match with regex_constants::match_any bug

[zmysysz]

In multiple overloads of regex_match, regex_constants::match_any is enabled by default, which can cause a drastic drop in matching performance (by a factor of tens to hundreds). This appears to be a bug in either the implementation or the way the function is invoked.

[jzmaddock]

You're going to have to give me a concrete example: the whole point of match_any is to improve performance, as in "I don't care what the match is, just stop when you find one".

[zmysysz]

Yes, the original intent of match_any was probably to improve performance, potentially by allowing optimizations such as non-greedy matching. However, those optimizations are more appropriate for regex_search. In contrast, regex_match is expected to perform a full match on the entire input string — a strict yes-or-no result. If performance improvements are needed, the implementation could, for example, short-circuit trailing .*. But in practice, this flag (match_any) clearly has the opposite effect, significantly degrading performance.

[jzmaddock]

Ca you please provide a specific example of a string and regex where matching is slower?

[zmysysz]

Sure, you can easily reproduce it with a random example.

#include <iostream>
#include <string>
#include <boost/regex.hpp>
#include <chrono>
#include <boost/version.hpp>

constexpr size_t ITERATIONS = 10'000'000;
constexpr size_t STRING_SIZE = 500;

int main() {
    // Create a string of approximately 1KB with a matchable pattern at the end
    std::string sample = "AAAAAAAAAAXYZ1234567" + std::string(STRING_SIZE, 'A');
    std::cout << "Boost version (numeric): " << BOOST_VERSION << std::endl;
    std::cout << "Boost version (string): " << BOOST_LIB_VERSION << std::endl;
    std::cout << "Sample str size: " << sample.size() << std::endl;
    // Regular expression to match the end pattern
    boost::regex pattern("^[A]{10}XYZ\\d{7}.*");
    // ----------- Overload with match result (boost::smatch) -----------
    {
        auto start = std::chrono::steady_clock::now();
        boost::smatch result;
        size_t matched = 0;

        for (size_t i = 0; i < ITERATIONS; ++i) {
            if (boost::regex_match(sample, result, pattern)) {
                ++matched;

            }
        }

        auto end = std::chrono::steady_clock::now();
        std::chrono::duration<double> elapsed = end - start;
        std::cout << "[With result]   Time: " << elapsed.count() << "s, Matched times: " << matched << std::endl;
    }

    // ----------- Overload without match result -----------
    {
        auto start = std::chrono::steady_clock::now();
        size_t matched = 0;

        for (size_t i = 0; i < ITERATIONS; ++i) {
            if (boost::regex_match(sample, pattern)) {
                ++matched;
            }
        }

        auto end = std::chrono::steady_clock::now();
        std::chrono::duration<double> elapsed = end - start;
        std::cout << "[Without result]     Time: " << elapsed.count() << "s, Matched: " << matched << std::endl;
    }

    return 0;
}

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