456";

    std::regex pattern("\\d+");  // match numbers

    std::sregex_iterator it(text.begin(), text.end(), pattern);
    std::sregex_iterator end;

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

123
456

What Does Iterator Hold?

Each iterator element is:

std::match_results<std::string::const_iterator>

✔️ Access Data

it->str();        // full match
it->position();   // position in string
it->length();     // length of match

When we use:

• log parsing
• extracting numbers
• parsing structured text
• simple DSL parsing
• config parsing

2. Capture Groups

Example

#include <iostream>
#include <regex>

int main()
{
    std::string text = "Name: Kang Age: 30";

    std::regex pattern("(\\w+):\\s*(\\w+)");

    std::sregex_iterator it(text.begin(), text.end(), pattern);
    std::sregex_iterator end;

    for (; it != end; ++it)
    {
        std::cout << "Key: " << (*it)[1] << "\n";
        std::cout << "Value: " << (*it)[2] << "\n";
    }
}

Key: Name
Value: Kang
Key: Age
Value: 30

(\\w+):\\s*(\\w+)

1. it[0]: (\\w+):\\s*(\\w+)
2. it[1]: first parentheses (\\w+)
2. it[2]: second parentheses (\\w+)

Type	Description
std::sregex_iterator	string iterator
std::cregex_iterator	C-string iterator
std::regex_iterator	generic template

3. Performance Considerations

Regex is Expensive

• Uses complex pattern engine
• Backtracking possible
• Not cache-friendly

Method	Speed
manual parsing	🔥 fastest
string functions	⚡ fast
regex_iterator	🐢 slow

3-1. Optimization Tips

• compile regex once

std::regex pattern("...");

• avoid inside loops

4. regex_iterator vs regex_search

regex_search

std::smatch match;
std::regex_search(text, match, pattern);

std::string text = "abc 123 def 456";
std::regex pattern("\\d+");

std::smatch match;
std::regex_search(text, match, pattern);

std::cout << match.str();

finds first match only

regex_iterator

for (...) { ... }

std::string text = "abc 123 def 456";
std::regex pattern("\\d+");

std::sregex_iterator it(text.begin(), text.end(), pattern);
std::sregex_iterator end;

for (; it != end; ++it)
    std::cout << it->str() << "\n";

finds all matches

5. Common Mistakes

❌ Recreating regex inside loop

for (...)
{
    std::regex pattern("...");  // ❌ slow
}

❌ Forgetting escape

"\d+"   // ❌ wrong
"\\d+"  // ✅ correct

❌ Assuming fast performance

👉 regex is not for high-frequency loops

6. Example

#include <iostream>
#include <regex>

int main()
{
    std::string text = "Contact: test@mail.com or hello@world.com";

    std::regex pattern("\\w+@\\w+\\.\\w+");

    std::sregex_iterator it(text.begin(), text.end(), pattern);
    std::sregex_iterator end;

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