Add Match and isMatch ReadOnlySpan<char> overloads as well as Regex protected methods which expose ReadOnlySpan<char> to avoid allocations.

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -222,7 +222,15 @@ private static ImmutableArray<Diagnostic> EmitRegexMethod(IndentedTextWriter wri
             DescribeExpression(writer, rm.Code.Tree.Root.Child(0), "            // ", rm.Code); // skip implicit root capture
             writer.WriteLine();
 
-            writer.WriteLine($"            protected override bool FindFirstChar()");
+            writer.WriteLine($"            protected override void Scan(global::System.ReadOnlySpan<char> text)");
+            writer.WriteLine($"            {{");
+            writer.Indent += 4;
+            EmitScan(writer, rm, id);
+            writer.Indent -= 4;
+            writer.WriteLine($"            }}");
+            writer.WriteLine();
+
+            writer.WriteLine($"            private bool FindFirstChar(global::System.ReadOnlySpan<char> inputSpan)");
             writer.WriteLine($"            {{");
             writer.Indent += 4;
             RequiredHelperFunctions requiredHelpers = EmitFindFirstChar(writer, rm, id);
@@ -233,7 +241,7 @@ private static ImmutableArray<Diagnostic> EmitRegexMethod(IndentedTextWriter wri
             {
                 writer.WriteLine($"            [global::System.Runtime.CompilerServices.SkipLocalsInit]");
             }
-            writer.WriteLine($"            protected override void Go()");
+            writer.WriteLine($"            private bool Go(global::System.ReadOnlySpan<char> inputSpan)");
             writer.WriteLine($"            {{");
             writer.Indent += 4;
             requiredHelpers |= EmitGo(writer, rm, id);
@@ -299,6 +307,43 @@ static void AppendHashtableContents(IndentedTextWriter writer, Hashtable ht)
             }
         }
 
+        private static void EmitScan(IndentedTextWriter writer, RegexMethod rm, string id)
+        {
+            using (EmitBlock(writer, "while (true)"))
+            {
+                using (EmitBlock(writer, "if (FindFirstChar(text))"))
+                {
+                    if (rm.MatchTimeout != Timeout.Infinite)
+                    {
+                        writer.WriteLine("base.CheckTimeout();");
+                        writer.WriteLine();
+                    }
+
+                    writer.WriteLine("// If we got a match, we're done.");
+                    using (EmitBlock(writer, "if (Go(text))"))
+                    {
+                        writer.WriteLine("return;");
+                    }
+                    writer.WriteLine();
+
+                    writer.WriteLine("// Reset state for another iteration.");
+                    writer.WriteLine("base.runtrackpos = base.runtrack!.Length;");
+                    writer.WriteLine("base.runstackpos = base.runstack!.Length;");
+                    writer.WriteLine("base.runcrawlpos = base.runcrawl!.Length;");
+                }
+                writer.WriteLine();
+
+                writer.WriteLine("// We failed to find a match. If we're at the end of the input, then we are done.");
+                using (EmitBlock(writer, "if (base.runtextpos == text.Length)"))
+                {
+                    writer.WriteLine("return;");
+                }
+                writer.WriteLine();
+
+                writer.WriteLine("base.runtextpos++;");
+            }
+        }
+
         /// <summary>Emits the body of the FindFirstChar override.</summary>
         private static RequiredHelperFunctions EmitFindFirstChar(IndentedTextWriter writer, RegexMethod rm, string id)
         {
@@ -347,7 +392,6 @@ private static RequiredHelperFunctions EmitFindFirstChar(IndentedTextWriter writ
                     {
                         case FindNextStartingPositionMode.LeadingPrefix_LeftToRight_CaseSensitive:
                             Debug.Assert(!string.IsNullOrEmpty(code.FindOptimizations.LeadingCaseSensitivePrefix));
-                            additionalDeclarations.Add("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
                             EmitIndexOf(code.FindOptimizations.LeadingCaseSensitivePrefix);
                             break;
 
