id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12797	Invalid regex recursion behavior	Lucas Trzesniewski <lucas.trzesniewski@…>	John Maddock	"I have found an unexpected behavior regarding recursion in regexes (in Perl mode). I've reduced the issue to the following test pattern:

{{{
(?(DEFINE)
    (?<prefix>)
    (?<dummy>x)
)
(?&prefix) unused | (?&prefix) match
}}}

If you try to match this against `foo match bar`, the `match` word should be found. This works in both Perl and PCRE, see the [https://regex101.com/r/emKagD/1 regex101 demo here].

* Removing or commenting out the `dummy` group causes the pattern to match
* Making the `dummy` group empty causes the pattern to match
* Replacing `(?&prefix)` with `(?&prefix)?` causes `Encountered an infinite recursion.`
* Replacing `(?&prefix)` with `(?&prefix)?` but making the definition of `pattern` non-empty causes the pattern to match

Here's the full test program:

{{{
#!cpp
#include <string>
#include <iostream>
#include <boost/regex.hpp>

static void test()
{
    boost::regex re(R""regex(
        (?(DEFINE)
          (?<prefix>)
          (?<dummy>x)
        )
        (?&prefix) unused | (?&prefix) match
    )regex"", boost::regex::perl | boost::regex::no_mod_s | boost::regex::mod_x | boost::regex::optimize);

    std::string subject(""foo match bar"");

    std::cout << boost::regex_replace(subject, re, ""[$&]"", boost::format_all) << std::endl;
}

int main(int argc, char **argv)
{
    try
    {
        test();
    }
    catch(std::exception ex)
    {
        std::cerr << ex.what() << std::endl;
    }

    return 0;
}
}}}

* Actual output is `foo match bar`
* Expected output is `foo [match] bar`

Tested with Boost 1.63.0 on MSVC 2015. The full pattern where the issue appeared can be found [http://stackoverflow.com/a/41910370/3764814 here].
"	Bugs	new	To Be Determined	regex	Boost 1.63.0	Problem		regex recursion