Fix handling of \G in Regex.Split/Replace (#44975)

In our optimized Regex.Split loop, we failed to update runtextstart, which means the \G anchor (aka starting where the previous match ended).

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

@@ -322,6 +322,10 @@ internal void Scan<TState>(Regex regex, string text, int textstart, ref TState s
                             return;
                         }
 
+                        // Now that we've matched successfully, update the starting position to reflect
+                        // the current position, just as Match.NextMatch() would pass in _textpos as textstart.
+                        runtextstart = runtextpos;
+
                         // Reset state for another iteration.
                         runtrackpos = runtrack!.Length;
                         runstackpos = runstack!.Length;

src/libraries/System.Text.RegularExpressions/tests/Regex.KnownPattern.Tests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Globalization;
+using System.Linq;
 using Xunit;
 
 namespace System.Text.RegularExpressions.Tests
@@ -873,6 +874,33 @@ public void Docs_EndOfLineComment(RegexOptions options)
         }
 
 
+        // https://docs.microsoft.com/en-us/dotnet/standard/base-types/anchors-in-regular-expressions#contiguous-matches-g
+        [Theory]
+        [InlineData(RegexOptions.None)]
+        [InlineData(RegexOptions.Compiled)]
+        public void Docs_Anchors_ContiguousMatches(RegexOptions options)
+        {
+            const string Input = "capybara,squirrel,chipmunk,porcupine";
+            const string Pattern = @"\G(\w+\s?\w*),?";
+            string[] expected = new[] { "capybara", "squirrel", "chipmunk", "porcupine" };
+
+            Match m = Regex.Match(Input, Pattern, options);
+
+            string[] actual = new string[4];
+            for (int i = 0; i < actual.Length; i++)
+            {
+                Assert.True(m.Success);
+                actual[i] = m.Groups[1].Value;
+                m = m.NextMatch();
+            }
+            Assert.False(m.Success);
+            Assert.Equal(expected, actual);
+
+            Assert.Equal(
+                ",arabypac,lerriuqs,knumpihcenipucrop",
+                Regex.Replace(Input, Pattern, m => string.Concat(m.Value.Reverse())));
+        }
+
 
         //
         // These patterns come from real-world customer usages

src/libraries/System.Text.RegularExpressions/tests/Regex.Replace.Tests.cs

@@ -100,6 +100,9 @@ public static IEnumerable<object[]> Replace_String_TestData()
 
             yield return new object[] { "([1-9])([1-9])([1-9])def", "abc123def!", "$+", RegexOptions.RightToLeft, -1, 10, "abc3!" };
             yield return new object[] { "([1-9])([1-9])([1-9])def", "abc123def!", "$_", RegexOptions.RightToLeft, -1, 10, "abcabc123def!!" };
+
+            // Anchors
+            yield return new object[] { @"\Ga", "aaaaa", "b", RegexOptions.None, 5, 0, "bbbbb" };
         }
 
         [Theory]

src/libraries/System.Text.RegularExpressions/tests/Regex.Split.Tests.cs

@@ -52,6 +52,9 @@ public static IEnumerable<object[]> Split_TestData()
             yield return new object[] { @"\d", "1a2b3c4d5e6f7g8h9i0k", RegexOptions.RightToLeft, 10, 20, new string[] { "1a", "b", "c", "d", "e", "f", "g", "h", "i", "k" } };
             yield return new object[] { @"\d", "1a2b3c4d5e6f7g8h9i0k", RegexOptions.RightToLeft, 2, 20, new string[] { "1a2b3c4d5e6f7g8h9i", "k" } };
             yield return new object[] { @"\d", "1a2b3c4d5e6f7g8h9i0k", RegexOptions.RightToLeft, 1, 20, new string[] { "1a2b3c4d5e6f7g8h9i0k" } };
+
+            // Anchors
+            yield return new object[] { @"(?<=\G..)(?=..)", "aabbccdd", RegexOptions.None, 8, 0, new string[] { "aa", "bb", "cc", "dd" } };
         }
 
         [Theory]
@@ -60,7 +63,7 @@ public static IEnumerable<object[]> Split_TestData()
         public void Split(string pattern, string input, RegexOptions options, int count, int start, string[] expected)
         {
             bool isDefaultStart = RegexHelpers.IsDefaultStart(input, options, start);
-            bool isDefaultCount = RegexHelpers.IsDefaultStart(input, options, count);
+            bool isDefaultCount = RegexHelpers.IsDefaultCount(input, options, count);
             if (options == RegexOptions.None)
             {
                 // Use Split(string), Split(string, string), Split(string, int) or Split(string, int, int)