@@ -356,13 +400,11 @@ private static RequiredHelperFunctions EmitFindFirstChar(IndentedTextWriter writ
                         case FindNextStartingPositionMode.LeadingSet_LeftToRight_CaseSensitive:
                         case FindNextStartingPositionMode.LeadingSet_LeftToRight_CaseInsensitive:
                             Debug.Assert(code.FindOptimizations.FixedDistanceSets is { Count: > 0 });
-                            additionalDeclarations.Add("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
                             EmitFixedSet();
                             break;
 
                         case FindNextStartingPositionMode.LiteralAfterLoop_LeftToRight_CaseSensitive:
                             Debug.Assert(code.FindOptimizations.LiteralAfterLoop is not null);
-                            additionalDeclarations.Add("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
                             EmitLiteralAfterAtomicLoop();
                             break;
 
@@ -463,7 +505,6 @@ bool EmitAnchors()
                         // the other anchors, which all skip all subsequent processing if found, with BOL we just use it
                         // to boost our position to the next line, and then continue normally with any searches.
                         writer.WriteLine("// Beginning-of-line anchor");
-                        additionalDeclarations.Add("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
                         additionalDeclarations.Add("int beginning = base.runtextbeg;");
                         using (EmitBlock(writer, "if (pos > beginning && inputSpan[pos - 1] != '\\n')"))
                         {
@@ -763,13 +804,15 @@ private static RequiredHelperFunctions EmitGo(IndentedTextWriter writer, RegexMe
                     writer.WriteLine($"int end = start + {(node.Kind == RegexNodeKind.Multi ? node.Str!.Length : 1)};");
                     writer.WriteLine("base.Capture(0, start, end);");
                     writer.WriteLine("base.runtextpos = end;");
+                    writer.WriteLine("return true;");
                     return requiredHelpers;
 
                 case RegexNodeKind.Empty:
                     // This case isn't common in production, but it's very common when first getting started with the
                     // source generator and seeing what happens as you add more to expressions.  When approaching
                     // it from a learning perspective, this is very common, as it's the empty string you start with.
                     writer.WriteLine("base.Capture(0, base.runtextpos, base.runtextpos);");
+                    writer.WriteLine("return true;");
                     return requiredHelpers;
             }
 
@@ -781,7 +824,6 @@ private static RequiredHelperFunctions EmitGo(IndentedTextWriter writer, RegexMe
 
             // Declare some locals.
             string sliceSpan = "slice";
-            writer.WriteLine("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
             writer.WriteLine("int pos = base.runtextpos, end = base.runtextend;");
             writer.WriteLine($"int original_pos = pos;");
             bool hasTimeout = EmitLoopTimeoutCounterIfNeeded(writer, rm);
@@ -826,7 +868,7 @@ private static RequiredHelperFunctions EmitGo(IndentedTextWriter writer, RegexMe
             }
             writer.WriteLine("base.runtextpos = pos;");
             writer.WriteLine("base.Capture(0, original_pos, pos);");
-            writer.WriteLine("return;");
+            writer.WriteLine("return true;");
             writer.WriteLine();
 
             // We only get here in the code if the whole expression fails to match and jumps to
@@ -837,6 +879,7 @@ private static RequiredHelperFunctions EmitGo(IndentedTextWriter writer, RegexMe
             {
                 EmitUncaptureUntil("0");
             }
+            writer.WriteLine("return false;");
 
             // We're done with the match.
 
