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

For regex patterns that produce zero-length matches, there is one
(imaginary) match in-between every character in the sequence being
searched (as well as before the first character and after the last
character). It's easiest to demonstrate using replacement:
`std::regex_replace("abc"s, "!", "")` should produce `!a!b!c!`, where
each exclamation mark makes a zero-length match visible.

Currently our implementation doesn't correctly set the prefix of each
zero-length match, "swallowing" the characters separating the imaginary
matches -- e.g. when going through zero-length matches within `abc`, the
corresponding prefixes should be `{'', 'a', 'b', 'c'}`, but before this
patch they will all be empty (`{'', '', '', ''}`). This happens in the
implementation of `regex_iterator::operator++`. Note that the Standard
spells out quite explicitly that the prefix might need to be adjusted
when dealing with zero-length matches in
[`re.regiter.incr`](http://eel.is/c++draft/re.regiter.incr):
> In all cases in which the call to `regex_search` returns `true`,
`match.prefix().first` shall be equal to the previous value of
`match[0].second`... It is unspecified how the implementation makes
these adjustments.

[Reproduction example](https://godbolt.org/z/8ve6G3dav)
```cpp
#include <iostream>
#include <regex>
#include <string>

int main() {
  std::string str = "abc";
  std::regex empty_matching_pattern("");

  { // The underlying problem is that `regex_iterator::operator++` doesn't update
    // the prefix correctly.
    std::sregex_iterator i(str.begin(), str.end(), empty_matching_pattern), e;
    std::cout << "\"";
    for (; i != e; ++i) {
      const std::ssub_match& prefix = i->prefix();
      std::cout << prefix.str();
    }
    std::cout << "\"\n";
    // Before the patch: ""
    // After the patch: "abc"
  }

  { // `regex_replace` makes the problem very visible.
    std::string replaced = std::regex_replace(str, empty_matching_pattern, "!");
    std::cout << "\"" << replaced << "\"\n";
    // Before the patch: "!!!!"
    // After the patch: "!a!b!c!"
  }
}
```

Fixes #64451

rdar://119912002

Author: var-const

libcxx/include/regex

@@ -4700,6 +4700,9 @@ private:
 
   template <class, class>
   friend class __lookahead;
+
+  template <class, class, class>
+  friend class regex_iterator;
 };
 
 template <class _BidirectionalIterator, class _Allocator>
@@ -5410,7 +5413,9 @@ template <class _BidirectionalIterator, class _CharT, class _Traits>
 regex_iterator<_BidirectionalIterator, _CharT, _Traits>&
 regex_iterator<_BidirectionalIterator, _CharT, _Traits>::operator++() {
   __flags_ |= regex_constants::__no_update_pos;
-  _BidirectionalIterator __start = __match_[0].second;
+  _BidirectionalIterator __start        = __match_[0].second;
+  _BidirectionalIterator __prefix_start = __start;
+
   if (__match_[0].first == __match_[0].second) {
     if (__start == __end_) {
       __match_ = value_type();
@@ -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_;
+    __prefix.first = __prefix_start;
+    __prefix.matched = __prefix.first != __prefix.second;
+  }
+
   return *this;
 }
 

libcxx/test/std/re/re.alg/re.alg.replace/zero_length_matches.pass.cpp

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <regex>
+
+// Test that replacing zero-length matches works correctly.
+
+#include <cassert>
+#include <regex>
+#include <string>
+#include "test_macros.h"
+
+int main(int, char**) {
+  // Various patterns that produce zero-length matches.
+  assert(std::regex_replace("abc", std::regex(""), "!") == "!a!b!c!");
+  assert(std::regex_replace("abc", std::regex("X*"), "!") == "!a!b!c!");
+  assert(std::regex_replace("abc", std::regex("X{0,3}"), "!") == "!a!b!c!");
+
+  // Replacement string has several characters.
+  assert(std::regex_replace("abc", std::regex(""), "[!]") == "[!]a[!]b[!]c[!]");
+
+  // Empty replacement string.
+  assert(std::regex_replace("abc", std::regex(""), "") == "abc");
+
+  // Empty input.
+  assert(std::regex_replace("", std::regex(""), "!") == "!");
+
+  // Not all matches are zero-length.
+  assert(std::regex_replace("abCabCa", std::regex("C*"), "!") == "!a!b!!a!b!!a!");
+
+  return 0;
+}

libcxx/test/std/re/re.iter/re.regiter/re.regiter.incr/post.pass.cpp

@@ -17,102 +17,151 @@
 #include <iterator>
 #include "test_macros.h"
 
