Fix balancing group inconsistency between IsMatched and Group.Success

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexRunner.cs

@@ -573,6 +573,12 @@ protected void TransferCapture(int capnum, int uncapnum, int start, int end)
             else if (end <= start2)
             {
                 start = start2;
+                // Ensure we don't create a capture with negative length
+                // When the balancing capture precedes the balanced group, end might be less than the new start
+                if (end < start)
+                {
+                    end = start;
+                }
             }
             else
             {

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

@@ -0,0 +1,189 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Linq;
+using System.Tests;
+using Xunit;
+
+namespace System.Text.RegularExpressions.Tests
+{
+    public class RegexBalancingGroupTests
+    {
+        /// <summary>
+        /// Tests for balancing groups where the balancing group's captured content
+        /// precedes the position of the group being balanced.
+        /// This tests the fix for https://github.com/dotnet/runtime/issues/XXXXX
+        /// </summary>
+        [Theory]
+        [MemberData(nameof(BalancingGroup_WithConditional_MemberData))]
+        public void BalancingGroup_WithConditional_ConsistentBehavior(RegexEngine engine, Regex regex, string input, bool expectedGroup2Matched, string expectedMatch)
+        {
+            _ = engine; // To satisfy xUnit analyzer
+            Match m = regex.Match(input);
+
+            Assert.True(m.Success, $"Match should succeed for input '{input}'");
+            Assert.Equal(expectedMatch, m.Value);
+
+            // Check that the group 2 state is consistent
+            bool group2Success = m.Groups[2].Success;
+            int group2CapturesCount = m.Groups[2].Captures.Count;
+
+            // The key test: Group.Success and Captures.Count should be consistent with the conditional behavior
+            Assert.Equal(expectedGroup2Matched, group2Success);
+            if (expectedGroup2Matched)
+            {
+                Assert.True(group2CapturesCount > 0, "If group 2 matched, it should have at least one capture");
+            }
+            else
+            {
+                Assert.Equal(0, group2CapturesCount);
+            }
+        }
+
+        public static IEnumerable<object[]> BalancingGroup_WithConditional_MemberData()
+        {
+            foreach (RegexEngine engine in RegexHelpers.AvailableEngines)
+            {
+                if (RegexHelpers.IsNonBacktracking(engine))
+                {
+                    // NonBacktracking engine doesn't support balancing groups
+                    continue;
+                }
+
+                var cases = new (string Pattern, string Input, bool ExpectedGroup2Matched, string ExpectedMatch)[]
+                {
+                    // Original bug report pattern
+                    // The balancing group (?'2-1'(?'x1'..)) captures content that comes BEFORE group 1's capture
+                    (@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))",
+                        "00123xzacvb1", true, "00123xzacvb1"),
+
+                    // Simpler test case: balancing group in forward context (normal case)
+                    (@"(a)(?'2-1'b)(?(2)c|d)", "abc", true, "abc"),
+
+                    // Balancing group in lookbehind where captured content comes after balanced group
+                    (@"(a)b(?<=(?'2-1'.))c(?(2)d|e)", "abcd", true, "abcd"),
+
+                    // Balancing group in lookbehind where captured content comes before balanced group (bug scenario)
+                    (@"a(b)c(?<=(?'2-1'a)..)d(?(2)e|f)", "abcde", true, "abcde"),
+                };
+
+                Regex[] regexes = RegexHelpers.GetRegexes(engine, cases.Select(c => (c.Pattern, (System.Globalization.CultureInfo?)null, (RegexOptions?)null, (TimeSpan?)null)).ToArray());
+                for (int i = 0; i < cases.Length; i++)
+                {
+                    yield return new object[] { engine, regexes[i], cases[i].Input, cases[i].ExpectedGroup2Matched, cases[i].ExpectedMatch };
+                }
+            }
+        }
+
+        /// <summary>
+        /// Tests that IsMatched() behavior is consistent with Group.Success and Group.Captures.Count
+        /// after TidyBalancing is called.
+        /// </summary>
+        [Theory]
+        [MemberData(nameof(BalancingGroup_IsMatched_Consistency_MemberData))]
+        public void BalancingGroup_IsMatched_Consistency(RegexEngine engine, Regex regex, string input, int groupNumber, bool expectedMatched)
+        {
+            _ = engine; // To satisfy xUnit analyzer
+            Match m = regex.Match(input);
+
+            Assert.True(m.Success, $"Match should succeed for input '{input}'");
+
+            // Check that the group state is consistent
+            bool groupSuccess = m.Groups[groupNumber].Success;