@@ -846,8 +889,6 @@ private static RequiredHelperFunctions EmitGo(IndentedTextWriter writer, RegexMe
             // And emit any required helpers.
             if (additionalLocalFunctions.Count != 0)
             {
-                writer.WriteLine("return;"); // not strictly necessary, just for readability
-
                 foreach (KeyValuePair<string, string[]> localFunctions in additionalLocalFunctions.OrderBy(k => k.Key))
                 {
                     writer.WriteLine();
@@ -2148,7 +2189,7 @@ void EmitBoundary(RegexNode node)
                     _ => "base.IsECMABoundary",
                 };
 
-                using (EmitBlock(writer, $"if ({call}(pos{(sliceStaticPos > 0 ? $" + {sliceStaticPos}" : "")}, base.runtextbeg, end))"))
+                using (EmitBlock(writer, $"if ({call}(inputSpan, pos{(sliceStaticPos > 0 ? $" + {sliceStaticPos}" : "")}, base.runtextbeg, end))"))
                 {
                     writer.WriteLine($"goto {doneLabel};");
                 }

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

@@ -171,6 +171,10 @@ public static void CompileToAssembly(System.Text.RegularExpressions.RegexCompila
         public string GroupNameFromNumber(int i) { throw null; }
         public int GroupNumberFromName(string name) { throw null; }
         protected void InitializeReferences() { }
+        public bool IsMatch(System.ReadOnlySpan<char> input) { throw null; }
+        public static bool IsMatch(System.ReadOnlySpan<char> input, [System.Diagnostics.CodeAnalysis.StringSyntaxAttribute(System.Diagnostics.CodeAnalysis.StringSyntaxAttribute.Regex)] string pattern) { throw null; }
+        public static bool IsMatch(System.ReadOnlySpan<char> input, [System.Diagnostics.CodeAnalysis.StringSyntaxAttribute(System.Diagnostics.CodeAnalysis.StringSyntaxAttribute.Regex, "options")] string pattern, System.Text.RegularExpressions.RegexOptions options) { throw null; }
+        public static bool IsMatch(System.ReadOnlySpan<char> input, [System.Diagnostics.CodeAnalysis.StringSyntaxAttribute(System.Diagnostics.CodeAnalysis.StringSyntaxAttribute.Regex, "options")] string pattern, System.Text.RegularExpressions.RegexOptions options, System.TimeSpan matchTimeout) { throw null; }
         public bool IsMatch(string input) { throw null; }
         public bool IsMatch(string input, int startat) { throw null; }
         public static bool IsMatch(string input, [System.Diagnostics.CodeAnalysis.StringSyntaxAttribute(System.Diagnostics.CodeAnalysis.StringSyntaxAttribute.Regex)] string pattern) { throw null; }
@@ -330,17 +334,20 @@ protected void DoubleCrawl() { }
         protected void DoubleStack() { }
         protected void DoubleTrack() { }
         protected void EnsureStorage() { }
-        protected abstract bool FindFirstChar();
-        protected abstract void Go();
-        protected abstract void InitTrackCount();
+        protected virtual bool FindFirstChar() { throw null; }
+        protected virtual void Go() { throw null; }
+        protected virtual void InitTrackCount() { throw null; }
         protected bool IsBoundary(int index, int startpos, int endpos) { throw null; }
+        protected bool IsBoundary(System.ReadOnlySpan<char> inputSpan, int index, int startpos, int endpos) { throw null; } // -> This is just temporary on the prototype. Method will be emitted by the generator engines
         protected bool IsECMABoundary(int index, int startpos, int endpos) { throw null; }
+        protected bool IsECMABoundary(System.ReadOnlySpan<char> inputSpan, int index, int startpos, int endpos) { throw null; } // -> This is just temporary on the prototype. Method will be emitted by the generator engines
         protected bool IsMatched(int cap) { throw null; }
         protected int MatchIndex(int cap) { throw null; }
         protected int MatchLength(int cap) { throw null; }
         protected int Popcrawl() { throw null; }
         protected internal System.Text.RegularExpressions.Match? Scan(System.Text.RegularExpressions.Regex regex, string text, int textbeg, int textend, int textstart, int prevlen, bool quick) { throw null; }
         protected internal System.Text.RegularExpressions.Match? Scan(System.Text.RegularExpressions.Regex regex, string text, int textbeg, int textend, int textstart, int prevlen, bool quick, System.TimeSpan timeout) { throw null; }
+        protected internal virtual void Scan(System.ReadOnlySpan<char> text) { throw null; }
         protected void TransferCapture(int capnum, int uncapnum, int start, int end) { }
         protected void Uncapture() { }
     }

