Fix NonBacktracking regex empty capture groups with newline at end of input

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Symbolic/SymbolicRegexMatcher.cs

@@ -474,7 +474,9 @@ public SymbolicMatch FindMatch(RegexRunnerMode mode, ReadOnlySpan<char> input, i
             }
             else
             {
-                Registers endRegisters = _containsAnyAnchor ?
+                // Use DefaultInputReader when pattern contains \Z anchor (needs special \n handling),
+                // otherwise use NoZAnchorOptimizedInputReader for correct capture tracking
+                Registers endRegisters = _containsEndZAnchor ?
                     FindSubcaptures<DefaultInputReader>(input, matchStart, matchEnd, perThreadData) :
                     FindSubcaptures<NoZAnchorOptimizedInputReader>(input, matchStart, matchEnd, perThreadData);
                 return new SymbolicMatch(matchStart, matchEnd - matchStart, endRegisters.CaptureStarts, endRegisters.CaptureEnds);
@@ -1481,9 +1483,9 @@ public static int GetPositionId(SymbolicRegexMatcher<TSet> matcher, ReadOnlySpan
             {
                 if ((uint)pos < (uint)input.Length)
                 {
-                    // Find the minterm, handling the special case for the last \n for states that start with a relevant anchor
+                    // Find the minterm, handling the special case for the last \n for \Z anchor
                     int c = input[pos];
-                    return c == '\n' && pos == input.Length - 1 ?
+                    return c == '\n' && pos == input.Length - 1 && matcher._containsEndZAnchor ?
                         matcher._minterms.Length : // mintermId = minterms.Length represents an \n at the very end of input
                         matcher._mintermClassifier.GetMintermID(c);
                 }

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Groups.Tests.cs

@@ -461,6 +461,17 @@ public static IEnumerable<object[]> Groups_MemberData()
             yield return (enUS, @"^(cat)\s+(dog)", "cat   \n\n\n   dog", RegexOptions.None, new string[] { "cat   \n\n\n   dog", "cat", "dog" });
             yield return (enUS, @"^(cat)\s+(dog)", "cat   \n\n\n   dog", RegexOptions.Multiline, new string[] { "cat   \n\n\n   dog", "cat", "dog" });
             yield return (enUS, @"(mouse)\s\n^(cat)\s+(dog)", "mouse\n\ncat   \n\n\n   dog", RegexOptions.Multiline, new string[] { "mouse\n\ncat   \n\n\n   dog", "mouse", "cat", "dog" });
+
+            // Regression test for https://github.com/dotnet/runtime/issues/120202
+            // Capture groups with beginning anchor and newline at end should work correctly with different end anchors
+            yield return (enUS, @"^(A)(\s)", "A\n", RegexOptions.None, new string[] { "A\n", "A", "\n" });
+            yield return (enUS, @"^(A)(\s)\z", "A\n", RegexOptions.None, new string[] { "A\n", "A", "\n" });
+            yield return (enUS, @"^(A)(\s)$", "A\n", RegexOptions.None, new string[] { "A\n", "A", "\n" });
+            yield return (enUS, @"^(A)(\s)\Z", "A\n", RegexOptions.None, new string[] { "A\n", "A", "\n" });
+            yield return (enUS, @"(A)(\s)\z", "A\n", RegexOptions.None, new string[] { "A\n", "A", "\n" }); // without beginning anchor
+            yield return (enUS, @"^(A)(\s)", "A\n", RegexOptions.Multiline, new string[] { "A\n", "A", "\n" });
+            yield return (enUS, @"^(A)(\s)", "A ", RegexOptions.None, new string[] { "A ", "A", " " });
+
             if (!RegexHelpers.IsNonBacktracking(engine)) // ECMAScript not supported
             {
                 yield return (enUS, @"^cat\s+dog", "cat   \n\n\n   dog", RegexOptions.ECMAScript, new string[] { "cat   \n\n\n   dog" });