[libcxx-commits] [libcxx] [libc++][regex] Correctly adjust match prefix for zero-length matches. (PR #94550)

[Konstantin Varlamov via libcxx-commits]

================
@@ -5424,9 +5429,21 @@ regex_iterator<_BidirectionalIterator, _CharT, _Traits>::operator++() {
     else
       ++__start;
   }
+
   __flags_ |= regex_constants::match_prev_avail;
-  if (!std::regex_search(__start, __end_, __match_, *__pregex_, __flags_))
+  if (!std::regex_search(__start, __end_, __match_, *__pregex_, __flags_)) {
     __match_ = value_type();
+
+  } else {
+    // The Standard mandates that if `regex_search` returns true ([re.regiter.incr]), "`match.prefix().first` shall be
+    // equal to the previous value of `match[0].second`... It is unspecified how the implementation makes these
+    // adjustments." The adjustment is necessary if we incremented `__start` above (the branch that deals with
+    // zero-length matches).
+    auto& __prefix = __match_.__prefix_;
----------------
var-const wrote:

There's another call to `regex_search` above, but I don't think prefix adjustment should ever be necessary there since it happens before `__start` is incremented.

https://github.com/llvm/llvm-project/pull/94550