Top-level recursion ignores top-level alternatives

[Davidebyzero]

A top-level recursive call, i.e. (?R) or (?0), only uses the first alternative, and ignores all subsequent ones.

As a simple example, a(?R)|z will only match z, but it is supposed to be able to also match az. The workaround is to use (?:a(?R)|z) or (a(?R)|z) or (a(?1)|z), which can match either az or z.

The regex which actually led to discovery of this bug was (?=(xx+?)\1*(?=\1$)((?R)))(?=(x+)\3*(?=\3$)((?R)))\2\4|x\B(?R)|, which calculates the OEIS A064097 number sequence in unary. But to make it work in the current version of Boost, the entire regex must be enclosed in parentheses.

I have tested and confirmed this to be happening in the latest version of Notepad++, v8.4.4 (released yesterday), which uses presumably the latest (or at least a very recent) version of Boost as its regex engine.

Sample program demonstrating the bug:

#include <boost/regex.hpp>
#include <iostream>
int main()
{
    boost::smatch what;
    if (boost::regex_search(std::string("az"), what, boost::regex(   "a(?R)|z" ))) std::cout << what[0] << '\n';
    if (boost::regex_search(std::string("az"), what, boost::regex("(?:a(?R)|z)"))) std::cout << what[0] << '\n';
    if (boost::regex_search(std::string("az"), what, boost::regex(  "(a(?1)|z)"))) std::cout << what[0] << '\n';
    return 0;
}

This should print three identical lines of az, but instead prints z followed by two lines of az.

[jzmaddock]

Thanks for the report, will investigate shortly!