-int main(int, char**)
-{
-    {
-        std::regex phone_numbers("\\d{3}-\\d{4}");
-        const char phone_book[] = "555-1234, 555-2345, 555-3456";
-        std::cregex_iterator i(std::begin(phone_book), std::end(phone_book), phone_numbers);
-        std::cregex_iterator i2 = i;
-        assert(i != std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i).size() == 1);
-        assert((*i).position() == 0);
-        assert((*i).str() == "555-1234");
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-        i++;
-        assert(i != std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i).size() == 1);
-        assert((*i).position() == 10);
-        assert((*i).str() == "555-2345");
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-        i++;
-        assert(i != std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i).size() == 1);
-        assert((*i).position() == 20);
-        assert((*i).str() == "555-3456");
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-        i++;
-        assert(i == std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-    }
-    {
-        std::regex phone_numbers("\\d{3}-\\d{4}");
-        const char phone_book[] = "555-1234, 555-2345, 555-3456";
-        std::cregex_iterator i(std::begin(phone_book), std::end(phone_book), phone_numbers);
-        std::cregex_iterator i2 = i;
-        assert(i != std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i).size() == 1);
-        assert((*i).position() == 0);
-        assert((*i).str() == "555-1234");
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-        ++i;
-        assert(i != std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i).size() == 1);
-        assert((*i).position() == 10);
-        assert((*i).str() == "555-2345");
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-        ++i;
-        assert(i != std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i).size() == 1);
-        assert((*i).position() == 20);
-        assert((*i).str() == "555-3456");
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-        ++i;
-        assert(i == std::cregex_iterator());
-        assert(i2!= std::cregex_iterator());
-        assert((*i2).size() == 1);
-        assert((*i2).position() == 0);
-        assert((*i2).str() == "555-1234");
-    }
-    { // https://llvm.org/PR33681
-        std::regex rex(".*");
-        const char foo[] = "foo";
+void validate_prefixes(const std::regex& empty_matching_pattern) {
+  const char source[] = "abc";
+
+  std::cregex_iterator i(source, source + 3, empty_matching_pattern);
+  assert(!i->prefix().matched);
+  assert(i->prefix().length() == 0);
+  assert(i->prefix().first == source);
+  assert(i->prefix().second == source);
+
+  ++i;
+  assert(i->prefix().matched);
+  assert(i->prefix().length() == 1);
+  assert(i->prefix().first == source);
+  assert(i->prefix().second == source + 1);
+  assert(i->prefix().str() == "a");
+
+  ++i;
+  assert(i->prefix().matched);
+  assert(i->prefix().length() == 1);
+  assert(i->prefix().first == source + 1);
+  assert(i->prefix().second == source + 2);
+  assert(i->prefix().str() == "b");
+
+  ++i;
+  assert(i->prefix().matched);
+  assert(i->prefix().length() == 1);
+  assert(i->prefix().first == source + 2);
+  assert(i->prefix().second == source + 3);
+  assert(i->prefix().str() == "c");
+
+  ++i;
+  assert(i == std::cregex_iterator());
+}
+
+void test_prefix_adjustment() {
+  // Check that we correctly adjust the match prefix when dealing with zero-length matches -- this is explicitly
+  // required by the Standard ([re.regiter.incr]: "In all cases in which the call to `regex_search` returns true,
+  // `match.prefix().first` shall be equal to the previous value of `match[0].second`"). For a pattern that matches
+  // empty sequences, there is an implicit zero-length match between every character in a string -- make sure the
+  // prefix of each of these matches (except the first one) is the preceding character.
+
+  // An empty pattern produces zero-length matches.
+  validate_prefixes(std::regex(""));
+  // Any character repeated zero or more times can produce zero-length matches.
+  validate_prefixes(std::regex("X*"));
+  validate_prefixes(std::regex("X{0,3}"));
+}
+
+int main(int, char**) {
+  {
+    std::regex phone_numbers("\\d{3}-\\d{4}");
+    const char phone_book[] = "555-1234, 555-2345, 555-3456";
+    std::cregex_iterator i(std::begin(phone_book), std::end(phone_book), phone_numbers);
+    std::cregex_iterator i2 = i;
+    assert(i != std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i).size() == 1);
+    assert((*i).position() == 0);
+    assert((*i).str() == "555-1234");
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+    i++;
+    assert(i != std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i).size() == 1);
+    assert((*i).position() == 10);
+    assert((*i).str() == "555-2345");
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+    i++;
+    assert(i != std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i).size() == 1);
+    assert((*i).position() == 20);
+    assert((*i).str() == "555-3456");
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+    i++;
+    assert(i == std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+  }
+  {
+    std::regex phone_numbers("\\d{3}-\\d{4}");
+    const char phone_book[] = "555-1234, 555-2345, 555-3456";
+    std::cregex_iterator i(std::begin(phone_book), std::end(phone_book), phone_numbers);
+    std::cregex_iterator i2 = i;
+    assert(i != std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i).size() == 1);
+    assert((*i).position() == 0);
+    assert((*i).str() == "555-1234");
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+    ++i;
+    assert(i != std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i).size() == 1);
+    assert((*i).position() == 10);
+    assert((*i).str() == "555-2345");
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+    ++i;
+    assert(i != std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i).size() == 1);
+    assert((*i).position() == 20);
+    assert((*i).str() == "555-3456");
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+    ++i;
+    assert(i == std::cregex_iterator());
+    assert(i2 != std::cregex_iterator());
+    assert((*i2).size() == 1);
+    assert((*i2).position() == 0);
+    assert((*i2).str() == "555-1234");
+  }
+  { // https://llvm.org/PR33681
+    std::regex rex(".*");
+    const char foo[] = "foo";
     //  The -1 is because we don't want the implicit null from the array.
-        std::cregex_iterator i(std::begin(foo), std::end(foo) - 1, rex);
-        std::cregex_iterator e;
-        assert(i != e);
-        assert((*i).size() == 1);
-        assert((*i).str() == "foo");
-
-        ++i;
-        assert(i != e);
-        assert((*i).size() == 1);
-        assert((*i).str() == "");
-
-        ++i;
-        assert(i == e);
-    }
+    std::cregex_iterator i(std::begin(foo), std::end(foo) - 1, rex);
+    std::cregex_iterator e;
+    assert(i != e);
+    assert((*i).size() == 1);
+    assert((*i).str() == "foo");
+
+    ++i;
+    assert(i != e);
+    assert((*i).size() == 1);
+    assert((*i).str() == "");
+
+    ++i;
+    assert(i == e);
+  }
+
+  test_prefix_adjustment();
 
   return 0;
 }