Custom regex literals in C++

23 Oct 2023

Regular expressions are powerful tools for pattern matching in string data. C++11 introduced the <regex> library, which provides built-in support for regular expressions. However, using regular expressions in C++ can be cumbersome, as the syntax requires escaping special characters and can become difficult to read and maintain.

To overcome these limitations, C++ allows users to define custom regex literals, which provide a more convenient and expressive way to work with regular expressions.

What are custom regex literals?

Custom regex literals enable you to define your own syntax for regular expressions in C++. By defining a custom literal suffix, you can create a shortcut for constructing regular expressions without the need for excessive escaping or verbose syntax.

Creating a custom regex literal

Let’s see how to create a custom regex literal in C++:

#include <regex>

constexpr std::regex operator"" _re(const char* pattern, std::size_t length) {
    return std::regex(pattern, length);
}

In the code snippet above, we define a constexpr function named operator"" _re. This function takes two parameters: pattern, which is a string representing the regular expression, and length, which is the length of the pattern string.

The function constructs and returns a std::regex object using the provided pattern and length. The constexpr keyword ensures that this function can be evaluated at compile-time.

Using the custom regex literal

Once the custom regex literal is defined, it can be used in your code just like any other literal. Consider the following example:

int main() {
    std::string text = "Hello, World!";
    std::regex pattern = "[A-Z]+";

    if (std::regex_search(text, pattern)) {
        // Perform some action
    }

    // Using the custom regex literal
    if (std::regex_search(text, "[A-Z]+"_re)) {
        // Perform the same action
    }

    return 0;
}

In the example above, we search for uppercase letters in the text string using both the traditional syntax and the custom regex literal. Both approaches yield the same result.

Benefits of custom regex literals

Using custom regex literals in C++ offers several benefits:

Improved readability: Custom regex literals allow you to write regular expressions in a more human-readable and expressive manner, without excessive escaping or cumbersome syntax.
Code consistency: By defining your own regex literal, you can ensure consistent regex usage across your codebase, making it easier to read and maintain.
Reduced errors: Custom literals can help reduce errors caused by misplacing or forgetting to escape special characters, as the defined syntax handles it automatically.

Conclusion

Custom regex literals in C++ provide a convenient and expressive way to work with regular expressions. By defining your own syntax, you can improve code readability, consistency, and reduce errors. Use custom regex literals to simplify your regular expression usage in C++.

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