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

@@ -9,7 +9,7 @@ namespace System.Text.RegularExpressions
     /// </summary>
     public class Capture
     {
-        internal Capture(string text, int index, int length)
+        internal Capture(string? text, int index, int length)
         {
             Text = text;
             Index = index;
@@ -19,27 +19,38 @@ internal Capture(string text, int index, int length)
         /// <summary>Returns the position in the original string where the first character of captured substring was found.</summary>
         public int Index { get; private protected set; }
 
+        /// <summary>
+        /// This method should only be called when the text for matching was sliced with a different beginning, so the resulting index of
+        /// the match is not from the start of the text, but instead the start of the slice. This method will add back that extra indices
+        /// to account for the original text beginning.
+        /// </summary>
+        /// <param name="beginning">The original text's beginning offset.</param>
+        internal void AddBeginningToIndex(int beginning)
+        {
+            Index += beginning;
+        }
+
         /// <summary>Returns the length of the captured substring.</summary>
         public int Length { get; private protected set; }
 
         /// <summary>The original string</summary>
-        internal string Text { get; set; }
+        internal string? Text { get; set; }
 
         /// <summary>Gets the captured substring from the input string.</summary>
         /// <value>The substring that is captured by the match.</value>
-        public string Value => Text.Substring(Index, Length);
+        public string Value => Text is string text ? text.Substring(Index, Length) : string.Empty;
 
         /// <summary>Gets the captured span from the input string.</summary>
         /// <value>The span that is captured by the match.</value>
-        public ReadOnlySpan<char> ValueSpan => Text.AsSpan(Index, Length);
+        public ReadOnlySpan<char> ValueSpan => Text is string text ? text.AsSpan(Index, Length) : ReadOnlySpan<char>.Empty;
 
         /// <summary>Returns the substring that was matched.</summary>
         public override string ToString() => Value;
 
         /// <summary>The substring to the left of the capture</summary>
-        internal ReadOnlyMemory<char> GetLeftSubstring() => Text.AsMemory(0, Index);
+        internal ReadOnlyMemory<char> GetLeftSubstring() => Text is string text ? text.AsMemory(0, Index) : ReadOnlyMemory<char>.Empty;
 
         /// <summary>The substring to the right of the capture</summary>
-        internal ReadOnlyMemory<char> GetRightSubstring() => Text.AsMemory(Index + Length, Text.Length - Index - Length);
+        internal ReadOnlyMemory<char> GetRightSubstring() => Text is string text ? text.AsMemory(Index + Length, Text.Length - Index - Length) : ReadOnlyMemory<char>.Empty;
     }
 }

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

@@ -5,20 +5,17 @@ namespace System.Text.RegularExpressions
 {
     internal sealed class CompiledRegexRunner : RegexRunner
     {
-        private readonly Action<RegexRunner> _goMethod;
-        private readonly Func<RegexRunner, bool> _findFirstCharMethod;
+        private readonly ScanDelegate _scanMethod;
 
-        public CompiledRegexRunner(Action<RegexRunner> go, Func<RegexRunner, bool> findFirstChar, int trackCount)
+        internal delegate void ScanDelegate(RegexRunner runner, ReadOnlySpan<char> text);
+
+        public CompiledRegexRunner(ScanDelegate scan, int trackCount)
         {
-            _goMethod = go;
-            _findFirstCharMethod = findFirstChar;
+            _scanMethod = scan;
             runtrackcount = trackCount;
         }
 
-        protected override void Go() => _goMethod(this);
-
-        protected override bool FindFirstChar() => _findFirstCharMethod(this);
-
-        protected override void InitTrackCount() { }
+        protected internal override void Scan(ReadOnlySpan<char> text)
+            => _scanMethod(this, text);
     }
 }