+            int capturesCount = m.Groups[groupNumber].Captures.Count;
+
+            Assert.Equal(expectedMatched, groupSuccess);
+            if (expectedMatched)
+            {
+                Assert.True(capturesCount > 0, $"If group {groupNumber} matched, it should have at least one capture");
+            }
+            else
+            {
+                Assert.Equal(0, capturesCount);
+            }
+        }
+
+        public static IEnumerable<object[]> BalancingGroup_IsMatched_Consistency_MemberData()
+        {
+            foreach (RegexEngine engine in RegexHelpers.AvailableEngines)
+            {
+                if (RegexHelpers.IsNonBacktracking(engine))
+                {
+                    continue;
+                }
+
+                var cases = new (string Pattern, string Input, int GroupNumber, bool ExpectedMatched)[]
+                {
+                    // Group 1 should be balanced out (no captures remaining)
+                    (@"(a)(?'2-1'b)", "ab", 1, false),
+
+                    // Group 2 should have a capture
+                    (@"(a)(?'2-1'b)", "ab", 2, true),
+
+                    // Balancing in lookbehind - group 1 should be balanced out
+                    (@"(a)b(?<=(?'2-1'.))c", "abc", 1, false),
+
+                    // Balancing in lookbehind - group 2 should have a capture
+                    (@"(a)b(?<=(?'2-1'.))c", "abc", 2, true),
+
+                    // Original bug pattern - group 1 should be balanced out
+                    (@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b", "00123xzacvb", 1, false),
+
+                    // Original bug pattern - group 2 should have a capture
+                    (@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b", "00123xzacvb", 2, true),
+                };
+
+                Regex[] regexes = RegexHelpers.GetRegexes(engine, cases.Select(c => (c.Pattern, (System.Globalization.CultureInfo?)null, (RegexOptions?)null, (TimeSpan?)null)).ToArray());
+                for (int i = 0; i < cases.Length; i++)
+                {
+                    yield return new object[] { engine, regexes[i], cases[i].Input, cases[i].GroupNumber, cases[i].ExpectedMatched };
+                }
+            }
+        }
+
+        /// <summary>
+        /// Tests various balancing group scenarios to ensure correct behavior.
+        /// </summary>
+        [Theory]
+        [MemberData(nameof(BalancingGroup_Various_MemberData))]
+        public void BalancingGroup_Various_Scenarios(RegexEngine engine, Regex regex, string input, string expectedValue, int expectedGroup1Count, int expectedGroup2Count)
+        {
+            _ = engine; // To satisfy xUnit analyzer
+            Match m = regex.Match(input);
+
+            Assert.True(m.Success);
+            Assert.Equal(expectedValue, m.Value);
+            Assert.Equal(expectedGroup1Count, m.Groups[1].Captures.Count);
+            Assert.Equal(expectedGroup2Count, m.Groups[2].Captures.Count);
+        }
+
+        public static IEnumerable<object[]> BalancingGroup_Various_MemberData()
+        {
+            foreach (RegexEngine engine in RegexHelpers.AvailableEngines)
+            {
+                if (RegexHelpers.IsNonBacktracking(engine))
+                {
+                    continue;
+                }
+
+                var cases = new (string Pattern, string Input, string ExpectedValue, int ExpectedGroup1Count, int ExpectedGroup2Count)[]
+                {
+                    // Basic balancing: group 1 captured, then balanced into group 2
+                    // Creates a zero-length capture in group 2
+                    (@"(a)(?'2-1'b)", "ab", "ab", 0, 1),
+
+                    // Multiple captures: group 2 is the second (a), then balancing transfers from group 1
+                    // Group 2 gets its own capture plus a zero-length capture from balancing
+                    (@"(a)(a)(?'2-1'b)", "aab", "aab", 0, 2),
+                };
+
+                Regex[] regexes = RegexHelpers.GetRegexes(engine, cases.Select(c => (c.Pattern, (System.Globalization.CultureInfo?)null, (RegexOptions?)null, (TimeSpan?)null)).ToArray());
+                for (int i = 0; i < cases.Length; i++)
+                {
+                    yield return new object[] { engine, regexes[i], cases[i].Input, cases[i].ExpectedValue, cases[i].ExpectedGroup1Count, cases[i].ExpectedGroup2Count };
+                }
+            }
+        }
+    }
+}

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/System.Text.RegularExpressions.Tests.csproj

@@ -21,6 +21,7 @@
     <Compile Include="GroupCollectionTests.cs" />
     <Compile Include="MatchCollectionTests.cs" />
     <Compile Include="MonoRegexTests.cs" />
+    <Compile Include="Regex.BalancingGroups.Tests.cs" />
     <Compile Include="Regex.CompileToAssembly.Tests.cs" />
     <Compile Include="Regex.Ctor.Tests.cs" />
     <Compile Include="Regex.Cache.Tests.cs" />