Is gcc 4.8 or earlier buggy about regular expressions?

Feature Detection

This is a snippet to detect if the libstdc++ implementation is implemented with C preprocessor defines:

#include <regex>
#if __cplusplus >= 201103L &&                             \
    (!defined(__GLIBCXX__) || (__cplusplus >= 201402L) || \
        (defined(_GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT) || \
         defined(_GLIBCXX_REGEX_STATE_LIMIT)           || \
             (defined(_GLIBCXX_RELEASE)                && \
             _GLIBCXX_RELEASE > 4)))
#define HAVE_WORKING_REGEX 1
#else
#define HAVE_WORKING_REGEX 0
#endif

Macros

  • _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT is defined in bits/regex.tcc in 4.9.x
  • _GLIBCXX_REGEX_STATE_LIMIT is defined in bits/regex_automatron.h in 5+
  • _GLIBCXX_RELEASE was added to 7+ as a result of this answer and is the GCC major version

Testing

You can test it with GCC like this:

cat << EOF | g++ --std=c++11 -x c++ - && ./a.out
#include <regex>

#if __cplusplus >= 201103L &&                             \
    (!defined(__GLIBCXX__) || (__cplusplus >= 201402L) || \
        (defined(_GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT) || \
         defined(_GLIBCXX_REGEX_STATE_LIMIT)           || \
             (defined(_GLIBCXX_RELEASE)                && \
             _GLIBCXX_RELEASE > 4)))
#define HAVE_WORKING_REGEX 1
#else
#define HAVE_WORKING_REGEX 0
#endif

#include <iostream>

int main() {
  const std::regex regex(".*");
  const std::string string = "This should match!";
  const auto result = std::regex_search(string, regex);
#if HAVE_WORKING_REGEX
  std::cerr << "<regex> works, look: " << std::boolalpha << result << std::endl;
#else
  std::cerr << "<regex> doesn't work, look: " << std::boolalpha << result << std::endl;
#endif
  return result ? EXIT_SUCCESS : EXIT_FAILURE;
}
EOF

Results

Here are some results for various compilers:

$ gcc --version
gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ./a.out
<regex> doesn't work, look: false

$ gcc --version
gcc (GCC) 6.2.1 20160830
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ./a.out
<regex> works, look: true

$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ./a.out
<regex> works, look: true

$ gcc --version
gcc (Ubuntu 6.2.0-5ubuntu12) 6.2.0 20161005
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ./a.out
<regex> works, look: true

$ gcc --version
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ./a.out
<regex> works, look: true

$ gcc --version
gcc (GCC) 6.2.1 20160830
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ clang --version
clang version 3.9.0 (tags/RELEASE_390/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
$ ./a.out  # compiled with 'clang -lstdc++'
<regex> works, look: true

Here be Dragons

This is totally unsupported and relies on the detection of private macros that the GCC developers have put into the bits/regex* headers. They could change and go away at anytime. Hopefully, they won't be removed in the current 4.9.x, 5.x, 6.x releases but they could go away in the 7.x releases.

If the GCC developers added a #define _GLIBCXX_HAVE_WORKING_REGEX 1 (or something, hint hint nudge nudge) in the 7.x release that persisted, this snippet could be updated to include that and later GCC releases would work with the snippet above.

As far as I know, all other compilers have a working <regex> when __cplusplus >= 201103L but YMMV.

Obviously this would completely break if someone defined the _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT or _GLIBCXX_REGEX_STATE_LIMIT macros outside of the stdc++-v3 headers.


<regex> was implemented and released in GCC 4.9.0.

In your (older) version of GCC, it is not implemented.

That prototype <regex> code was added when all of GCC's C++0x support was highly experimental, tracking early C++0x drafts and being made available for people to experiment with. That allowed people to find problems and give feedback to the standard committee before the standard was finalised. At the time lots of people were grateful to have had access to bleeding edge features long before C++11 was finished and before many other compilers provided any support, and that feedback really helped improve C++11. This was a Good ThingTM.

The <regex> code was never in a useful state, but was added as a work-in-progress like many other bits of code at the time. It was checked in and made available for others to collaborate on if they wanted to, with the intention that it would be finished eventually.

That's often how open source works: Release early, release often -- unfortunately in the case of <regex> we only got the early part right and not the often part that would have finished the implementation.

Most parts of the library were more complete and are now almost fully implemented, but <regex> hadn't been, so it stayed in the same unfinished state since it was added.

Seriously though, who though that shipping an implementation of regex_search that only does "return false" was a good idea?

It wasn't such a bad idea a few years ago, when C++0x was still a work in progress and we shipped lots of partial implementations. No-one thought it would remain unusable for so long so, with hindsight, maybe it should have been disabled and required a macro or built-time option to enable it. But that ship sailed long ago. There are exported symbols from the libstdc++.so library that depend on the regex code, so simply removing it (in, say, GCC 4.8) would not have been trivial.

Tags:

C++

Regex

Gcc

C++11

Libstdc++

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