[libc++] Fix regex_search to match $ alone with match_default flag

[SanjayMarreddi]

Using regex_search with the regex_constant match_default and a simple regex pattern $ is expected to match general strings such as "a", "ab", "abc"... at [last, last) positions. But, the current implementation fails to do so.

Fixes #75042

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Sanjay Marreddi (SanjayMarreddi)

Changes

Using regex_search with the regex_constant match_default and a simple regex pattern $ is expected to match general strings such as "a", "ab", "abc"... at [last, last) positions. But, the current implementation fails to do so.

Fixes #75042

---

Full diff: https://github.com/llvm/llvm-project/pull/78845.diff

2 Files Affected:

• (modified) libcxx/include/regex (+8)
• (modified) libcxx/test/std/re/re.const/re.matchflag/match_not_eol.pass.cpp (+28)

diff --git a/libcxx/include/regex b/libcxx/include/regex
index b575a267583b5f..48af5b8b57fd64 100644
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -5124,6 +5124,14 @@ bool basic_regex<_CharT, _Traits>::__search(
       }
       __m.__matches_.assign(__m.size(), __m.__unmatched_);
     }
+    __m.__matches_.assign(__m.size(), __m.__unmatched_);
+    if (__match_at_start(__first, __last, __m, __flags, false)) {
+      __m.__prefix_.second  = __m[0].first;
+      __m.__prefix_.matched = __m.__prefix_.first != __m.__prefix_.second;
+      __m.__suffix_.first   = __m[0].second;
+      __m.__suffix_.matched = __m.__suffix_.first != __m.__suffix_.second;
+      return true;
+    }
   }
   __m.__matches_.clear();
   return false;
diff --git a/libcxx/test/std/re/re.const/re.matchflag/match_not_eol.pass.cpp b/libcxx/test/std/re/re.const/re.matchflag/match_not_eol.pass.cpp
index edeea517d2537a..a696d815dda5a8 100644
--- a/libcxx/test/std/re/re.const/re.matchflag/match_not_eol.pass.cpp
+++ b/libcxx/test/std/re/re.const/re.matchflag/match_not_eol.pass.cpp
@@ -47,5 +47,33 @@ int main(int, char**)
     assert( std::regex_search(target, re, std::regex_constants::match_not_eol));
     }
 
+    {
+      std::string target = "foo";
+      std::regex re("$");
+      assert(std::regex_search(target, re));
+      assert(!std::regex_search(target, re, std::regex_constants::match_not_eol));
+
+      std::smatch match;
+      assert(std::regex_search(target, match, re));
+      assert(match.position(0) == 3);
+      assert(match.length(0) == 0);
+      assert(!std::regex_search(target, match, re, std::regex_constants::match_not_eol));
+      assert(match.length(0) == 0);
+    }
+
+    {
+      std::string target = "foo";
+      std::regex re("$");
+      assert(!std::regex_match(target, re));
+      assert(!std::regex_match(target, re, std::regex_constants::match_not_eol));
+    }
+
+    {
+      std::string target = "a";
+      std::regex re{"^b*$"};
+      assert(!std::regex_search(target, re));
+      assert(!std::regex_search(target, re, std::regex_constants::match_not_eol));
+    }
+
   return 0;
 }

[SanjayMarreddi]

• I can see that std/re/re.const/re.matchflag/match_not_eol.pass.cpp is failing in generic-cxx03.
• However locally, when I ran the tests using sudo libcxx/utils/libcxx-lit build/ -sv libcxx/test/std/re/re.const, the test passed.
• How do I replicate the above behaviour locally to fix the failing tests on CI? Thanks!

[H-G-Hristov]

Suggested change

	std::regex re{"^b*$"};
	std::regex re("^b*$");

Did you test in C++03 mode?

https://libcxx.llvm.org/TestingLibcxx.html#usage

sudo libcxx/utils/libcxx-lit build/ -sv libcxx/test/std/re/re.const --param=c++03

It is interesting, this test doesn't have // UNSUPPORTED: c++03. Regex is a C++11 feature. Maybe it is supported as an extension.

[SanjayMarreddi]

• Updated the test.
• Yeah, I tested in the C++03 mode using the above command specifying the param. The tests are passing locally.

[ldionne]

LGTM -- I checked and this should fix all reported regressions around this.