C++ std::regex syntax incompatibility

[lemire]

std::regex in C++ supports a modified ECMAScript syntax which differs from the actual ECMAScript regex syntax, causing compatibility issues with JavaScript-like patterns, notably in URL pattern matching.

This problem arises in the tests of following PR: Node.js Pull Request #56452

The implementation of std::regex rejects certain regex constructs that are valid in JavaScript ECMAScript syntax:

Example Pattern: "/([\\d&&[0-1]])"

Reproducible Example:

{
  "pattern": [{ "pathname": "/([\\d&&[0-1]])" }],
  "inputs": [{ "pathname": "/0" }],
  "expected_match": {
    "pathname": { "input": "/0", "groups": { "0": "0" } }
  }
}

C++ Code to Simulate Failure:

#include <iostream>
#include <regex>
#include "ada.h" // Assuming ada is a library for URL parsing

int main() {
    ada::url_pattern_init init{.pathname = "/([\\d&&[0-1]])"};
    auto url_pattern = ada::parse_url_pattern(init, nullptr, nullptr);
    if (!url_pattern) {
        std::cout << "URL pattern parsing failure" << std::endl;
    }

    // Simplified regex test
    try {
        std::regex regexPattern("[[0-1]]");
    } catch (const std::regex_error& e) {
        std::cout << "Regex construction error: " << e.what() << std::endl;
    }

    return 0;
}

The regex is somewhat confusing: [[0-1] is a character class made of three characters. Followed by ].

JavaScript handles this by interpreting the final ] as a literal character as if it does not close a valid character class.

C++ rejects the regex.

To achieve the intended regex behavior in C++, the pattern needs to be adjusted:

std::regex regexPattern("[[0-1]\\]");

This discrepancy can lead to failures when C++ and JavaScript regex compatibility is assumed or required

[anonrig]

I recommend keeping std::regex solution and on top of that using the regex engine proposal for others to properly support ecmascript regex through the plugin support.