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

@@ -7,25 +7,21 @@ namespace System.Text.RegularExpressions
 {
     internal sealed class CompiledRegexRunnerFactory : RegexRunnerFactory
     {
-        private readonly DynamicMethod _goMethod;
-        private readonly DynamicMethod _findFirstCharMethod;
+        private readonly DynamicMethod _scanMethod;
         private readonly int _trackcount;
 
         // Delegates are lazily created to avoid forcing JIT'ing until the regex is actually executed.
-        private Action<RegexRunner>? _go;
-        private Func<RegexRunner, bool>? _findFirstChar;
+        private CompiledRegexRunner.ScanDelegate? _scan;
 
-        public CompiledRegexRunnerFactory(DynamicMethod goMethod, DynamicMethod findFirstCharMethod, int trackcount)
+        public CompiledRegexRunnerFactory(DynamicMethod scanMethod, int trackcount)
         {
-            _goMethod = goMethod;
-            _findFirstCharMethod = findFirstCharMethod;
+            _scanMethod = scanMethod;
             _trackcount = trackcount;
         }
 
         protected internal override RegexRunner CreateInstance() =>
             new CompiledRegexRunner(
-                _go ??= _goMethod.CreateDelegate<Action<RegexRunner>>(),
-                _findFirstChar ??= _findFirstCharMethod.CreateDelegate<Func<RegexRunner, bool>>(),
+                _scan ??= _scanMethod.CreateDelegate<CompiledRegexRunner.ScanDelegate>(),
                 _trackcount);
     }
 }

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

@@ -16,7 +16,7 @@ public class Group : Capture
         internal int _capcount;
         internal CaptureCollection? _capcoll;
 
-        internal Group(string text, int[] caps, int capcount, string name)
+        internal Group(string? text, int[] caps, int capcount, string name)
             : base(text, capcount == 0 ? 0 : caps[(capcount - 1) * 2], capcount == 0 ? 0 : caps[(capcount * 2) - 1])
         {
             _caps = caps;

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

@@ -50,7 +50,7 @@ public class Match : Group
         internal bool _balancing;        // whether we've done any balancing with this match.  If we
                                          // have done balancing, we'll need to do extra work in Tidy().
 
-        internal Match(Regex? regex, int capcount, string text, int begpos, int len, int startpos) :
+        internal Match(Regex? regex, int capcount, string? text, int begpos, int len, int startpos) :
             base(text, new int[2], 0, "0")
         {
             _regex = regex;
@@ -66,7 +66,7 @@ internal Match(Regex? regex, int capcount, string text, int begpos, int len, int
         /// <summary>Returns an empty Match object.</summary>
         public static Match Empty { get; } = new Match(null, 1, string.Empty, 0, 0, 0);
 
-        internal void Reset(Regex regex, string text, int textbeg, int textend, int textstart)
+        internal void Reset(Regex regex, string? text, int textbeg, int textend, int textstart)
         {
             _regex = regex;
             Text = text;
@@ -94,6 +94,7 @@ internal void Reset(Regex regex, string text, int textbeg, int textend, int text
         public Match NextMatch()
         {
             Regex? r = _regex;
+            Debug.Assert(Text != null);
             return r != null ?
                 r.Run(false, Length, Text, _textbeg, _textend - _textbeg, _textpos)! :
                 this;
@@ -338,7 +339,7 @@ internal sealed class MatchSparse : Match
     {
         private new readonly Hashtable _caps;
 
-        internal MatchSparse(Regex regex, Hashtable caps, int capcount, string text, int begpos, int len, int startpos) :
+        internal MatchSparse(Regex regex, Hashtable caps, int capcount, string? text, int begpos, int len, int startpos) :
             base(regex, capcount, text, begpos, len, startpos)
         {
             _caps = caps;